Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
The document provides an introduction to the geological time scale and methods for dating rocks and events in Earth's history. It discusses that the geological time scale is a system that relates geological strata and events to periods of time in Earth's past. Relative dating methods look at principles like superposition to determine the sequence of rock layers, while absolute dating provides numerical ages using radiometric dating techniques like uranium-lead dating. The document outlines important divisions of the geological time scale including eons, eras, periods, and epochs, and summarizes some key boundaries that indicate changes in life forms, like the Precambrian-Cambrian boundary marking the rise of multicellular life.
Beyond the Drake Equation: A Time-Dependent Inventory of Habitable Planets an...Sérgio Sacani
We introduce a mathematical framework for statistical exoplanet population and astrobiology studies
that may help directing future observational efforts and experiments. The approach is based on a
set of differential equations and provides a time-dependent mapping between star formation, metal
enrichment, and the occurrence of exoplanets and potentially life-harboring worlds over the chemopopulation history of the solar neighborhood. Our results are summarized as follows: 1) the formation
of exoplanets in the solar vicinity was episodic, starting with the emergence of the thick disk about
11 Gyr ago; 2) within 100 pc from the Sun, there are as many as 11, 000 (η⊕/0.24) Earth-size planets
in the habitable zone (“temperate terrestrial planets” or TTPs) of K-type stars. The solar system is
younger than the median TTP, and was created in a star formation surge that peaked 5.5 Gyr ago and
was triggered by an external agent; 3) the metallicity modulation of the giant planet occurrence rate
results in a later typical formation time, with TTPs outnumbering giant planets at early times; 4) the
closest, life-harboring Earth-like planet would be ∼
< 20 pc away if microbial life arose as soon as it did
on Earth in ∼
> 1% of the TTPs around K stars. If simple life is abundant (fast abiogenesis), it is also
old, as it would have emerged more than 8 Gyr ago in about one third of all life-bearing planets today.
Older Earth analogs are more likely to have developed sufficiently complex life capable of altering the
environment and producing detectable oxygenic biosignatures.
This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.
Publication date: 13th September 2023
Describe the paths of water through the hydrologic cycle. Explain.docxfrancese2
Describe the paths of water through the hydrologic cycle. Explain the processes and the energy gains and losses involved in the changes of water between its 3 states. Operationally, we often most concerned with water does when it reaches the solid earth, both on the surface and in the sub-surface. Explain the relationship between the saturated zone, the water table, a ground water well and the cone of depression, all within the sub-surface.
The food chain is a valuable concept in biogeography. Give an example of a specific food chain, labeling the various levels of the food chain. After looking at characteristics of food chains, explain how a geographer’s approach to the study of organisms might be different than biologist’s study of organisms; what would each try to emphasize more than the other? What exactly is a biome? Compare/contrast the concept of the biome with that of the zoogeographic region. Compare/contrast the floral characteristics of 2 of the following biomes: Desert, Tundra, Midlatitude Grassland and Boreal Forest.
Theorize the difference in soil development in adjoining soils developed on forested, sloped area versus a grassed flat area. What are the soil-forming factors? Explain the importance of the nature of the parent material to soil formation and type. Then, cite at least 2 examples in which the influence of parent materials might be outweighed by other soil-forming factors. Explain the “struggle” between the internal and external processes in shaping the Earth’s surface. What are the different ways that the surface of the Earth is changed over time?
Describe the general sequence of events in continental drift since the time of 5 separate continents 450 million years ago. What is the difference between the older continental drift theory by Wegener and the more recent plate tectonic theory? Plate tectonics theory explains many seemingly unrelated phenomena. Explain how the patterns of volcanoes and earthquakes related to plate tectonics. Explain several pieces of evidence that combine to make the theory of plate tectonics the one that is generally accepted.
Provide a reason why some scientists believe the Pleistocene is over and a reason why other scientists believe we are now in an interglacial stage. Some believe, for example, that since areas of pack ice and glacial ice still exist we are still in an ice age. Others, on the other hand, seeing the rapid retreat of ice and snow pack in many areas, believes that this period of glaciation has ended. So, using some other justifications, why do we see some differences in interpretation? Is there some scientific data available that can support both sides view? If so, provide it. Why hasn’t this controversy been solved? What impact does this division of views have on the public policies that are enacted by state, national and international bodies?
.
1) Geotectonics refers to the structure and arrangement of rock masses in the Earth's crust resulting from folding and faulting. It involves the processes that control the structure and properties of the Earth's crust and its evolution over time.
2) The theory of plate tectonics states that the Earth's outer layer is made up of large rigid plates that move relative to each other, driven by heat loss from the Earth's interior.
3) Plate tectonics helped explain long-standing questions about earthquakes, volcanoes, and mountain formation and revolutionized understanding of Earth's dynamics.
The document summarizes key concepts in geology, including:
1) The formation of the solar system from a nebula of dust and gas around 4.5 billion years ago, which led to the formation of the Sun and planets through gravitational attraction and other processes.
2) Methods for determining the age of the Earth including absolute radiometric dating techniques that measure radioactive decay and relative dating principles like superposition and cross-cutting relationships.
3) The use of fossils to provide temporal and paleoecological information about Earth's history and environmental conditions of different periods. Index fossils in particular help date and classify rock layers.
Earth Expansion Tectonics - The New Paradigm (Part 4)Proyecto Matriz
“Que cese su guerra de todos contra todos y que en lugar de esto se unan en la conquista
de las rocas. Que el ser humano, en lugar de ir en busca del oro, en busca de fama
o malgastando su fuerza productiva en labores infructíferas, escoja la mejor parte:
la cooperación pacífica en la investigación y el descubrimiento del rumbo de las fuerzas
naturales con el fin de desarrollar productos nutritivos, y el apacible deleite
de las frutas que la tierra puede producir en abundancia para todos..”
JULIUS HENSEL (1892)
The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. Geologists have divided Earth's history into a series of time intervals. These time intervals are not equal in length like the hours in a day. Instead the time intervals are variable in length. This is because geologic time is divided using significant events in the history of the Earth.
The document provides an introduction to the geological time scale and methods for dating rocks and events in Earth's history. It discusses that the geological time scale is a system that relates geological strata and events to periods of time in Earth's past. Relative dating methods look at principles like superposition to determine the sequence of rock layers, while absolute dating provides numerical ages using radiometric dating techniques like uranium-lead dating. The document outlines important divisions of the geological time scale including eons, eras, periods, and epochs, and summarizes some key boundaries that indicate changes in life forms, like the Precambrian-Cambrian boundary marking the rise of multicellular life.
Beyond the Drake Equation: A Time-Dependent Inventory of Habitable Planets an...Sérgio Sacani
We introduce a mathematical framework for statistical exoplanet population and astrobiology studies
that may help directing future observational efforts and experiments. The approach is based on a
set of differential equations and provides a time-dependent mapping between star formation, metal
enrichment, and the occurrence of exoplanets and potentially life-harboring worlds over the chemopopulation history of the solar neighborhood. Our results are summarized as follows: 1) the formation
of exoplanets in the solar vicinity was episodic, starting with the emergence of the thick disk about
11 Gyr ago; 2) within 100 pc from the Sun, there are as many as 11, 000 (η⊕/0.24) Earth-size planets
in the habitable zone (“temperate terrestrial planets” or TTPs) of K-type stars. The solar system is
younger than the median TTP, and was created in a star formation surge that peaked 5.5 Gyr ago and
was triggered by an external agent; 3) the metallicity modulation of the giant planet occurrence rate
results in a later typical formation time, with TTPs outnumbering giant planets at early times; 4) the
closest, life-harboring Earth-like planet would be ∼
< 20 pc away if microbial life arose as soon as it did
on Earth in ∼
> 1% of the TTPs around K stars. If simple life is abundant (fast abiogenesis), it is also
old, as it would have emerged more than 8 Gyr ago in about one third of all life-bearing planets today.
Older Earth analogs are more likely to have developed sufficiently complex life capable of altering the
environment and producing detectable oxygenic biosignatures.
This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.
Publication date: 13th September 2023
Describe the paths of water through the hydrologic cycle. Explain.docxfrancese2
Describe the paths of water through the hydrologic cycle. Explain the processes and the energy gains and losses involved in the changes of water between its 3 states. Operationally, we often most concerned with water does when it reaches the solid earth, both on the surface and in the sub-surface. Explain the relationship between the saturated zone, the water table, a ground water well and the cone of depression, all within the sub-surface.
The food chain is a valuable concept in biogeography. Give an example of a specific food chain, labeling the various levels of the food chain. After looking at characteristics of food chains, explain how a geographer’s approach to the study of organisms might be different than biologist’s study of organisms; what would each try to emphasize more than the other? What exactly is a biome? Compare/contrast the concept of the biome with that of the zoogeographic region. Compare/contrast the floral characteristics of 2 of the following biomes: Desert, Tundra, Midlatitude Grassland and Boreal Forest.
Theorize the difference in soil development in adjoining soils developed on forested, sloped area versus a grassed flat area. What are the soil-forming factors? Explain the importance of the nature of the parent material to soil formation and type. Then, cite at least 2 examples in which the influence of parent materials might be outweighed by other soil-forming factors. Explain the “struggle” between the internal and external processes in shaping the Earth’s surface. What are the different ways that the surface of the Earth is changed over time?
Describe the general sequence of events in continental drift since the time of 5 separate continents 450 million years ago. What is the difference between the older continental drift theory by Wegener and the more recent plate tectonic theory? Plate tectonics theory explains many seemingly unrelated phenomena. Explain how the patterns of volcanoes and earthquakes related to plate tectonics. Explain several pieces of evidence that combine to make the theory of plate tectonics the one that is generally accepted.
Provide a reason why some scientists believe the Pleistocene is over and a reason why other scientists believe we are now in an interglacial stage. Some believe, for example, that since areas of pack ice and glacial ice still exist we are still in an ice age. Others, on the other hand, seeing the rapid retreat of ice and snow pack in many areas, believes that this period of glaciation has ended. So, using some other justifications, why do we see some differences in interpretation? Is there some scientific data available that can support both sides view? If so, provide it. Why hasn’t this controversy been solved? What impact does this division of views have on the public policies that are enacted by state, national and international bodies?
.
1) Geotectonics refers to the structure and arrangement of rock masses in the Earth's crust resulting from folding and faulting. It involves the processes that control the structure and properties of the Earth's crust and its evolution over time.
2) The theory of plate tectonics states that the Earth's outer layer is made up of large rigid plates that move relative to each other, driven by heat loss from the Earth's interior.
3) Plate tectonics helped explain long-standing questions about earthquakes, volcanoes, and mountain formation and revolutionized understanding of Earth's dynamics.
The document summarizes key concepts in geology, including:
1) The formation of the solar system from a nebula of dust and gas around 4.5 billion years ago, which led to the formation of the Sun and planets through gravitational attraction and other processes.
2) Methods for determining the age of the Earth including absolute radiometric dating techniques that measure radioactive decay and relative dating principles like superposition and cross-cutting relationships.
3) The use of fossils to provide temporal and paleoecological information about Earth's history and environmental conditions of different periods. Index fossils in particular help date and classify rock layers.
Earth Expansion Tectonics - The New Paradigm (Part 4)Proyecto Matriz
“Que cese su guerra de todos contra todos y que en lugar de esto se unan en la conquista
de las rocas. Que el ser humano, en lugar de ir en busca del oro, en busca de fama
o malgastando su fuerza productiva en labores infructíferas, escoja la mejor parte:
la cooperación pacífica en la investigación y el descubrimiento del rumbo de las fuerzas
naturales con el fin de desarrollar productos nutritivos, y el apacible deleite
de las frutas que la tierra puede producir en abundancia para todos..”
JULIUS HENSEL (1892)
The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. Geologists have divided Earth's history into a series of time intervals. These time intervals are not equal in length like the hours in a day. Instead the time intervals are variable in length. This is because geologic time is divided using significant events in the history of the Earth.
Large-scale Volcanism and the Heat Death of Terrestrial WorldsSérgio Sacani
This document discusses the potential for large igneous provinces (LIPs) to cause the "heat death" of terrestrial planets through massive volcanic eruptions that overwhelm the climate system. It examines the timing of LIP events on Earth to estimate the likelihood of nearly simultaneous eruptions. Statistical analysis of Earth's LIP record finds that eruptions within 0.1-1 million years of each other are likely. Simultaneous LIPs could have driven Venus into a runaway greenhouse effect like its current state. The timing of LIP events on Earth provides insight into potential past LIP activity on Venus that may have ended its hypothesized earlier temperate climate.
1. The document discusses the quest to detect extra-terrestrial life and the consequences this would have for science and society. It explores both the scientific and societal implications.
2. While only life on Earth is currently known, conditions suitable for harboring life are thought to exist on other worlds like Mars, Europa, and Enceladus. The discovery of life elsewhere would have fundamental impacts and challenge existing views of life's uniqueness.
3. There is much unknown about the origin and distribution of life in the universe. Key questions remain unanswered about whether life follows universal principles or if Earth's life arose from highly improbable chances. Detecting life elsewhere could help answer these questions.
Venus and Earth have remarkably diferent
surface conditions, yet the lithospheric
thickness and heat fow on Venus may be
Earth-like. This fnding supports a tectonic
regime with limited surface mobility and
dominated by intrusive magmatism.
unraveling the secrets of plate tectonics.pptxShubash Sah
Plate tectonics is the scientific theory that explains the movement and interaction of Earth's lithospheric plates, which are large slabs of the planet's crust and upper mantle. This theory has revolutionized our understanding of geological processes, including the formation of mountains, earthquakes, and volcanic activity. Here's an overview of the key aspects and developments in the field of plate tectonics:
Historical Background
Continental Drift Theory:
Proposed by Alfred Wegener in 1912, the idea suggested that continents were once connected in a supercontinent called Pangaea and have since drifted apart.
Wegener's theory was based on the fit of the continents, fossil correlations, and geological similarities across continents.
Development of Plate Tectonics:
In the mid-20th century, technological advances such as seafloor mapping and the study of magnetic anomalies led to the formulation of the plate tectonics theory.
The discovery of mid-ocean ridges and the symmetrical patterns of magnetic stripes on the ocean floor provided evidence for seafloor spreading.
Structure and Movement of Plates
Lithospheric Plates:
Earth's lithosphere is divided into several major and minor plates, including the Pacific, North American, Eurasian, and African plates.
These plates float on the semi-fluid asthenosphere beneath them.
Plate Boundaries:
Divergent Boundaries: Plates move apart, forming new crust, as seen at mid-ocean ridges.
Convergent Boundaries: Plates collide, leading to subduction (one plate sinking beneath another) or mountain building.
Transform Boundaries: Plates slide past each other, causing earthquakes.
Mechanisms of Plate Movement
Mantle Convection:
Heat from Earth's interior causes the mantle to convect, driving the movement of plates.
Hot material rises at mid-ocean ridges, cools, and then sinks at subduction zones.
Ridge Push and Slab Pull:
Ridge Push: Elevated mid-ocean ridges push plates away due to gravity.
Slab Pull: The weight of a subducting plate pulls the rest of the plate along.
Geological Implications
Mountain Building:
At convergent boundaries, the collision of continental plates can create mountain ranges, such as the Himalayas.
Oceanic-continental convergence leads to volcanic mountain ranges.
Earthquakes and Volcanoes:
Most earthquakes occur along plate boundaries due to the release of stress accumulated from plate movements.
Volcanic activity is prominent at divergent and convergent boundaries and hotspots.
Ocean Basin Formation:
Divergent boundaries lead to the creation of new oceanic crust, expanding ocean basins over time.
Recent Advances and Research
GPS and Satellite Technology:
Modern GPS technology allows precise measurement of plate movements, enhancing our understanding of tectonic processes.
Seismic Tomography:
This technique provides images of the Earth's interior, revealing details about mantle convection and plate interactions.
Computer Modeling:
Advanced simulations help scientists predict plate
The document discusses astrobiology and the Search for Extraterrestrial Intelligence (SETI) Institute. It describes the three centers at SETI that focus on researching life in the universe, studying conditions for life on and off Earth, and education/outreach. SETI uses radio telescopes like the Allen Telescope Array to search for signals from intelligent civilizations, but detecting credible signals is challenging. The Drake Equation aims to estimate the number of detectable civilizations but has many unknown variables. While the search for life continues, no conclusive evidence of extraterrestrial intelligence has been found so far.
The document discusses current research in the field of astrobiology. It covers research being done on Mars, Enceladus, and Titan to discover evidence of ancient or current life. On Mars, methane pulses detected by the Curiosity rover could potentially be produced by methanogens living underground. Enceladus shows signs of hydrothermal activity and a subsurface ocean, conditions that could support life. Titan, with its liquid hydrocarbon seas, may be able to support a hypothetical form of life called an azotosome, which could use nitrogen-based membranes instead of cell walls. The document argues that expanding our understanding of possible forms life could take is important for continuing progress in the search for life beyond Earth.
Lecture 2 : Evolutionary Patterns, Rates And Trendsguest42a8fbf
The document discusses evidence for evolution from various fields including biogeography, fossils, comparative anatomy, embryology, biochemistry and genetics. It explains key concepts in evolution like macroevolutionary events, morphological divergence and convergence, reproductive isolation mechanisms, molecular clocks and models of speciation. Continental drift theory and plate tectonics helped explain distribution of fossils and living species, while radiometric dating techniques helped establish geological timescales.
Page 65 4.1 InTroduCTIonIn chapter one, we reviewed th.docxalfred4lewis58146
Page | 65
4.1 InTroduCTIon
In chapter one, we reviewed the scientific method and the exact meaning of a
theory, which is a well-supported explanation for a natural phenomenon that still
cannot be completely proven. A Grand Unifying Theory is a set of ideas that
is central and essential to a field of study such as the theory of gravity in physics
or the theory of evolution in biology. The Grand Unifying Theory of geology is the
theory of Plate Tectonics, which defines the outer portion of the earth as a brit-
tle outer layer that is broken into moving pieces called tectonic plates. This the-
ory is supported by many lines of evidence including the shape of the continents,
the distribution of fossils and rocks, the distribution of environmental indicators,
as well as the location of mountains, volcanoes, trenches, and earthquakes. The
movement of plates can be observed on human timescales and easily measured
using GPS satellites.
Plate tectonics is integral to the study of geology because it aids in reconstruct-
ing earth’s history. This theory helps to explain how the first continents were built,
how oceans formed, and even helps inform hypotheses for the origin of life. The
theory also helps explain the geographic distribution of geologic features such as
mountains, volcanoes, rift valleys, and trenches. Finally, it helps us assess the po-
tential risks of geologic catastrophes such as earthquakes and volcanoes across
the earth. The power of this theory lies in its ability to create testable hypotheses
regarding Earth’s history as well as predictions regarding its future.
4.1.1 learning outcomes
After completing this chapter, you should be able to:
• Explain several lines of evidence supporting the movement of tectonic plates
• Accurately describe the movement of tectonic plates through time
• Describe the progression of a Hawaiian Island and how it relates to the
Theory of Plate Tectonics
4Plate TectonicsBradley Deline
Page | 66
Introductory GeoloGy Plate tectonIcs
• Describe the properties of tectonics plates and how that relates to the
proposed mechanisms driving plate tectonics
• Be able to describe and identify the features that occur at different plate
boundaries
4.1.2 Key Terms
4.2 evIdenCe of The movemenT of ConTInenTs
The idea that the continents appear to have been joined based on their shapes is
not new, in fact this idea first appeared in the writings of Sir Francis Bacon in 1620.
The resulting hypothesis from this observation is rather straightforward: the shapes
of the continents fit together because they were once connected and have since bro-
ken apart and moved. This hypothesis is discussing a historical event in the past and
cannot be directly tested without a time machine. Therefore, geoscientists reframed
the hypothesis by assuming the continents used to be connected and asking what
other patterns we would expect to find. This is exactly how turn of the century earth
scientists (.
The document provides an overview of the field of astrobiology, including its history and goals. It discusses the search for life on Mars, Europa and beyond Earth. The goals of astrobiology include understanding the origin and distribution of life, characterizing habitable environments in the solar system and beyond, and determining how life emerges and evolves in different environments. Both long term goals like searching for life on other worlds and short term goals like Mars exploration are mentioned.
This summary provides the key points from the document in 3 sentences:
The document discusses a study that proposes a new hypothesis for the origin of intraplate volcanism. The study suggests that some intraplate volcanic activity, like the Steens-Columbia River flood basalt, can be explained by tearing of subducting tectonic plates as they slow down, which allows rapid mantle upwelling. While this model may explain some cases of intraplate magmatism, it does not fully account for all aspects of some systems like Yellowstone, so the relationship between plate tectonics and intraplate volcanism remains an area of ongoing research.
The CARCACE project deepwater platforms - modular designs for in situ experim...Ædel Aerospace GmbH
This document describes the CARCACE project which aims to study ecosystems created by large organic falls in the deep Atlantic Ocean. The project involves deploying cow carcasses at 1000m depth in the Setubal Canyon and Azores to study community succession over time. New platform designs were developed to deploy and monitor the carcasses, including a floating platform and reinforced concrete platform anchored to the seafloor. The concrete platform was deployed in March 2011 to begin the first experiment of the CARCACE project.
This document discusses several complications that the fossil record poses for the theory of evolution. It notes that species seem to endure in the fossil record for around a million years on average, which conflicts with the idea of continuous branching and succession of species over long periods of time. It also discusses "living fossils" - species that appear very similar to fossils from millions of years ago. This challenges the idea of constant evolutionary change. Additionally, it describes the Cambrian explosion, where most animal phyla appear suddenly in the Cambrian period, which is inconsistent with a gradual process of evolution. The document argues that the fossil record provides evidence against key assumptions of evolutionary theory.
This document discusses several topics related to earthquakes and earthquake engineering:
- Earthquakes are caused by the sudden movement of tectonic plates along fault lines in the earth's crust.
- The interior structure of the earth is layered, with a solid crust, highly viscous mantle, liquid outer core, and solid inner core.
- Earthquake ground motions can be amplified by local surface geology and sediment thickness, influencing seismic damage. Proper understanding of ground conditions is important for hazard assessment and mitigation.
The document discusses several theories for the origin of life, including the primordial soup hypothesis proposed by Alexander Oparin, the iron-sulfur world hypothesis proposed by Gunter Wachtershäuser, and the hypothesis that life began around hydrothermal vents. It notes that while the primordial soup hypothesis involves chemicals combining in an ocean or pond, the iron-sulfur world and hydrothermal vent hypotheses propose that life began in environments near hydrothermal vents, where chemical reactions could have been driven by thermal energy.
This document reviews exoplanet habitability. It discusses how the conventionally habitable planet requires surface liquid water, but the diversity of known exoplanets suggests planets can be habitable even if different from Earth. Over 1000 exoplanets are now known, and thousands more candidates have been identified. The habitable zone is defined as the region where a planet can have surface temperatures allowing liquid water. However, a planet in the habitable zone is not guaranteed to be habitable, as Venus and Earth demonstrate. Considering the diversity of exoplanets, a broader view of planetary habitability will maximize the chances of identifying a habitable world.
An ESA Study For The Search For Life On MarsFaith Brown
This document proposes a hypothetical strategy to search for life on Mars using robotic missions. It discusses the following key points:
1. Mars and early Earth had similar environments that could have supported the development of life, so life may have arisen on Mars as well in a primitive prokaryotic form.
2. A robotic mission should carefully select landing sites with good exobiological potential to find chemical or morphological biosignatures. Samples would be collected and analyzed using integrated instruments on a lander and rover.
3. The goal is to obtain environmental data, look for microscopic evidence of life, analyze biogeochemistry, and identify potential niches for extant life to increase the chances of detecting past or present
The document summarizes Earth's geologic history condensed into one calendar year. Key events include:
- By March, oceans formed but no life existed on the barren planet.
- First life emerged in April in the form of single-celled organisms near ocean vents.
- By December, more complex sea creatures evolved and the first plants colonized land despite heavy rains.
- On December 31st, early humans appeared in the last hour of the year along with Neanderthals and cave drawings. Modern civilizations emerged in the final minutes.
There are few places left on the planet where the impact of people has not been felt. We have explored and left our footprint on nearly every corner of the globe. As our population and needs grow, we are leaving less and less room for wildlife.
Wildlife are under threat from many different kinds of human activities, from directly destroying habitat to spreading invasive species and disease. Most ecosystems are facing multiple threats. Each new threat puts additional stress on already weakened ecosystems and their wildlife.
Artigo relata como a Terra sofreu com os impactos de ateroides a 4 bilhões de anos atrás, e como a superfície do planeta foi remodelada e os oceanos formados.
This document provides an overview of plate tectonics and its history. It discusses how the theory of plate tectonics emerged in the 1960s and revolutionized earth sciences by explaining phenomena like earthquakes and mountain building. It also summarizes early ideas around continental drift proposed by scientists like Alfred Wegener in the early 20th century, and how plate tectonics unified and built upon these early concepts.
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
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This document discusses the potential for large igneous provinces (LIPs) to cause the "heat death" of terrestrial planets through massive volcanic eruptions that overwhelm the climate system. It examines the timing of LIP events on Earth to estimate the likelihood of nearly simultaneous eruptions. Statistical analysis of Earth's LIP record finds that eruptions within 0.1-1 million years of each other are likely. Simultaneous LIPs could have driven Venus into a runaway greenhouse effect like its current state. The timing of LIP events on Earth provides insight into potential past LIP activity on Venus that may have ended its hypothesized earlier temperate climate.
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Venus and Earth have remarkably diferent
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unraveling the secrets of plate tectonics.pptxShubash Sah
Plate tectonics is the scientific theory that explains the movement and interaction of Earth's lithospheric plates, which are large slabs of the planet's crust and upper mantle. This theory has revolutionized our understanding of geological processes, including the formation of mountains, earthquakes, and volcanic activity. Here's an overview of the key aspects and developments in the field of plate tectonics:
Historical Background
Continental Drift Theory:
Proposed by Alfred Wegener in 1912, the idea suggested that continents were once connected in a supercontinent called Pangaea and have since drifted apart.
Wegener's theory was based on the fit of the continents, fossil correlations, and geological similarities across continents.
Development of Plate Tectonics:
In the mid-20th century, technological advances such as seafloor mapping and the study of magnetic anomalies led to the formulation of the plate tectonics theory.
The discovery of mid-ocean ridges and the symmetrical patterns of magnetic stripes on the ocean floor provided evidence for seafloor spreading.
Structure and Movement of Plates
Lithospheric Plates:
Earth's lithosphere is divided into several major and minor plates, including the Pacific, North American, Eurasian, and African plates.
These plates float on the semi-fluid asthenosphere beneath them.
Plate Boundaries:
Divergent Boundaries: Plates move apart, forming new crust, as seen at mid-ocean ridges.
Convergent Boundaries: Plates collide, leading to subduction (one plate sinking beneath another) or mountain building.
Transform Boundaries: Plates slide past each other, causing earthquakes.
Mechanisms of Plate Movement
Mantle Convection:
Heat from Earth's interior causes the mantle to convect, driving the movement of plates.
Hot material rises at mid-ocean ridges, cools, and then sinks at subduction zones.
Ridge Push and Slab Pull:
Ridge Push: Elevated mid-ocean ridges push plates away due to gravity.
Slab Pull: The weight of a subducting plate pulls the rest of the plate along.
Geological Implications
Mountain Building:
At convergent boundaries, the collision of continental plates can create mountain ranges, such as the Himalayas.
Oceanic-continental convergence leads to volcanic mountain ranges.
Earthquakes and Volcanoes:
Most earthquakes occur along plate boundaries due to the release of stress accumulated from plate movements.
Volcanic activity is prominent at divergent and convergent boundaries and hotspots.
Ocean Basin Formation:
Divergent boundaries lead to the creation of new oceanic crust, expanding ocean basins over time.
Recent Advances and Research
GPS and Satellite Technology:
Modern GPS technology allows precise measurement of plate movements, enhancing our understanding of tectonic processes.
Seismic Tomography:
This technique provides images of the Earth's interior, revealing details about mantle convection and plate interactions.
Computer Modeling:
Advanced simulations help scientists predict plate
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The document discusses current research in the field of astrobiology. It covers research being done on Mars, Enceladus, and Titan to discover evidence of ancient or current life. On Mars, methane pulses detected by the Curiosity rover could potentially be produced by methanogens living underground. Enceladus shows signs of hydrothermal activity and a subsurface ocean, conditions that could support life. Titan, with its liquid hydrocarbon seas, may be able to support a hypothetical form of life called an azotosome, which could use nitrogen-based membranes instead of cell walls. The document argues that expanding our understanding of possible forms life could take is important for continuing progress in the search for life beyond Earth.
Lecture 2 : Evolutionary Patterns, Rates And Trendsguest42a8fbf
The document discusses evidence for evolution from various fields including biogeography, fossils, comparative anatomy, embryology, biochemistry and genetics. It explains key concepts in evolution like macroevolutionary events, morphological divergence and convergence, reproductive isolation mechanisms, molecular clocks and models of speciation. Continental drift theory and plate tectonics helped explain distribution of fossils and living species, while radiometric dating techniques helped establish geological timescales.
Page 65 4.1 InTroduCTIonIn chapter one, we reviewed th.docxalfred4lewis58146
Page | 65
4.1 InTroduCTIon
In chapter one, we reviewed the scientific method and the exact meaning of a
theory, which is a well-supported explanation for a natural phenomenon that still
cannot be completely proven. A Grand Unifying Theory is a set of ideas that
is central and essential to a field of study such as the theory of gravity in physics
or the theory of evolution in biology. The Grand Unifying Theory of geology is the
theory of Plate Tectonics, which defines the outer portion of the earth as a brit-
tle outer layer that is broken into moving pieces called tectonic plates. This the-
ory is supported by many lines of evidence including the shape of the continents,
the distribution of fossils and rocks, the distribution of environmental indicators,
as well as the location of mountains, volcanoes, trenches, and earthquakes. The
movement of plates can be observed on human timescales and easily measured
using GPS satellites.
Plate tectonics is integral to the study of geology because it aids in reconstruct-
ing earth’s history. This theory helps to explain how the first continents were built,
how oceans formed, and even helps inform hypotheses for the origin of life. The
theory also helps explain the geographic distribution of geologic features such as
mountains, volcanoes, rift valleys, and trenches. Finally, it helps us assess the po-
tential risks of geologic catastrophes such as earthquakes and volcanoes across
the earth. The power of this theory lies in its ability to create testable hypotheses
regarding Earth’s history as well as predictions regarding its future.
4.1.1 learning outcomes
After completing this chapter, you should be able to:
• Explain several lines of evidence supporting the movement of tectonic plates
• Accurately describe the movement of tectonic plates through time
• Describe the progression of a Hawaiian Island and how it relates to the
Theory of Plate Tectonics
4Plate TectonicsBradley Deline
Page | 66
Introductory GeoloGy Plate tectonIcs
• Describe the properties of tectonics plates and how that relates to the
proposed mechanisms driving plate tectonics
• Be able to describe and identify the features that occur at different plate
boundaries
4.1.2 Key Terms
4.2 evIdenCe of The movemenT of ConTInenTs
The idea that the continents appear to have been joined based on their shapes is
not new, in fact this idea first appeared in the writings of Sir Francis Bacon in 1620.
The resulting hypothesis from this observation is rather straightforward: the shapes
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cannot be directly tested without a time machine. Therefore, geoscientists reframed
the hypothesis by assuming the continents used to be connected and asking what
other patterns we would expect to find. This is exactly how turn of the century earth
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The document discusses a study that proposes a new hypothesis for the origin of intraplate volcanism. The study suggests that some intraplate volcanic activity, like the Steens-Columbia River flood basalt, can be explained by tearing of subducting tectonic plates as they slow down, which allows rapid mantle upwelling. While this model may explain some cases of intraplate magmatism, it does not fully account for all aspects of some systems like Yellowstone, so the relationship between plate tectonics and intraplate volcanism remains an area of ongoing research.
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The document discusses several theories for the origin of life, including the primordial soup hypothesis proposed by Alexander Oparin, the iron-sulfur world hypothesis proposed by Gunter Wachtershäuser, and the hypothesis that life began around hydrothermal vents. It notes that while the primordial soup hypothesis involves chemicals combining in an ocean or pond, the iron-sulfur world and hydrothermal vent hypotheses propose that life began in environments near hydrothermal vents, where chemical reactions could have been driven by thermal energy.
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An ESA Study For The Search For Life On MarsFaith Brown
This document proposes a hypothetical strategy to search for life on Mars using robotic missions. It discusses the following key points:
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2. A robotic mission should carefully select landing sites with good exobiological potential to find chemical or morphological biosignatures. Samples would be collected and analyzed using integrated instruments on a lander and rover.
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Artigo relata como a Terra sofreu com os impactos de ateroides a 4 bilhões de anos atrás, e como a superfície do planeta foi remodelada e os oceanos formados.
This document provides an overview of plate tectonics and its history. It discusses how the theory of plate tectonics emerged in the 1960s and revolutionized earth sciences by explaining phenomena like earthquakes and mountain building. It also summarizes early ideas around continental drift proposed by scientists like Alfred Wegener in the early 20th century, and how plate tectonics unified and built upon these early concepts.
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The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
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EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
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The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
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photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
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with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
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The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
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at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
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Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
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Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
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We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
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The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
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The ambient solar wind that flls the heliosphere originates from multiple
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to one active region (12961) and then across to another region (12957). This
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a coronal mass ejection. Our results describe solar wind variability at 0.5 au
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Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
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composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
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confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
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star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
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list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
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We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
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In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
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like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
The technology uses reclaimed CO₂ as the dyeing medium in a closed loop process. When pressurized, CO₂ becomes supercritical (SC-CO₂). In this state CO₂ has a very high solvent power, allowing the dye to dissolve easily.
When I was asked to give a companion lecture in support of ‘The Philosophy of Science’ (https://shorturl.at/4pUXz) I decided not to walk through the detail of the many methodologies in order of use. Instead, I chose to employ a long standing, and ongoing, scientific development as an exemplar. And so, I chose the ever evolving story of Thermodynamics as a scientific investigation at its best.
Conducted over a period of >200 years, Thermodynamics R&D, and application, benefitted from the highest levels of professionalism, collaboration, and technical thoroughness. New layers of application, methodology, and practice were made possible by the progressive advance of technology. In turn, this has seen measurement and modelling accuracy continually improved at a micro and macro level.
Perhaps most importantly, Thermodynamics rapidly became a primary tool in the advance of applied science/engineering/technology, spanning micro-tech, to aerospace and cosmology. I can think of no better a story to illustrate the breadth of scientific methodologies and applications at their best.
(June 12, 2024) Webinar: Development of PET theranostics targeting the molecu...Scintica Instrumentation
Targeting Hsp90 and its pathogen Orthologs with Tethered Inhibitors as a Diagnostic and Therapeutic Strategy for cancer and infectious diseases with Dr. Timothy Haystead.
Authoring a personal GPT for your research and practice: How we created the Q...Leonel Morgado
Thematic analysis in qualitative research is a time-consuming and systematic task, typically done using teams. Team members must ground their activities on common understandings of the major concepts underlying the thematic analysis, and define criteria for its development. However, conceptual misunderstandings, equivocations, and lack of adherence to criteria are challenges to the quality and speed of this process. Given the distributed and uncertain nature of this process, we wondered if the tasks in thematic analysis could be supported by readily available artificial intelligence chatbots. Our early efforts point to potential benefits: not just saving time in the coding process but better adherence to criteria and grounding, by increasing triangulation between humans and artificial intelligence. This tutorial will provide a description and demonstration of the process we followed, as two academic researchers, to develop a custom ChatGPT to assist with qualitative coding in the thematic data analysis process of immersive learning accounts in a survey of the academic literature: QUAL-E Immersive Learning Thematic Analysis Helper. In the hands-on time, participants will try out QUAL-E and develop their ideas for their own qualitative coding ChatGPT. Participants that have the paid ChatGPT Plus subscription can create a draft of their assistants. The organizers will provide course materials and slide deck that participants will be able to utilize to continue development of their custom GPT. The paid subscription to ChatGPT Plus is not required to participate in this workshop, just for trying out personal GPTs during it.
The cost of acquiring information by natural selectionCarl Bergstrom
This is a short talk that I gave at the Banff International Research Station workshop on Modeling and Theory in Population Biology. The idea is to try to understand how the burden of natural selection relates to the amount of information that selection puts into the genome.
It's based on the first part of this research paper:
The cost of information acquisition by natural selection
Ryan Seamus McGee, Olivia Kosterlitz, Artem Kaznatcheev, Benjamin Kerr, Carl T. Bergstrom
bioRxiv 2022.07.02.498577; doi: https://doi.org/10.1101/2022.07.02.498577
PPT on Direct Seeded Rice presented at the three-day 'Training and Validation Workshop on Modules of Climate Smart Agriculture (CSA) Technologies in South Asia' workshop on April 22, 2024.
Immersive Learning That Works: Research Grounding and Paths ForwardLeonel Morgado
We will metaverse into the essence of immersive learning, into its three dimensions and conceptual models. This approach encompasses elements from teaching methodologies to social involvement, through organizational concerns and technologies. Challenging the perception of learning as knowledge transfer, we introduce a 'Uses, Practices & Strategies' model operationalized by the 'Immersive Learning Brain' and ‘Immersion Cube’ frameworks. This approach offers a comprehensive guide through the intricacies of immersive educational experiences and spotlighting research frontiers, along the immersion dimensions of system, narrative, and agency. Our discourse extends to stakeholders beyond the academic sphere, addressing the interests of technologists, instructional designers, and policymakers. We span various contexts, from formal education to organizational transformation to the new horizon of an AI-pervasive society. This keynote aims to unite the iLRN community in a collaborative journey towards a future where immersive learning research and practice coalesce, paving the way for innovative educational research and practice landscapes.
ESA/ACT Science Coffee: Diego Blas - Gravitational wave detection with orbita...Advanced-Concepts-Team
Presentation in the Science Coffee of the Advanced Concepts Team of the European Space Agency on the 07.06.2024.
Speaker: Diego Blas (IFAE/ICREA)
Title: Gravitational wave detection with orbital motion of Moon and artificial
Abstract:
In this talk I will describe some recent ideas to find gravitational waves from supermassive black holes or of primordial origin by studying their secular effect on the orbital motion of the Moon or satellites that are laser ranged.
The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations
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The importance of continents,
oceans and plate tectonics
for the evolution of complex
life: implications for finding
extraterrestrial civilizations
Robert J. Stern 1
&TarasV. Gerya 2*
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs).These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation:
foc (the fraction of habitable exoplanets
with significant continents and oceans) and
fpt (the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of
foc and
fpt is very small (<0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A most important scientific question is whether there is life elsewhere in the universe and how to find this. We
are particularly interested to find exoplanets with civilizations that can communicate with us via radio waves
or other ways. Our search for habitable exoplanets is now focused on our galaxy, where we hope to find active,
communicative civilizations (ACCs) among its many billion star systems. Within the uncertainties of involved
astronomical and biological parameters, the Drake Equation typically predicts that there should be many (<~100
to millions)1
exoplanets in our galaxy hosting ACCs. These optimistic calculations are however not supported
by any significant evidence, which is often referred to as the Fermi
Paradox1
. This implies that some important
variables are missing from the Drake Equation or their magnitudes are incorrectly estimated. The inconsistency
has been repeatedly analyzed and various solutions have been offered to reduce the number of ACCs, in par-
ticular due to the rareness of complex
life2
(multi-cellular life with complex cells and functions, such as algae,
land plants, fungi, animals on Earth)3–7
compared to primitive single-cell life in our galaxy. Among others, plate
tectonics has been repeatedly proposed as one of the rare conditions for complex
life2
.
Our approach to address this long-standing enigma is to re-examine the history of life on Earth. It is widely
accepted that this began by 3800 Ma and that complex multicellular heterotrophs (animals) did not evolve until
after 1000 Ma. Ward and
Brownlee2
note “Over and over again the same question arises, why did it take so long
for animals to emerge on planet Earth? Was it due to external environmental factors, such as the lack of oxygen
for so long in the history of this planet, or to biological factors, such as the absence of key morphological or
physiological innovations?” These insights build on our new understanding that the explosion of complex life
(algae, land plants, animals)3–7
in Late Neoproterozoic time (1000–541 Ma) leading to the development of our
OPEN
1
Department of Sustainable Earth Systems Science, University of Texas at Dallas, Richardson, TX 75083‑0688,
USA. 2
Department of Earth Sciences, ETH-Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland. *
email: tgerya@
ethz.ch
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own ACC was a consequence of a prolonged and profound transformation of Earth’s global tectonic regime from
single lid to plate tectonics (“Methods”). This time period is characterized by extreme variability in atmospheric
oxygen levels7
and records the transition from a largely bacterial toward (to a large extent) an eukaryotic pho-
totrophic world5,6
.
In this paper, we build on the previous studies suggesting the importance of plate tectonics for the develop-
ment of complex
life2
. Firstly, we discuss the importance of the late (Neoproterozoic) onset of the modern plate
tectonics regime. We begin by exploring what kinds of tectonics an active silicate body can have and then explain
that such bodies likely have complex tectonic histories as they age and cool. Then we address three key aspects
concerning reconstructing the evolution of the only planet with life and an ACC that we know of—Earth. Sec-
ondly, we demonstrate and explain how and why the late (Neoproterozoic) onset of the modern plate tectonics
regime accelerated complex life evolution, which is the reason for us to introduce the respective
fpt term to the
Drake equation. Thirdly, we explore for the first time why the very rare long-lasting presence of large expanses
of both continents and oceans on planets with plate tectonics is an essential and restrictive condition for the
evolution of ACCs, which is the reason for us to introduce the respective foc term to the Drake equation. Finally,
we quantify
fpt and
foc terms in the modified Drake Equation and show how this addresses the Fermi Paradox.
The onset of plate tectonics in the Neoproterozoic
On the need to consider complex tectonic histories for Earth and other active silicate bodies
Better understanding of complex physical–chemical processes associated with and possibly stimulating the major
changes in life
evolution4,7
requires detailed reconstruction, understanding and quantification of complex tectonic
histories for Earth and other silicate planetary bodies. From this prospective, we discriminate two major styles
of global tectonics for these bodies (“Methods”): (1) plate tectonics (PT) and (2) single lid (SL). We also propose
that these bodies may experience transitions between different SL styles (e.g., Mars-style vs. Venus-style) or
that PT episodes might alternate with SL
episodes8–10
. In this paper, we only go back to the beginning of Meso-
proterozoic time at 1.6 Ga thereby embracing the entire period of accelerating life
evolution3,5,11
. We combine
available geological indicators and their data biases (“Methods”) allowing identification of PT and SL episodes.
What is the evidence that a transition from SL to the modern episode of PT occurred in Neo-
proterozoic time?
The time for the onset of plate tectonics on Earth remains
controversial12
. Whereas many researchers advocate
that modern plate tectonic regime operated since the
Archean13
, several recent studies argue that the present
regime started in the
Neoproterozoic10,14–16
, although earlier plate tectonic episodes also may have
occurred17
.
The arguments also depend on what definition of plate tectonic regime (strict, or broad, “Methods”) is assumed
by respective studies. Geoscientists agree that PT processes of seafloor spreading, subduction, and continental
collision make distinctive minerals, rocks, and rock assemblages, called “Plate Tectonic Indicators”14
. It should
be noted, that the controversy for the onset of plate
tectonics12
is in part related to uncertainties in interpretation
of available natural data. In particular, the absence of certain PT indicators, like blueschists, may be attributed to
factors such as higher mantle potential temperature and crustal composition
differences18
. Also, paleomagnetic
data before 1.2 Gyr may not necessarily provide robust constraints on the continental
configuration19
. Therefore,
a combined approach relying on several (rather than any single) PT indicators should be
preferred14
. Stern14
identified three groups of PT indicators: 1) Seafloor Spreading and Subduction Initiation Indicators; 2) Subduc-
tion Indicators; and 3) Collision Indicators (Fig. 1). These three groups of PT indicators occur overwhelmingly
in Neoproterozoic and Phanerozoic time. This empirical evidence is also consistent with physical considera-
tions built on 1) our understanding that long-lasting plate tectonics is mostly driven by the continued sinking
of oceanic lithosphere in subduction zones and 2) that oceanic lithosphere was not dense and strong enough to
subduct in a long-lasting continued manner as a coherent layer until mantle potential temperature cooled to less
than 100–150 °C above present
values20
, which roughly correspond to Neoproterozoic mantle
temperatures21
.
The abundance of PT indicators should scale with the growth of the PT mosaic, which should take 100’s of
millions of years to accomplish. Therefore, we expect the abundance of PT indicators to increase with time during
the transition from SL to PT; this agrees with the Neoproterozoic record. Building on our understanding that
the sinking of oceanic lithosphere in subduction zones drives plate motions, we expect that evidence of subduc-
tion initiation (Group 1 PT indicators) would appear earlier than evidence of ongoing subduction (Group 2 PT
indicators); this is observed, with ophiolites appearing~870 Ma and Group 2 PT indicators appearing~750 Ma.
Subduction would have to operate for tens of millions of years to close an ocean and cause two continents to col-
lide. That is also observed, with Group 3 PT indicators appearing~600 Ma. So the sequence of appearance of PT
indicators in the Neoproterozoic – Group 1 before Group 2 before Group 3—is as expected. Another test of the
hypothesis that the modern episode of PT began in Neoproterozoic time is that the preceding Mesoproterozoic
Era was characterized by a SL regime. This prediction is explored in the next section.
What is the evidence that a SL episode occurred in Mesoproterozoic time?
Given the conclusion that an active silicate body has either PT or SL (“Methods”), a requirement of the hypothesis
that PT started in Neoproterozoic time is that the immediately preceding epoch – the Mesoproterozoic – was
a SL episode.
Stern10
identified three SL indicators: (1) elevated thermal regime; (2) abundance of unusual dry
magmas such as A-type granites and anorthosites; and (3) paucity of new passive continental margins. Negative
evidence is the lack of PT indicators. Figure 1 shows that the Mesoproterozoic was not only when few PT indi-
cators were produced and preserved, it was characterized by abundant SL indicators. In contrast, PT indicators
are documented from older Paleoproterozoic
terranes17
, suggesting adequate preservation of geologic evidence
for at least the last 2 Gyr of Earth history.
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A similar conclusion is reached based on types of mineral deposits, which should also be sensitive to tectonic
regime. For example, orogenic gold and porphyry copper deposits, which are common in Neoproterozoic and
younger times, are missing from the
Mesoproterozoic22
. In contrast, different ore types such as iron oxide copper
gold (IOCG) deposits and Fe–Ti–V-P deposits associated with anorthosites are common in Mesoproterozoic
terranes23,24
.
Finally, the paleomagnetic data do not show large dispersions between continental blocks in the Mesopro-
terozoic, in contrast to what is documented for Neoproterozoic and younger times and for the Paleoprotero-
zoic. In particular, the supercontinent Nuna/Columbia formed in the Paleoproterozoic and persisted through
Mesoproterozoic time. For example, Evans and
Mitchell25
noted minimal paleogeographic changes during the
Mesoproterozoic. Pisarevsky et al.26
concluded that the supercontinent Nuna/Columbia was assembled by the
beginning of Mesoproterozoic time, with some relative motion between continental blocks beginning in the
mid-Mesoproterozoic.
How long would it take for the Mesoproterozoic single lid to transform into a plate tectonic
global mosaic?
PT requires a global mosaic and this would take some hundreds million years to emerge from SL (Fig. 2). Time
is required after the first subduction zone and associated transforms and divergent plate boundary form to
propagate laterally and grow the mosaic. The rate of this “infection” is limited by how fast new subduction zones
can form and lengthen. Trench lengthening rates accompanying Cretaceous and younger subduction initiation
episodes based on observations and thermomechanical
models27
—vary from~100 to~600 km/Myr (100–600
mm/y). With these rates, 92 to 550 Myr would be needed to expand from a single subduction initiation point to
a global plate network with~55,000 km of convergent plate margins.
We can also consider the major C isotope excursions and glaciation episodes that happened in Neoprotero-
zoic time (Fig. 1) as due to strong disruption of surface Earth systems caused by large-scale new plate boundary
formation episodes. Such disturbances reflect environmental changes associated with the prolonged climate crisis
called Neoproterozoic Snowball Earth. Many explanations have been offered for what caused these changes, but
Figure 1.
Evolution of Earth’s tectonic regime over the past 1.6 Ga. (a) Single lid tectonic indicators, from
Stern10
. (b) Plate tectonic indicators cumulative plot, modified from
Stern14
; (c) Simplified climate history, from
Stern and
Miller28
. “Boring Billion” from
Holland37
; (d) Simplified biological evolution. See text for further
discussion.
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nearly all of these could ultimately have reflected the transition from SL to
PT28
. For example, the formation of
new subduction zones and continent movements would have disrupted climate via explosive
volcanism29
and/
or true polar
wander30
. The oldest Neoproterozoic C isotope excursion is the 811 Ma Bitter Springs
event31
and
the youngest is the~570 Ma Shuram
event32
, indicating a SL-PT transition that took 241 Myr. Using the first
Neoproterozoic Snowball Earth glacial episode (Sturtian) which began~720 Ma and the last (Gaskiers) which
occurred~580 Ma as marking the climate disruption due to the tectonic transition gives a slightly shorter (140
Myr) transition. C isotope excursions and evidence for glaciations in the sedimentary rock record further suggest
that the tectonic transition was episodic, not smoothly continuous. It should be however noted that geochemical
data also suggest the Paleoproterozoic glaciations, rise in oxygen (Great Oxidation Event, GOE) and the carbon
isotope excursion at 2.5–2.05
Ga33
, which predate the recently proposed relatively short-lived 2.05–1.8 Ga epi-
sode of plate
tectonics17
. It therefore remains partly uncertain what are the exact causal relationships between
the different geochemical and climate events and the onset of plate
tectonics17,33
; the end of the plate tectonic
episode seems to have been a result of forming the supercontinent Nuna, which terminated many subduction
zones34
. Biological consequences of this ancient PT episode also need investigation and better understanding.
The impact of plate tectonics on biological evolution acceleration
What is the evidence that biological evolution accelerated in Neoproterozoic time?
Life began sometime prior to~3.8
Ga35,36
. Evolution was slow for the first 3 billion years, dominated by microbes
(Bacteria and Archaea), single-cell organisms that lack the membrane-bound organelles of eukaryotes, espe-
cially the nucleus, mitochondria, and chloroplasts. All complex, multicellular life is eukaryotic so single-cell
eukaryotes had to evolve before multicellular algae, land plants and animals. Eukaryote fossils go back to late
Paleoproterozoic time and perhaps earlier (Fig. 1). Because of the importance of oxygen to animal metabolism,
multicellular animals and oxygenation of the atmosphere and ocean co-evolved. Rising oxygen concentrations
due to the GOE 2.4 billion years ago facilitated eukaryotic emergence.
There are no “big events” to define when Mesoproterozoic time began and ended and what are its natural
subdivisions (periods). The Mesoproterozoic Era is the heart of the “Boring Billion” (between~1800 and 800 Ma;
Figs. 1, 2). This term was coined by
Holland37
because atmospheric oxygen levels did not change much during
this time but now describes a protracted episode of geobiological stasis, including a remarkably stable carbon
isotope record. Other indications of extended environmental stability are captured in S, Mo, Cr, Sr isotopes, and
by low trace element concentrations and P in marine black shales. This protracted stable period—~20% of Earth
history – was also a prolonged episode of low nutrient
supply38
.
The Neoproterozoic contrasts with the Mesoproterozoic by being a time of climate instability and rapidly
evolving life. Neoproterozoic strata host evidence of global “Snowball Earth” glaciations, large perturbations to
the carbon cycle, oceanic oxygenation, the diversification of microscopic eukaryotes, and the rise of
metazoans39
(Figs. 1, 2). The Neoproterozoic Era is subdivided into 3 periods: the Tonian (1 Ga-720 Ma), Cryogenian
(720–635 Ma) and Ediacaran (635–541 Ma). There is no question that the pace of biological evolution acceler-
ated in Neoproterozoic
time3–5
, leading to the appearance of large multicellular organisms (or, metazoans). The
much longer Tonian period was more stable, more like the Mesoproterozoic era than the much shorter and more
dramatic Cryogenian and Ediacaran
periods40
. Acceleration of biological evolution characterized Cryogenian
and Ediacaran time. Molecular clocks agree that animal multicellularity arose by 800 Ma (Tonian), a bilaterian
body plan by 650 Ma (Cryogenian), and divergences between related phyla by 560–540 Ma (late Ediacaran)41
.
All animal phyla arose in Neoproterozoic
time42
.
Figure 2.
The last 1.6 Gyr of Earth’s tectonic history. See text for further discussion.
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How could the Neoproterozoic tectonic transition accelerate biological evolution?
Five processes were likely
involved43
(Fig. 3): 1) Increased nutrient supply; 2) Increased oxygenation of atmos-
phere and ocean; 3) Climate amelioration; 4) Increased rate of habitat formation and destruction; and 5) Moder-
ate, sustained pressure from incessant environmental change.
Nutrient supply is essential for life, especially key compounds—organic carbon, ammonium, ferrous iron and
phosphate—containing C, N, Fe, and P bioactive elements
respectively44
. Phosphorus is essential because it is
a globally limiting nutrient and plays a unique role in marine biogeochemistry, ecology and, hence, evolution.
Researchers agree that the Mesoproterozoic biosphere was significantly less productive than
today45,46
. Because P
is derived from weathering of continental crust and delivered to the ocean by
rivers47
, this suggests that decreased
nutrient supply due to reduced erosion and weathering was responsible. Tectonic processes exposing fresh rocks
on the surface are crucial for enhancing delivery of P and other inorganic nutrients, because shielding of fresh
rock surfaces by soil reduces nutrient fluxes due to chemical
weathering48
. Rapid uplift and orogeny (Pan-African
event49
, Transgondwanan
Supermountains50
, Circum-Gondwanan
Orogens51
) at convergent plate boundaries
associated with the tectonic transition would have greatly enhanced erosion, weathering and P delivery to the
oceans. In this context, microbial enhancement of carbon and sulfate acid
weathering52
acted as an important
driver of nutrient delivery to the oceans from rivers. After the rise of oxygen, more weathering from more
exposed land and more active bio-modulated chemical
weathering52
resulted in enhanced erosion and enhanced
burial of the organic carbon, besides delivery of P and other nutrients. As the result, P depletion of paleosols rose
during the Neoproterozoic Oxidation Event (NOE) similarly to the Paleoproterozoic
GOE52
.
Support for the interpretation of unprecedented uplift, erosion, and weathering in Ediacaran time comes
from the seawater Sr curve. In marine carbonates (seawater proxies), 87
Sr/86
Sr increased rapidly through Neo-
proterozoic time from near mantle-like values of~0.7055 in the Tonian to the highest values in Earth history
of~0.7095 in early Paleozoic
time53
. Increased seawater 87
Sr/86
Sr reflects increased flux of radiogenic Sr from the
continents, principally Pan-African uplifts, including the Transgondwanan Supermountains. Such strong uplifts
require continental collision and did not occur during the Mesoproterozoic single lid episode, as shown by low
marine carbonate 87
Sr/86
Sr record, including the~1.0 Ga Grenville
Orogeny54
. The addition of P, Fe and other
nutrients from erosion and weathering of Ediacaran collisional mountains broke the Mesoproterozoic nutrient
drought, stimulating life and evolution. Greatly increased nutrient supply from the continents to the oceans
during Neoproterozoic time is consistent with a protracted Neoproterozoic transition from Mesoproterozoic
SL to Phanerozoic PT.
Free oxygen in ocean and atmosphere increased with time because of the proliferation of photosynthetic
cyanobacteria (Fig. 1) combined with the efficient burial of organic carbon. Large, complex animals (metazoans)
could not evolve during the Mesoproterozoic because they require more oxygen for respiration than was available.
Minimum oxygen thresholds depend on animal size, mobility, nervous system, etc., but there is general agree-
ment that the Mesoproterozoic atmosphere and shallow ocean contained much less than the 0.1 – 0.25 present
oxygen level needed to support Cambrian
metazoa55
. A Neoproterozoic Oxygenation Event (NOE) proposed
based on a range of isotopic proxies led to a much more oxygenated environment by Late Ediacaran time. There
are several explanations for the NOE. One is that an increased supply of nutrients into the oceans stimulated
phytoplankton growth, which converted
CO2 into organic matter. This was further stimulated by the evolution of
new plants such as algae in late Cryogenian time (659–645 Ma)56
, which transformed the base of the food chain
and produced more free oxygen. Another explanation is that enhanced chemical weathering of continents was
responsible57
. Central to all these explanations is that more dead cyanobacteria and algae – organic carbon – must
Figure 3.
Summary diagram43
showing how plate tectonics stimulates life and evolution whereas a single lid
tectonic style retards life and evolution. See text for further discussion.
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be buried. Increased organic carbon burial reflected enhanced sediment supply and formation of new rift basins
and passive continental margins accompanying the tectonic transition.
Climate is especially important for metazoans. Primitive life can exist between temperatures near the freezing
of water and~120 °C, but metazoans thrive between 5° and 35 °C. Plate tectonics and single lid tectonics control
climate differently. PT and the supercontinent cycle controls Earth’s climate in 4 main ways. First, gases released
from magmas can either warm or cool the surface, depending on their composition, which is controlled by plate
tectonic setting. CO2 emissions associated with especially mid-ocean ridge and mantle plume igneous activity
encourages atmospheric warming whereas explosive volcanism associated with convergent margins injects
SO2
into the stratosphere to cause short-term
cooling58
. Second, proportions of Earth’s surface covered by water
exert a strong control on climate, more temperate when the proportion is high and harsher when it is low.
Long-term sea level rise and fall (tectono-eustasy) mostly reflects the mean age of seafloor, which changes sys-
tematically over Wilson and Supercontinent cycles. Consequently, PT Earth experienced systematic changes in
climate, with warmer (greenhouse) climates about 100 m.y. after continental breakup as new oceans
widen59
. It
is unknown what would control seafloor depth on SL Earth and thus how sea level would behave, but it is likely
to change much less than for PT. Third, weathering of silicate rocks consumes atmospheric
CO2 so mountain
building—which exposes more silicate rocks—leads to atmospheric
cooling60
. Enhanced erosion and weather-
ing associated with PT uplifts accompanying rifting and orogenesis releases more nutrients like P and Fe that
stimulate photosynthetic life which, if sufficient dead organic C is buried, sequester
CO2 to cool climate. Uplifts
and erosion on a SL Earth should be lower so nutrient flux should be reduced and climate affected less. Fourth,
subduction removes large volumes of marine carbonate rocks and organic carbon, removing
CO2 from the
near-surface and sequestering it in the mantle, leading to climate
cooling61
. Such PT controls, modified by
Milankovich cycles operating over much shorter
timescales62
, are largely responsible for Earth’s climate during
the time that PT has operated.
It remains debatable what are the carbon cycle and climate controls for SL planets in
general63–65
and for
Mesoproterozoic Earth in particular. The two active SL planets in our Solar System – Venus and Mars—have
atmospheres that are>95%
CO2 thereby suggesting a reduced efficiency of
CO2 recycling on these planets com-
pared to Earth. On the other hand, global-scale models show that the carbon cycle could be efficiently maintained
on SL
planets63,64
, where carbon recycling out of and into the mantle could occur through continuous volcanism,
weathering, burial, sinking and delamination of carbonated crust. It has therefore been suggested that plate
tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable
climate63
. This is supported by both
observations66
and geodynamic
models67
showing that crustal formation
and recycling also occurred throughout non-plate tectonic processes during the early Earth’s evolution, imply-
ing significant mass fluxes from the mantle to the surface and back during SL tectonic regime. In particular, the
protracted Mesoproterozoic SL episode on Earth experienced a relatively stable warm climate, with no evidence
for glaciation despite the Sun being~5%–20% less luminous than
today68
. Elevated concentrations of greenhouse
gases CO2 and methane
(CH4) in the atmosphere likely kept Mesoproterozoic climate
warm69
, which needs to
be reconciled with global-scale carbon cycle
models63,64
used for planetary exploration.
Habitat formation and destruction is an integral part of PT via the Wilson and Supercontinent cycles governing
landscape and climate evolution. Ever since Darwin visited the Galapagos in 1835, scientists have appreciated
the essential role that isolated habitats play in allopatric speciation. PT makes and destroys habitats much faster
and more efficiently than can active single lid tectonic regimes. The pace of evolution as a function of continental
fragmentation has also been
confirmed70,71
.
Moderate sustained pressure on organisms from continuous environmental change happens with PT, much
less so for SL. Nutrient fluxes, topography, climate, and habitats change continuously with time for PT. Strong
tectonic-erosion coupling produces complex and variable landscape, climate and precipitation patterns that
are especially pronounced along active plate margins. This complexity stimulates
biodiversity72
. Continental
rifting and plate divergence produce large continental shelves with robust sediment and nutrient delivery from
adjacent continents. The nutrients are efficiently redistributed in shelves by currents and tides, creating favorable
environments for marine
life72
. All these processes were stimulated by the transition to modern PT, causing life
to rapidly diversify (Fig. 3). SL tectonics is incapable of exerting moderate, sustained environmental pressure,
except through the action of mantle plumes – especially when they first reach the surface and form large igne-
ous provinces (LIPs). The most dramatic climatic effect is global warming due to increased greenhouse gases.
Subsequent cooling can be caused by
CO2 drawdown through weathering of LIP-related basalts. Other strong
stresses on the biosphere include oceanic anoxia, ocean acidification, and toxic metal input73
. It should however
be mentioned that some of the feedbacks characteristic for plate tectonics may also be present during SL epi-
sodes, due to various regional-scale tectono-magmatic
activities74
driving topographic changes and landscape
and climate evolution.
Why is the acceleration of evolution by plate tectonics critical for the development of ACCs?
Accelerated evolution of complex life by the onset of PT is critical mainly due to the general slowness of biological
evolution. Timescales of biological evolution estimated on the basis of the analysis of phylogenies and/or fossils
take hundreds of millions of years, comparable to timescales of major PT processes of Wilson and Supercontinent
cycles75,76
. In a constant rate birth–death
model77
, species originate with speciation rate, and become extinct with
extinction rate, typically expressed as rates per lineage (L) per million years
(L−1
Myr−1
). Estimated speciation
and extinction rates typically
range76
from 0 to 1
L−1
Myrv1
and rarely exceed 1
L−1
Myr−1
, except during crisis
intervals75
. This implies that during the ca. 500–1000 Myr of modern PT, only up to few hundred new com-
plex species can have been sequentially generated (which can be truncated by species extinction) potentially (but
not necessarily, due to many other possible important parameters such as reproductive style, trophic structure,
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social type, habitat stability, availability of resources, stability of climate, nature of predation, etc.) leading to the
appearance of ACC-forming species on Earth. If one assumes this chain of new species evolution is one of the
important prerequisites for the appearance of ACC-forming species on other planets, then time needed for such
biological evolution will strongly depend on average effective speciation and extinction rates. We can therefore
speculate that slowing speciation rates and/or increasing extinction rates (truncating evolutionary chains) by a
non-stimulating global tectono-magmatic environment, such as a SL
episode78,79
, would not leave enough time
for complex life to develop.
The prerequisites of life related to oceans and continents
Why is the presence of significant oceans and continents important for the evolution of intel-
ligent life and civilizations?
We do not know where and how life on Earth began; furthermore with the present state of our knowledge the
life origin problem cannot be
solved80
. But life existed by 3.8
Ga35,36
and evolved for more than 3 Gyr in the
oceans. However, the presence of exposed land may also be crucial for life origin and
evolution81,82
. In particular,
thermodynamic considerations indicate that the absence of exposed land would cause the total hydrolysis of all
polymers and
metabolites83,84
. In addition, it has also been suggested that life originated and initially evolved
in hydrated paleosols (regolith) on land rather than in the
ocean81
. On the other hand, there are at least three
reasons that life evolution up to the point of metazoans must occur in water. First, seawater contains dissolved
nutrients that life requires and organisms are bathed in this, making nutrients easy to absorb through initially
primitive and then increasingly sophisticated evolving cell walls. Second, seawater protects organisms from
deadly ultraviolet radiation; it wasn’t until about 2.4 Ga that photosynthetic cyanobacteria oxygenated Earth’s
atmosphere to form a protective stratospheric ozone layer that the flux of UV radiation reaching Earth’s surface
diminished significantly85
. Third, all complex, multicellular life is eukaryotic. Single-cell eukaryotes had to evolve
before multicellular plants and animals could evolve from them, and this had to happen in water because of the
structural support that seawater offers. This structural support was needed for especially early metazoans, which
first evolved as soft-bodied creatures in Ediacaran time. These organisms could not thrive on land until hard,
strong exo- and endo-skeletons that allowed creatures to contend with gravity evolved in early Paleozoic time.
Although primitive life must evolve in the sea, advanced communicative civilizations must evolve on dry
land78
. First, changing landscape provide more varied habitats than do seascapes, and this is needed for accel-
erating the evolution and diversity of complex species. Consequentially, regions with high tectonic complex-
ity, predominantly located at the confluence of major lithospheric plates such as the circum-Mediterranean,
Mesoamerica, Madagascar and South East Asia, provided especially favorable sites for allopatric speciation
and the emergence of new land species across
straits72
. This correlation is much less pronounced for marine
species, mainly because this realm is more permeable to the movement of
organisms72,86
. Furthermore, dry
land stimulated adaptations necessary for survival in harsh terrestrial
environments87
: water retention, special-
ized gas exchange structures, reproduction by predominantly internal fertilization, locomotion in the absence
of structural support, adapted eyes and newly developed senses. These conditions stimulated development of
diverse animal appendages adapted for locomotion, feeding, manipulation and other
functions88,89
, and helped to
adapt eyes and other senses and the central nervous system to the new environment and functions. The resulting
sophisticated bioassets allowed increasingly intelligent creatures to populate and examine the extremely variable
terrestrial environments, which is one (but not the only) prerequisite to increasingly develop and transfer vari-
ous experiences (i.e., knowledge and information) about these environments within biological populations. This
may potentially (but not necessarily) result in the beginning of abstract thinking leading to the development of
religion, science and the noosphere of
Vernadsky90
. Technology arises from the exigencies of daily living such
as tool-making, agriculture, clothing, and weapons, but the pace of innovation accelerates once science evolves.
Using and understanding of fire and
electricity91,92
is essential for development of ACCs and this is unlikely in
the seawater environment. On the other hand, from the planetary formation and evolution prospective, the long-
term coexistence of continents and ocean on planets with plate tectonics (which are favorable for development
of ACCs) is a restrictive requirement and this has to be taken into account in the Drake equation.
Implications for the Drake equation
The Drake equation
The Drake equation estimates how many ACCs there are in our galaxy. It is formulated
as93
:
where R*=number of new stars formed per year,
fp =the fraction of stars with planetary systems,
ne =the average
number of planets that could support life (habitable planets) per planetary system,
fl =the fraction of habitable
planets that develop primitive life,
fi =the fraction of planets with life that evolve intelligent life and civilizations,
fc =the fraction of civilizations that become ACCs, L=the length of time that ACCs broadcast radio into space.
There is considerable disagreement on the values of these parameters, but the ‘educated guesses’ used by
Drake and his colleagues in 1961 were:
• R*=1/year (1 star forms per year in the galaxy)
• fp =0.2–0.5 (one fifth to one half of all stars formed will have planets)
• ne =1–5
• fl =1 (100% of planets will develop life)
• fi =1 (100% of which will develop intelligent life and civilizations)
ACCs = R∗
· fp · ne · fl · fi · fc · L
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• fc =0.1–0.2 (10–20% of civilizations become ACCs)
• L=1000–100,000,000 years
Drake93
acknowledged the great uncertainties (ACCs=200 – 50,000,000) and inferred that there were prob-
ably between 1000 and 100,000,000 ACCs in our galaxy. Other scientists used different estimates for these vari-
ables, resulting in a range of estimated ACCs from<100 to several
million94
. The Fermi Paradox points out that
all these estimates seem to be much too high. We focus on
fi, the fraction of planets with life that evolve intel-
ligent life and civilizations and propose to break this into two variables,
foc and
fpt, such that
foc ·
fpt = fi, where
foc
is the fraction of habitable exoplanets with significant continents and oceans and
fpt is the fraction of habitable
exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga.
Why is the presence of continents and oceans unusual?
Relatively small topographic variations of terrestrial planets (< 10–20 km) suggest that the presence of both
continents and oceans is restrictive in term of the required thickness (volume) and long-term stability of the
surface water layer. Topographic variations are mainly driven by isostasy and are therefore expected to be nearly
independent of planetary size (and surface gravity) variations. In particular, on Earth, the difference between
the average elevation of continents (~0.835 km above sea level) and the average depth of oceans (3.7 km) is
due mainly to the difference in their average vertical density profiles caused by systematic differences in their
lithospheric and crustal thickness, composition and thermal structure. This imposes strict requirements in
term of the optimal volume of surface water needed to satisfy stability of plate tectonics in the presence of both
oceans and dry land masses. The mass fraction of surface water on Earth is 0.0224% (1 Earth ocean) and can
only vary within less than one order of magnitude (from 0.007% to 0.027%, 0.3–1.2 Earth oceans) not to violate
this requirement. The minimum surface water fraction (0.007%, or 0.3 Earth oceans) is given by the requirement
of predominantly submarine conditions at mid-ocean ridges (average water depth 2.5 km), which is needed to
produce hydrated oceanic crust that can stabilize asymmetric (one sided) subduction and plate tectonics95,96
. On
the other hand, the maximum surface water fraction (0.027%, or 1.2 Earth oceans) will flood nearly all continents
without violating the condition of the presence of significant (>5–10% of the planetary surface) land masses (such
as mountain ranges, volcanic islands, rift flanks, etc.). Based on their continental freeboard model, Korenaga
et al.97
suggested that a water world could exist on Earth with two to three oceans of water for the present and
the Archean hypsometry, respectively. This could notably widen the upper bound of the permitted water mass
variability to 0.045–0.067% (2–3 Earth oceans). We however prefer our more conservative upper bound 0.027%
(1.2 Earth oceans) in order to ensure that significant land masses remain.
Some topographic variations are also expected with changes in planetary mass, gravity and
composition98,99
.
Isostatically compensated topography should not depend on the planetary surface gravity but on lateral density
changes induced by thermal and compositional variations in the crust and the mantle. The later will likely be of
similar magnitude as on Earth and other terrestrial planets due to the similar nature of thermal and magmatic
processes involved in crust and lithosphere formation. Some compositional variations can be expected as func-
tions of variability in stellar and planetary
compositions98,99
, which will however remain on the same order
(some hundreds kg/m3
) as observed on Earth. Indeed, some flattening of topography with increasing planetary
mass and gravity can be expected due to the incomplete isostatic compensation and lithospheric flexure effects
(especially relevant for smaller planets). This may in particular further reduce permitted water mass variability
on super-Earths. The expected moderate topographic variability will however likely be within the wide range of
uncertainties that we will obtain by combining water mass ranges from Mars-size and Earth-size planets and the
largest observed super-Earths. The water mass fraction limits should scale simultaneously in inverse proportion
with the planetary radius: a Mars size planet (0.5 Earth’s radius, same density) requires surface water mass frac-
tions of 0.015–0.055% (0.1–0.3 Earth oceans), whereas the largest known super-Earth (2.35 Earth radius, same
density) requires 0.003–0.012% (2–7 Earth oceans).
It has also been suggested that Earth’s mantle contains significant water and the respective mantle water
mass fraction is on the order of 0.008–0.08% (0.36–3.6 Earth oceans100
). However, the long-term stability of the
surface water volume also requires stability of the water mass hosted by the mantle. This is likely caused by the
mantle saturation and subsequent long-term global-scale equilibrium in the partitioning of volatiles between the
interior and surface affected by the atmospheric composition, temperature and pressure97,101–103
. The retention of
water in the planetary interior could pose more restrictive conditions on the ocean formation and
depth97,103
. It
should also be stressed that an addition of some stable water mass hosted in the crust and the
mantle97,101–103
to
the mass of the surface ocean will not widen the range of permitted variability of the total water mass delivered
to a planet, which would allow the long-term coexistence of continents, oceans and plate tectonics.
Mass-balance calculations indicate that a cometary contribution to Earth’s water was probably limited to ≤
1%104
. On the other hand, meteorite data and planetary formation models suggest that some variable amount of
water can be delivered to relatively dry terrestrial planets by water-rich planetesimals formed in the outer Solar
System beyond the “planetary snowline” and scattered inwards during the growth, migration, and dynamic evolu-
tion of the giant
planets105,106
. In such planetary formation scenarios, the amount of the delivered water can be
highly variable105
. Assuming 0–90% volatile lost during
impacts105,107
, and 5–10% water content in the water-rich
planetesimals total delivered water mass fraction can range from 0.008–3.8%105
, which is a much broader range
than the required optimal water mass variability. The potential planetary water mass fraction variability can be
further broadened by considering the possible existence of ocean
worlds108
(such as Europa and Callisto, mass
water fraction 6–55%109
). The expected large variability of planetary water mass fractions (0–55%) makes the
requirement of the long-tem existence of the optimal surface water volume to be a kind of “Goldilocks condi-
tion”. By comparing the permitted variability ranges for planets of different size (0.009–0.04%) and the expected
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variability due to different planetary accretion scenarios (up to 3.8–55%) we can evaluate
foc as the probability
for a planet to have the optimal surface water volume to be on the order of 0.00016 – 0.011. This probability
range can be tested by using the recent work of Kimura and
Ikoma110
, which predicted diversity in water content
of terrestrial exoplanets orbiting M dwarfs. Based on the available data from their
simulations110
, the results for
planets with 0.5–2.35 Earth radii suggest variability of water mass fraction to range from 0–56%. The fraction of
the planets with the optimal water volume is very small and ranges from
foc =0.0006–0.0022, which is thus well
within the range of our estimate.
Why plate tectonics operating for at least 0.5 Gyr is unusual
Plate tectonics is unique to Earth and no other terrestrial planet or satellite in the Solar system has plate tecton-
ics, although episodic regional-scale subduction processes have been identified on
Venus111,112
. The presence
and sufficient depth of the surface ocean seems to be necessary to ensure the long-term stability of continued
subduction95,96
, and is already included in our
foc estimate. An important additional restriction comes from
the stellar composition. Unterborn et al.98
found that only 1/3 of the range of stellar compositions observed in
our galaxy is likely to host planets able of sustaining density-driven tectonics (such as plate tectonics), which
sets an upper limit of
fpt to 0.33. This relatively strong reduction, does not however take into account possible
devolatilization effect for estimating rocky exoplanet
compositions99
. Spargaren et al.99
have recently quantified
these effects and obtained notably modified planetary compositions compared to earlier works98
. They, however
pointed out that their obtained planetary compositions rich in Na and Si will have more buoyant crusts than
Earth, which may render subduction and hence plate tectonics less
efficient98,99
. Therefore, in the absence of any
other estimates of the likelihood of sustaining density-driven tectonics on exoplanets, the value proposed by
Unterborn et al.98
will have to serve for our estimation of
fpt.
Further reduction of
fpt may come from the consideration that relatively small terrestrial planets such as
Mars or Mercury should have strongly lowered convection vigor due to their low gravity and smaller mantle
thickness, which make them unlikely candidates for the long-term stability of a plate tectonic regime. Planetary
accretion models113
show that the fraction of such small terrestrial planets can be large (ca. 50%), which implies
respective reduction of
fpt to 0.17. It is also not clear if plate tectonics is more likely or less
likely114–117
on large
terrestrial planets (super-Earths). Therefore,
fpt may have to be further reduced to exclude super-Earths, which are
common in our
galaxy113
. Another restriction may come from the fact that continued (rather than intermittent)
subduction requires a limited range of mantle potential
temperatures20
, which can only be realized in part of the
planetary cooling
history118
. In the case when planetary evolution starts from cooler mantle temperature than
Earth, plate tectonics may never start and single lid tectonics may operate for the entire planetary
history118
. This
should thus further reduce the
fpt value to exclude planets with insufficiently hot mantles during their evolution.
Unfortunately, this reduction cannot be easily quantified and simply implies that
fpt <0.17.
Possible solution to the Fermi Paradox
Based on the modified Drake Equation, we suggest that the Fermi Paradox may be resolved if the product of
foc
and fpt is very small. Our preliminary estimates show that
foc can be on the order of 0.0002 – 0.01 whereas
fPT
is<0.17, which makes their product
foc ·
fpt = fi to be extremely small (<0.00003 –<0.002). This estimate drasti-
cally reduces the potential number of ACCs (to<0.006 –<100,000) in our galaxy calculated with the modified
Drake equation. Further significant reduction may come from the re-evaluation of the characteristic length
of time for ACCs communication activities (L). Values of L can be limited to 400–7,800,000 years by societal
collapse119
and biological species
survival120
, which again reduces the potential number of ACCs in our galaxy to
even lower numbers (<0.0004 –<20,000). The value less than 1 of the lower bound implies that the probability
to find at least one ACC (including ourselves) in our galaxy can be as low as<0.04% (this lower limit is however
strongly dependent on the large remaining uncertainties of parameters in our modified Drake equation). As
the result, it may be that primitive life is quite common in the galaxy. However, due to the extreme rareness of
long-term (several hundred of million years) coexistence of continents, oceans and plate tectonics on planets
with life, ACCs may be very rare.
On the other hand, the chances of finding planets with life, continents oceans and plate tectonics (i.e., COPT
planets) in our galaxy, which are potentially suitable for ACCs, by remote sensing are relatively high. They can
be evaluated on the basis of the Drake equation modified for the purpose of remote sensing as
where: LCOPT =500,000,000 yr is the characteristic time of the long-term coexistence of ocean continents and
plate tectonics in Earth history needed for the accelerated development of advanced life. The resulting expected
number of COPT planets in our galaxy ranges between 500 and ca. 1,000,000, which create reasonable chances
of finding them by future exoplanetary exploration.
Methods
Single lid vs. plate tectonics
Based on the presence/absence of an active global plate
mosaic121
, silicate planetary bodies of the Solar System
show two major types of tectonics: plate tectonics (PT, the global plate mosaic is present, modern Earth) and
single lid (SL, the global mosaic is absent, Mars, Moon, Io, Venus, perhaps Archean-Hadean Earth). SL behavior
can be further subdivided into three main sub-types characteristic for different planetary bodies depending on
their size and interior temperature. From most convectively active to dead silicate bodies, these include: (1)
volcanic heat pipe (small bodies with hot interior, Io), (2) squishy lid (large bodies with hot interior, Venus,
COPTs = R∗
· fp · ne · fl · foc · fpt · LCOPT
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Archean-Hadean Earth122
); (3) stagnant lid (small bodies with little convection, Mercury, Mars). We use a strict
definition of PT, which requires the presence of a global plate mosaic driven by long-lasting subduction123
. This
allows discrimination of modern Earth’s tectonic regime from other types of mobile planetary surface behavior
(like Venus)111,112,124
, in which (i) localized plate boundaries do not exist or do not form a global plate mosaic
and (ii) horizontal surface motions are not predominantly driven by oceanic plate subduction (we classify this
surface behavior as single lid
tectonics125
). Three out of four actively convecting silicate bodies in our Solar System
have SL tectonics, so this is likely to dominate the tectonic styles of active silicate bodies in our galaxy. Because
PT does not occur on any other planet, it may be that the PT regime is also unusual in Earth’s tectonic history.
Is the geologic record too biased to reconstruct Earth’s tectonic history?
The geologic record is biased to younger
rocks126
but how far back in time is the record good enough to allow
Earth’s tectonic history to be reconstructed? Some argue that it is too incomplete for Earth’s tectonic history to
be reconstructed far into the Precambrian. There is strong evidence that deep erosion in Late Neoproterozoic
time to cut the Great Unconformity, removing 3–5 km of rock, was caused by extensive
glaciation127
. Such deep
erosion could have removed older group 1 PT indicators (ophiolites) but this does not seem to have happened
because early Neoproterozoic (Tonian) ophiolites are well
preserved128
. Also, several well-preserved ophiolites
that formed 1.9–2.1 Ga are known (Fig. 1), indicating that preservation is good enough and suggesting that an
episode of proto-PT occurred in Paleoproterozoic
time17
. One occurrence of 3.8 Ga
ophiolites129
may suggest
viability of oceanic spreading and episodic regional subduction in squishy-lid Archean Earth in agreement with
recent numerical
models74
. Even if some evidence from supracrustal rocks like ophiolites has been removed,
deep erosion would not remove groups 2 and 3 of PT indicators, which are metamorphic rocks and exist deep
in the crust. On the basis of these arguments, we think that Earth’s tectonic history can be reconstructed back
to at least 2.5 Ga, the beginning of Proterozoic time.
Data availability
We analyzed results of Monte Carlo simulations of Kimura and
Ikoma110
, which are publically available via
GitHub at https://github.com/TadahiroKimura/Kimura-Ikoma2022.
Received: 3 February 2023; Accepted: 14 February 2024
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