5938665(1).ppt

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© Sierra College Astronomy Department
Life in the Universe
Life on Earth
 Is there life beyond Earth?
 Great for science fiction plots, but at present there is
no undeniable evidence that aliens have been here.
 Many astronomers of the past have suggested
that life existed elsewhere.
 Kepler – thought there were inhabitants on the Moon.
 Herschel – claimed life existed on nearly all the
planets.
 Lowell – thought he saw canals on Mars.
 Before searching for life elsewhere in the
universe we must first look at how life arose on
the Earth.

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Life on Earth
 Three recent developments have led to the idea
that life may not be so uncommon beyond Earth.
 Life arose quite early in Earth’s history.
 Laboratory experiments show that the chemical make-
up of the young Earth could readily combine to form
organic molecules, suggesting that life formed from
naturally occurring chemistry.
 We have discovered microorganisms which live in
extreme Earth conditions which may be similar to
those found on other worlds

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Life on Earth
When did life arise on the Earth?
 After the Earth formed 4.5 billion years ago, life
had little chance to survive.
 First several hundred million years was era of heavy
bombardment.
 These bombardments could vaporize oceans killing
any life that might have formed.
 Studies of the craters on the Moon suggest that the
heavy bombardment ended 4.2 to 3.9 billion years
ago.
 Remarkably life may have been thriving 3.85 billion
years ago – consequently it appears lifer arose in a
geological blink-of-an-eye once conditions became
hospitable.

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Life on Earth
Fossils and Geologic Time Scale
 The key to understanding ancient life is to look for fossils,
relics of organisms which lived and died long ago.
 These fossils can be found under layers of sediments
which are carried by rivers or lie under ocean floors.
 Grand Canyon shows billions of years of Earth’s history.
 Relative ages of rocks and fossils are easy to determine: each
deeper layer formed earlier.
 Radiometric dating confirms this and estimates the absolute ages
of the material.
 Based of the layering of the rocks and fossils Earth’s
history can be divided into several distinct intervals or
geological time scales.

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Life on Earth
Fossil Evidence of Early Life
 Since the Earth is so geologically active, it is difficult
to find rocks which are over 3 billion years old.
 The evidence of the existence of life 3.5 billion years
ago is suggestive and subtle since microscopic
fossils are difficult to detect.
 What we see today is what was left behind by ancient
bacteria in rocks called stromatolites.
 Also, the ratio of carbon-13 to carbon-12 in rocks
containing fossils has been found to be lower relative
to rocks that do not contain fossils.
 Some rocks dating back to 3.85 billion years ago have the
lower ratio and this is suggestive that ancient life formed at
this distant time in the past.

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Life on Earth
How did life progress on Earth?
 Theory of Evolution
 In the 19th century, most biologists agreed that species
change through time.
 In 1859, Charles Darwin explained how life might
undergo those changes based on two observations:
 Overproduction and struggle for survival.
 Individual variation.
 Conclusion: unequal reproductive success  what is
often termed natural selection or survival of the fittest.

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Life on Earth
The Mechanism of Life
 DNA (deoxyribonucleic acid) is the genetic
material of all life on Earth.
 It consists of 4 chemical bases: adenine (A), thymine
(T), guanine (G), cytosine (C) which pair up in two
long strands and wind together in a double helix.
 DNA is self-replicating, which is the key to heredity.
 Any change (small or large) to this sequence from
imperfect duplication is called a mutation.
 Most mutations are lethal and kill the cell with the mutation.
 But some are beneficial and these can be passed to offspring.

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Life on Earth
The First Living Organism
 DNA has the same basic chemical nature in all life.
 Also, all organisms build proteins from the same set of
amino acids.
 This suggests that all life had a common ancestor which
arose some 3.85 billion years ago.
 The present day black smokers which thrive deep underwater at
high temperatures may resemble this early form of life.
 High temperatures may promote faster and more diverse
chemistry which allows life to form quickly.
 Looking at DNA from all life, biologists have composed a
“tree of life”, which suggests how the DNA changed to
other types of life.
 Consists of 3 main domains: bacteria, archaea, and eukarya.

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Life on Earth
The Origin of The First Living Organism
 Where did this first life come from?
 Some experiments done in the 1950s (and since)
showed a mixture of early-Earth organic molecules plus
lightning can produce all the major molecules of life
including amino acids and DNA bases.
 Strands of RNA (ribonucleic acid) which resemble single
strands of DNA have been reproduced in the laboratory.
 Many biologists presume that RNA came first followed by DNA.
 Microscopic enclosed membranes can form which may have
surrounded self-replicating RNA on the early Earth.

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Life on Earth
Could life have migrated from elsewhere?
 The idea that life started elsewhere and
then came to the Earth (via meteor
impacts) is called panspermia.
 While the prospect of life traveling and
surviving through space seems difficult, we
have evidence of organic molecules in
meteorites and tests that show microbes
can survive space for years.

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Life on Earth
A Brief History of Life on Earth
 As Earth formed, chemical reactions form the
first organic molecules.
 After the heavy bombardment ended, the
common ancestor of life formed.
 Life rapidly grew and diversified, but remained
single cell organisms for 1 billion years.
 The land was still inhospitable until the ozone
layer formed – this required atmospheric oxygen

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Life on Earth
 Nearly all of the original oxygen was released via
photosynthesis from single-celled cyanobacteria some
3.5 billion years ago.
 For more than 1 billion years, this oxygen reacted with
surface rocks and little stayed in the atmosphere.
 Eventually, some 2 billion years ago, the oxygen began
to accumulate, but would not be “breathable” until just a
few hundred million years ago.
 This allowed new species (e.g. plants and animals) to form.
 Many other species did not need oxygen or even found oxygen to
be toxic.

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Life on Earth
 About 540 million years ago, tiny plants and
animal organisms changed dramatically in about
40 million years and formed into all the basic
plants (phyla) that we find on Earth today.
 The dramatic change in the diversity of life is
called the Cambrian explosion.
 Early dinosaurs and mammals arose at the same
time (225 to 250 million years ago).
 Dinosaurs died off 65 million years ago (by a meteor
impact?) and the mammals then rose to dominance.
 The earliest humans formed only a few million
years ago (after 99.9% of Earth’s history).

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Life on Earth
What is necessary for Life to exist?
 While animals need moderate temperatures and
abundant oxygen, simpler life can live under
much more extreme conditions and locations
(extremophiles).
 Underground, high and low temperatures.
 There seem to be 3 basic requirements.
 A source of nutrients
 Energy to fuel the activities of life
 Liquid water (the biggest constraint)

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Life in the Solar System
 To look for life elsewhere, we need to
search for places where the basic
necessities of life exist – the habitable
worlds.
 This eliminates most of worlds in our solar
system.
 Moon and Mercury are barren and dry.
 Venus too hot for liquid water.
 Jovian planets are gaseous.
 This leaves Mars and a few of the moons
orbiting the Jovian planets, notably Europa.

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Mars
 Percival Lowell thought he saw canals on
Mars, but we are quite confident now that
there are no civilizations on Mars.
 Nevertheless, we have good evidence the
liquid water once flowed on the Martian
surface.
 Today it contains subsurface ice which
could be heated to form areas of liquid
water underground.

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Missions to Mars, looking for life
 The Viking missions took soil samples and
looked for chemical changes that could be
attributed to biological processes.
 3 experiments suggested that life may be
present, but also ordinary chemical reactions
could have caused the same results.
 A fourth experiment found little organic
material, the opposite of what one would
expect if life were present.

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 The Mars Phoenix mission detected water under
the surface, though soil was basic and may have
trouble harboring life (perchrolates)
 Pathfinder, Spirit and Opportunity studied the
Martian conditions to see if life might have
existed.
 The Mars Express orbiter detected methane gas.
 Methane should disappear within a few centuries due
to chemical reactions.
 So, something is supplying Mars with methane.
 It could come from comet impacts, volcanoes, or life.
 Volcanism seems to be the most likely candidate.

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Martian Meteorites
 One meteorite which landed in Antarctica 13,000
years ago and found in 1984 was clearly of
Martian origin.
 Inside the meteorite were complex organic
materials and structures which looked like
nanobacteria , very small bacteria which have
been discovered on Earth.
 These structures can also be made by chemical
and geological means.
 Contamination from being on the Earth may also
explain the presence of organic materials.

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Life on Europa
 Europa has enough tidal heating to possibly form
a subsurface ocean underneath its icy crust.
 Life there could form like the “black smokers” on
Earth.
 Larger life forms could exist in the vast oceans,
but energy sources are limited and this would
tend to limit the size of any life there.

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Life on Ganymede, Callisto and Titan
 Ganymede and Callisto might have
subsurface oceans, but their internal heat
is small and liquid water would not be
terribly abundant.
 Titan has no native liquid water, but an
abundance of organic materials.
 Could life evolve from the lakes of methane?
 Water might be brought in from comets, but
this would eventually freeze.

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Life Around Other Stars
Beyond the Solar System
 Where in the Galaxy might we find life?
 Since technology might allow us to obtain
surface pictures or spectra, we restrict ourselves
to considering extrasolar planets with habitable
surfaces.
 So far all detected extrasolar planets (except
maybe one or two) are gaseous giants and are
unlikely to have surface life.
 However, they may be surrounded by moons
which may support life.

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Constraints on Star Systems
 A star must be stable and live long enough
to allow a planet to develop life.
 Stars greater than a few solar masses are
ruled out (but this is only about 1% of all
stars).
 A star must allow stable planet orbits.
 Binary and multiple star systems are much
less likely to have this – about 50% of all star
systems.

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 A third constraint is a planet must form in
the habitable zone.
 This is a region where a terrestrial type planet
would have the right surface temperature for
liquid water to exist.
 Stars less massive than the Sun have smaller
zones.
 A star like the Sun (or more massive) would
have the largest zone.
 Even if we restricted our search to Sun-like stars,
we would still have to consider billions of stars in
our Galaxy.

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Finding Habitable Planets
 Two upcoming missions may be able spot Earth
sized planets.
 Kepler will look for transits of planets across other
stars.
 The Space Interferometer Mission (SIM) may be
able to detect Earth sized planets.
 A decade or so from now, the Terrestrial Planet
Finder (TPF) or something like it may be able to
image extrasolar planets.
 Infrared spectra from future telescopes can look for
signatures of life

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Rare Earth?
 Some feel that an Earth type planet (with its
complex type of life) is rare:
 Galactic constraints
 Too close to the galaxy’s center and the rate of supernovae
are too great.
 Too far from the center and “metal” content is too low.
 This leaves about 10% of the galaxy’s disk that might be
habitable.
 A stellar system needs a Jupiter-like planet to sweep-
out and deflect meteors that might wipe out life on
Earth.
 Climate stability
 Plate tectonics and the carbon dioxide cycle.
 Earth’s large Moon keeps axial tilt relatively stable.

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Rare Earth?
 Counterarguments to the Rare Earth
Hypothesis
 The above conditions may not affect the
creation and advancement of complex life as
much as we think.
 There may be other overlooked conditions and
processes that could assist the creation and
advancement of complex life.

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The Search for Extraterrestrial Intelligence
What About “Intelligent” Life beyond the Solar System?
 SETI (Search for Extraterrestrial Intelligence) is trying to
find signs of alien communication
 How many civilizations out there?
 The (modified) Drake Equation suggests the number of
civilizations we might be able to contact:
# of civilizations = NHP × flife × fciv × fnow
where NHP is the number of habitable worlds
flife is the fraction of these world which actually have life
fciv is the fraction of these worlds which have
interstellar communications
fnow is the fraction of these worlds which have
a civilization at the present time
 It is hard to know exactly what any of these numbers are at the
present time.

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Interstellar Travel
 If one is restricted to going no faster than
the speed of light, then interstellar travel
will be difficult.
 In any event, vast new energy sources
must be used to propel a ship.
 Hydrogen scoopers.
 Nuclear bombs or nuclear power
 Matter-antimater.

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Interstellar Travel
 If there is advanced alien life out there, why
haven’t we seen them (Fermi Paradox)?
 We are alone and there is no other advanced life out
there.
 Civilizations are common, but no one has colonized
the galaxy because
 Technology prevents a widespread travel.
 The desire to explore is unusual.
 Civilizations destroy themselves before they can colonize the
stars.
 There is a galactic civilization, but it has not revealed
itself!!!

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