La historia de la vida en la Tierra y en otras plataformas planetarios potenciales portadoras de vida están profundamente ligadas a la historia del Universo. Puesto que la vida, tal como la conocemos, se basa en elementos químicos forjados en estrellas pesadas moribundas, el Universo tiene que ser lo suficientemente antiguo para que las estrellas se formaran y evolucionaran. La teoría cosmológica actual indica que el Universo es de 13,7 ± 0,13 mil millones de años y que las primeras estrellas se formaron cientos de millones de años después del Big Bang. En este trabajo, se argumenta que podemos dividir la historia cosmológica en cuatro edades, desde el Big Bang a la vida inteligente.
La historia de la vida en la Tierra y en otras plataformas planetarios potenciales portadoras de vida están profundamente ligadas a la historia del Universo. Puesto que la vida, tal como la conocemos, se basa en elementos químicos forjados en estrellas pesadas moribundas, el Universo tiene que ser lo suficientemente antiguo para que las estrellas se formaran y evolucionaran. La teoría cosmológica actual indica que el Universo es de 13,7 ± 0,13 mil millones de años y que las primeras estrellas se formaron cientos de millones de años después del Big Bang. En este trabajo, se argumenta que podemos dividir la historia cosmológica en cuatro edades, desde el Big Bang a la vida inteligente.
Uma equipe formada por astrônomos de Israel, da Europa, da Coreia e dos EUA, anunciou a descoberta de um exoplaneta gigante gasoso circumbinário, na zona habitável de seu par de estrelas, uma ocorrência surpreendentemente comum para os exoplanetas circumbinários descobertos pela missão Kepler/K2 da NASA.
Lembrando o planeta da ficção, Tatooine, exoplanetas circumbinários orbitam duas estrelas e assim têm dois sóis em seu céu.
O exoplaneta circumbinário, recém-descoberto, denominado de Kepler-453b, leva 240.5 dias para orbitar suas estrelas, enquanto as estrelas orbitam uma com relação a outra a cada 27.3 dias.
A estrela maior, a Kepler-453A, é similar ao nosso Sol, contendo 94% da massa do Sol, enquanto que a estrela menor, a Kepler-453B, tem cerca de 20% da massa e é mais fria e mais apagada.
O sistema binário, localiza-se na constelação de Lyra, e está a aproximadamente 1400 anos-luz de distância da Terra. Estima-se que esse sistema tenha entre 1 e 2 bilhões de anos de vida, sendo bem mais novo que o nosso Sistema Solar.
Também conhecido como KIC 9632895b, o Kepler-453b tem um raio 6.2 vezes maior que o da Terra. Sua massa não foi medida nos dados atuais, mas provavelmente ele deve ter cerca de 16 vezes a massa da Terra.
De acordo com os astrônomos, o Kepler-453b, é o terceiro planeta circumbinário da missão Kepler, descoberto na zona habitável de um par de estrelas.
Devido ao seu tamanho, e a sua natureza gasosa, o planeta pouco provavelmente deve abrigar a vida como nós a conhecemos. Contudo, ele pode, como os gigantes gasosos do Sistema Solar, ter grandes luas, e essas luas poderiam ser habitáveis. Sua órbita se manterá estável por 10 milhões de anos, aumentando a possibilidade da vida se formar nas suas luas.
Com o número de exoplanetas circumbinários conhecidos agora em dez, os cientistas podem começar a comparar diferentes sistemas e procurar uma tendência. Os sistemas tendem a ser bem compactos e podem aparecer num grande número de configurações.
Uma vez pensados como sendo raros e até mesmo impossíveis de existir, essa e outras descobertas do Kepler, confirmam que esses planetas são comuns na nossa Via Láctea.
“A diversidade e complexidade desses sistemas circumbinários é algo maravilhoso. Cada novo planeta circumbinário, é uma joia, revelando algo inesperado e desafiador”, disse o Prof. William Welsh da Universidade Estadual de San Diego, e o primeiro autor do artigo que descreve a descoberta, publicado no Astrophysical Journal.
Fonte:
http://www.sci-news.com/astronomy/science-kepler453b-circumbinary-exoplanet-03117.html
NEUTRON STARS - UNIQUE COMPACT OBJECTS OF THEIR OWNIJRST Journal
This paper outlines the study of neutron stars right from the early
theoretical predictions and observations by various astrophysicists which
gradually aroused huge interests among the scientific community, to the
latest developments in the scientific analysis of the behavior of the different
categories of compact objects. Although white dwarfs, neutron stars, brown
dwarfs, Black Holes etc.fall under the category of compact objects, each of
them is unique in its own way.
An extreme starburst_in_the_core_of_a_rich_galaxy_cluster_at_z_1_7Sérgio Sacani
Os astrônomos descobriram um raro tipo de aglomerado de galáxias que tem seu coração repleto de novas estrelas. A descoberta inesperada, aconteceu com a ajuda dos telescópios espaciais da NASA Spitzer e Hubble, e sugere que galáxias gigantescas no núcleo desses aglomerados massivos podem crescer de maneira significante se alimentando de gás roubado de outras galáxias.
“Normalmente, as estrelas no centro dos aglomerados de galáxias, são velhas e mortas, essencialmente fósseis estelares”, disse Tracy Webb da Universidade McGill, em Montreal, no Canadá, principal autor do artigo que descreve as descobertas publicado na edição de 20 de Agosto do Teh Astrophysical Journal. “Mas nós acreditamos que a gigantesca galáxia no centro desse aglomerado está gerando novas estrelas de maneira furiosa depois de realizar uma fusão com uma galáxia menor”.
Os aglomerados de galáxias são grandes famílias de galáxias unidas e agrupadas pela gravidade. A nossa Via Láctea reside em um pequeno grupo de galáxias, chamado de Grupo Local, que por sua vez reside na periferia do gigantesco superaglomerado de galáxias Lanikea com 100000 galáxias (Lanikea signifiva “céu imensurável” em havaiano).
O aglomerado nesse novo estudo, é chamado pelos astrônomos de SpARCS1049+56, e tem no mínimo 27 galáxias, e uma massa combinada equivalente a 400 trilhões sóis. Ele está localizado a cerca de 9.8 bilhões de anos-luz na direção da constelação de Ursa Major. O objeto foi inicialmente descoberto usando o Spitzer e o Telescópio Canadá-França-Havaí, localizado no monte Mauna Kea no Havaí, e confirmado, usando o Observatório W.M. Keck, também no Mauna Kea.
This article aims to present the origin and evolution of Universe, Sun and Earth as well as alternative solutions for the survival of humanity with the end of Earth planet, Sun and Universe.
Uma equipe formada por astrônomos de Israel, da Europa, da Coreia e dos EUA, anunciou a descoberta de um exoplaneta gigante gasoso circumbinário, na zona habitável de seu par de estrelas, uma ocorrência surpreendentemente comum para os exoplanetas circumbinários descobertos pela missão Kepler/K2 da NASA.
Lembrando o planeta da ficção, Tatooine, exoplanetas circumbinários orbitam duas estrelas e assim têm dois sóis em seu céu.
O exoplaneta circumbinário, recém-descoberto, denominado de Kepler-453b, leva 240.5 dias para orbitar suas estrelas, enquanto as estrelas orbitam uma com relação a outra a cada 27.3 dias.
A estrela maior, a Kepler-453A, é similar ao nosso Sol, contendo 94% da massa do Sol, enquanto que a estrela menor, a Kepler-453B, tem cerca de 20% da massa e é mais fria e mais apagada.
O sistema binário, localiza-se na constelação de Lyra, e está a aproximadamente 1400 anos-luz de distância da Terra. Estima-se que esse sistema tenha entre 1 e 2 bilhões de anos de vida, sendo bem mais novo que o nosso Sistema Solar.
Também conhecido como KIC 9632895b, o Kepler-453b tem um raio 6.2 vezes maior que o da Terra. Sua massa não foi medida nos dados atuais, mas provavelmente ele deve ter cerca de 16 vezes a massa da Terra.
De acordo com os astrônomos, o Kepler-453b, é o terceiro planeta circumbinário da missão Kepler, descoberto na zona habitável de um par de estrelas.
Devido ao seu tamanho, e a sua natureza gasosa, o planeta pouco provavelmente deve abrigar a vida como nós a conhecemos. Contudo, ele pode, como os gigantes gasosos do Sistema Solar, ter grandes luas, e essas luas poderiam ser habitáveis. Sua órbita se manterá estável por 10 milhões de anos, aumentando a possibilidade da vida se formar nas suas luas.
Com o número de exoplanetas circumbinários conhecidos agora em dez, os cientistas podem começar a comparar diferentes sistemas e procurar uma tendência. Os sistemas tendem a ser bem compactos e podem aparecer num grande número de configurações.
Uma vez pensados como sendo raros e até mesmo impossíveis de existir, essa e outras descobertas do Kepler, confirmam que esses planetas são comuns na nossa Via Láctea.
“A diversidade e complexidade desses sistemas circumbinários é algo maravilhoso. Cada novo planeta circumbinário, é uma joia, revelando algo inesperado e desafiador”, disse o Prof. William Welsh da Universidade Estadual de San Diego, e o primeiro autor do artigo que descreve a descoberta, publicado no Astrophysical Journal.
Fonte:
http://www.sci-news.com/astronomy/science-kepler453b-circumbinary-exoplanet-03117.html
NEUTRON STARS - UNIQUE COMPACT OBJECTS OF THEIR OWNIJRST Journal
This paper outlines the study of neutron stars right from the early
theoretical predictions and observations by various astrophysicists which
gradually aroused huge interests among the scientific community, to the
latest developments in the scientific analysis of the behavior of the different
categories of compact objects. Although white dwarfs, neutron stars, brown
dwarfs, Black Holes etc.fall under the category of compact objects, each of
them is unique in its own way.
An extreme starburst_in_the_core_of_a_rich_galaxy_cluster_at_z_1_7Sérgio Sacani
Os astrônomos descobriram um raro tipo de aglomerado de galáxias que tem seu coração repleto de novas estrelas. A descoberta inesperada, aconteceu com a ajuda dos telescópios espaciais da NASA Spitzer e Hubble, e sugere que galáxias gigantescas no núcleo desses aglomerados massivos podem crescer de maneira significante se alimentando de gás roubado de outras galáxias.
“Normalmente, as estrelas no centro dos aglomerados de galáxias, são velhas e mortas, essencialmente fósseis estelares”, disse Tracy Webb da Universidade McGill, em Montreal, no Canadá, principal autor do artigo que descreve as descobertas publicado na edição de 20 de Agosto do Teh Astrophysical Journal. “Mas nós acreditamos que a gigantesca galáxia no centro desse aglomerado está gerando novas estrelas de maneira furiosa depois de realizar uma fusão com uma galáxia menor”.
Os aglomerados de galáxias são grandes famílias de galáxias unidas e agrupadas pela gravidade. A nossa Via Láctea reside em um pequeno grupo de galáxias, chamado de Grupo Local, que por sua vez reside na periferia do gigantesco superaglomerado de galáxias Lanikea com 100000 galáxias (Lanikea signifiva “céu imensurável” em havaiano).
O aglomerado nesse novo estudo, é chamado pelos astrônomos de SpARCS1049+56, e tem no mínimo 27 galáxias, e uma massa combinada equivalente a 400 trilhões sóis. Ele está localizado a cerca de 9.8 bilhões de anos-luz na direção da constelação de Ursa Major. O objeto foi inicialmente descoberto usando o Spitzer e o Telescópio Canadá-França-Havaí, localizado no monte Mauna Kea no Havaí, e confirmado, usando o Observatório W.M. Keck, também no Mauna Kea.
This article aims to present the origin and evolution of Universe, Sun and Earth as well as alternative solutions for the survival of humanity with the end of Earth planet, Sun and Universe.
NGTS-1b: A hot Jupiter transiting an M-dwarfSérgio Sacani
We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf
host (Teff,∗=3916 +71
−63 K) in a P = 2.647 d orbit discovered as part of the Next Generation
Transit Survey (NGTS). The planet has a mass of 0.812 +0.066
−0.075MJ and radius
of 1.33 +0.61
−0.33 RJ , making it the largest and most massive planet discovered transiting
any M-dwarf. NGTS-1b is the third transiting giant planet found around an M-dwarf,
reinforcing the notion that close-in gas giants can form and migrate similar to the
known population of hot Jupiters around solar type stars. The host star shows no
signs of activity, and the kinematics hint at the star being from the thick disk population.
With a deep (2.5%) transit around a K = 11.9 host, NGTS-1b will be a strong
candidate to probe giant planet composition around M-dwarfs via JWST transmission
spectroscopy.
Chiotelis Ioannis, Theodoropoulou Maria, “Searching for Black Holes. Photometry in our Classrooms”, Hellenic Conference on Innovating STEM Education, 16-18 December 2016, Athens, Greece.
An infrared transient from a star engulfing a planetSérgio Sacani
Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun1,2. Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star3–5. However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae6,7—a class of eruptions now confirmed8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 1035 erg s−1) and radiated energy (approximately 6.5 × 1041 erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.
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
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
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
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
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.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
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.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
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
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
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
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
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.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
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 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
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
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
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
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.
X-rays from a Central “Exhaust Vent” of the Galactic Center ChimneySérgio Sacani
Using deep archival observations from the Chandra X-ray Observatory, we present an analysis of
linear X-ray-emitting features located within the southern portion of the Galactic center chimney,
and oriented orthogonal to the Galactic plane, centered at coordinates l = 0.08◦
, b = −1.42◦
. The
surface brightness and hardness ratio patterns are suggestive of a cylindrical morphology which may
have been produced by a plasma outflow channel extending from the Galactic center. Our fits of the
feature’s spectra favor a complex two-component model consisting of thermal and recombining plasma
components, possibly a sign of shock compression or heating of the interstellar medium by outflowing
material. Assuming a recombining plasma scenario, we further estimate the cooling timescale of this
plasma to be on the order of a few hundred to thousands of years, leading us to speculate that a
sequence of accretion events onto the Galactic Black Hole may be a plausible quasi-continuous energy
source to sustain the observed morphology
X-rays from a Central “Exhaust Vent” of the Galactic Center Chimney
Delivery of dark_material_to_vesta_via_carbonaceous_chondritic_impacts
1. Delivery of Dark Material to Vesta via Carbonaceous Chondritic
Impacts
Vishnu Reddy
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
Department of Space Studies, University of North Dakota, Grand Forks, USA
Email: reddy@mps.mpg.de
Lucille Le Corre
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
David P. O’Brien
Planetary Science Institute, Tucson, Arizona, USA
Andreas Nathues
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
Edward A. Cloutis
Department of Geography, University of Winnipeg, Manitoba, Canada
Daniel D. Durda
Southwest Research Institute, Boulder, Colorado, USA
William F. Bottke
Southwest Research Institute, Boulder, Colorado, USA
Megha U. Bhatt
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
David Nesvorny
Southwest Research Institute, Boulder, Colorado, USA
Debra Buczkowski
Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA
Jennifer E. C. Scully
Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles
California, USA
Elizabeth M. Palmer
Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles
California, USA
Holger Sierks
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
Paul J. Mann
Department of Geography, University of Winnipeg, Manitoba, Canada
! "!
2. Kris J. Becker
Astrogeology Science Center, U.S. Geological Survey, Flagstaff, Arizona, USA
Andrew W. Beck
Department of Mineral Sciences, Smithsonian National Museum of Natural History, 10th and Constitution
NW, Washington, DC, USA
David Mittlefehldt
Astromaterials Research Office, NASA Johnson Space Center, Mail Code KR, Houston, Texas, USA
Jian-Yang Li
Department of Astronomy, University of Maryland, College Park, Maryland, USA
Robert Gaskell
Planetary Science Institute, Tucson, Arizona, USA
Christopher T. Russell
Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles
California, USA
Michael J. Gaffey
Department of Space Studies, University of North Dakota, Grand Forks, USA
Harry Y. McSween
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
Thomas B. McCord
Bear Fight Institute, Winthrop, Washington, USA
Jean-Philippe Combe
Bear Fight Institute, Winthrop, Washington, USA
David Blewett
Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA
Pages: 58
Figures: 15
Tables: 2
Proposed Running Head: Dawn at Vesta, Dark Material
Editorial correspondence to:
Vishnu Reddy
Max-Planck Institute for Solar System Research,
37191 Katlenburg-Lindau,
Germany.
(808) 342-8932 (voice)
reddy@mps.mpg.de
! #!
3. Abstract
NASA’s Dawn spacecraft observations of asteroid (4) Vesta reveal a surface with the
highest albedo and color variation of any asteroid we have observed so far. Terrains rich
in low albedo dark material (DM) have been identified using Dawn Framing Camera
(FC) 0.75 µm filter images in several geologic settings: associated with impact craters (in
the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or
rays commonly associated with topographic highs; and as dark spots (likely secondary
impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or
exogenic in origin. We report that the majority of the spectra of DM are similar to
carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using
high-resolution seven color images we compared DM color properties (albedo, band
depth) with laboratory measurements of possible analog materials. Band depth and
albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite
Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite
and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM.
Modeling of carbonaceous chondrite abundance in DM (1-6 vol%) is consistent with
howardite meteorites. We find no evidence for large-scale volcanism (exposed
dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater
scaling laws and numerical models of ejecta reaccretion suggest the delivery and
emplacement of this DM on Vesta during the formation of the ~400 km Veneneia basin
by a low-velocity (<2 km/sec) carbonaceous impactor. This discovery is important
because it strengthens the long-held idea that primitive bodies are the source of carbon
and probably volatiles in the early Solar System.
! $!
4. 1. Introduction
NASA’s Dawn spacecraft entered orbit around asteroid (4) Vesta in July 2011 for
a year-long mapping project using its suite of three instruments. The Dawn Framing
Cameras are a pair of identical instruments (one for cold redundancy) (Sierks et al. 2011)
that have imaged the entire visible surface of the asteroid in clear (panchromatic) and
seven colors (0.44-1.0 µm) with a resolution up to ~20 meter/pixel. During the approach
phase, the entire surface was imaged in three successive rotational characterizations
(RC1-3) at resolutions of 9.06 km/pixel, 3.38 km/pixel and 487 m/pixel, respectively.
Following orbit insertion, Vesta was mapped from Survey, high-altitude mapping orbit
(HAMO), and low altitude mapping orbit (LAMO) at resolutions of 250 m/pixel, 60
m/pixel, and 20 m/pixel, respectively.
The surface of Vesta as imaged by Dawn FC revealed a surface that is unlike any
asteroid we have visited so far with a spacecraft. Albedo and color variations of Vesta are
the most diverse among the objects in the asteroid belt, with a majority of these variations
linked to distinct compositional units on the asteroid’s surface (Reddy et al. 2012a).
Whereas Hubble Space Telescope (Thomas et al. 1997, Binzel et al. 1997) and ground
based (Gaffey, 1997) observations of Vesta had shown a large hemispherical scale
dichotomy (Reddy et al. 2010), Dawn FC color data showed smaller regional and local
scale albedo variations. Vesta’s Western hemisphere on average has lower albedo than
the Eastern hemisphere (Reddy et al 2012a). The large low albedo region extends from
90° to 200° longitude and 64° S to 16° N latitude and was originally interpreted as
ancient howarditic terrain from ground-based spectral observations (Gaffey, 1997). All
coordinate used in this paper conform to the Claudia system used by the Dawn science
! %!
5. team. Bright material extending on either side of this dark region has been interpreted as
impact ejecta from the formation of the Rheasilvia basin at the South Pole (Reddy et al.
2012a).
Preliminary analysis by Reddy et al. (2012a) showed carbonaceous chondrite
contaminants as a possible source of the dark material. This line of evidence is supported
by laboratory study of HED meteorites (e.g., Buchanan et al. 1993; Zolensky et al. 1996),
and ground based spectral observations that suggested the presence of a 3-µm feature
probably due to OH- in carbonaceous chondrites (Hasegawa et al. 2003; Rivkin et al.
2006). A preliminary geomorphological description of DM on Vesta is presented in
Jaumann et al. (2012). More recently McCord et al. (2012) confirmed this hypothesis that
dark material on Vesta is indeed related to carbonaceous chondritic clasts in HED
meteorites. Here we presented a more detailed analysis testing this hypothesis and further
constrain the origin, abundance and delivery mechanism for DM on Vesta. We also
investigated alternate compositional affinities for DM.
2. Description of Dark Material
The Dawn Framing Camera (FC) discovered DM on Vesta (Reddy et al. 2012a).
These enigmatic surface features in the form of low albedo units were first observed
during RC3 at a resolution of ~487 m/pixel. Whereas they are ubiquitous in the low
albedo hemisphere, their distribution is global with several occurring on the putative
Rheasilvia ejecta units. Features associated with DM can be broadly classified into (A)
those associated with impact craters (in the ejecta material and/or on the crater walls and
rims); (B) flow-like deposits or streaks/rays commonly associated with topographic
highs; and (C) dark spots. Figures 1-3 show examples of each type of feature using
! &!
6. images obtained during HAMO and LAMO phases at a resolution of 20 to 60 m/pixel.
Here we describe each group of features associated with DM to provide geological
context for understanding their origin. We selected the 15-km diameter impact crater
Cornelia (9.3S°, 225.5°E), which has a dark ejecta blanket and a prominent DM deposit
on the crater walls and floor, as a test site for our analysis.
2.1 Impact Craters
A majority of DM on Vesta is found in and around impact craters. Figures 1 and 2
show the first of the two types of impact craters. In Fig. 1A, DM appears in the ejecta
blanket left of the rim of the 58-km Marcia crater and also along the inner walls (see
white arrows). It appears that the inner crater wall was likely covered by the DM seen on
the ejecta blanket on the left of the crater rim, but subsequent downslope movements of
fresher bright material has buried and mixed with the original DM. The exact cause for
this mass wasting is unknown but seismic shaking triggered by nearby impacts could be a
possible explanation. Dark material on the crater walls appears to have a ‘tadpole’ shape
(see white arrows in Fig. 1B) due to bright material (likely more recent and less space
weathered) flowing around them and covering part of their corresponding debris flow.
Topographic features (i.e. pile of boulders or bright outcrops of brecciated material)
along the crater walls seem to divert the bright granular material in channel-like features
around the DM near the rim of the crater. Further down the slope, the bright debris flows
spread out (indicated by dashed white arrows in Fig. 1B) forming lobate deposits (talus
cones) that cover older dark deposits causing these ‘tadpole’ shaped features. Similar
morphology can also be observed on the Moon: a classic example is the Diophantus
crater on the western edge of Mare Imbrium that exhibits avalanche scars due to down
! '!
7. slope movement of darker pyroclastic material. The DM ‘tadpole’ features are protected
by the topography (in this case, outcrops of bright material) and in this way preserved
from being completely covered by the subsequent debris flow. The layer forming the
bright outcrops and the underlying dark debris flows produce this appearance of a
lithology constituted by sub-surface deposit of DM with a boundary more or less
contiguous or parallel to the crater rim. Characteristics of the bright layer might cause the
variations in position and inclination of this apparent stratification clearly visible in the
case of Marcia and partially visible in Numisia (Fig. 2). This layer might represent either
an ancient DM lithology or the remnants of material deposited by the impact event and
covered afterwards by debris flows from the crater wall.
Figure 2A shows another example of DM associated with an impact crater
(Numisia). Unlike Marcia crater, some DM debris flows appear almost intact on the
Northern part of the crater wall. Dark material is also visible in the ejecta blanket (see
white arrows in Fig. 2A) and seems to have contributed to the DM falling along the crater
wall by mass wasting. The intact DM debris flows (Fig. 2B) are originating from the
northern rim of the crater and are flowing downslope around the topographic highs
composed of bright material outcrops. In another part of the crater wall, smaller dark
deposits are detectable and have the same ‘tadpole’ morphology that is found in Marcia
crater (indicated by white arrows in Fig 1B). As seen for Marcia, these features are
surrounded by bright and medium albedo debris flows coming from the rim crest (see
dashed white arrows in Fig. 1C). These subsequent mass-wasting features have covered
and mixed with the pre-existing DM deposits, forming lobes when they reached the crater
! (!
8. floor. They are able to cover part of the bright outcrops as well and efficiently bury some
parts of the crater wall.
Small impact craters on the eject blanket give us an indication of the thickness of
the DM (Fig 2B). The interiors of some small craters seem to have the same brightness as
the underlying ejecta blanket and some of them appear to have even darker albedo inside
the craters and in their ejecta (see black arrows in Fig. 2A), exposing material that has not
been buried by regolith or altered by space weathering. This suggests that either the DM,
in this area, has a depth at least as deep as the small craters or the small craters were
preexisting and then buried by the ejecta material. In contrast, other small craters of
similar diameter located outside the DM ejecta have brighter interiors (see dashed white
arrows in Fig. 2A). In addition, further away from the rim, a few small craters that
formed on the dark ejecta blanket have bright ejecta indicating that the dark ejecta
blanket is likely thinner with increasing distance from the crater rim (see dashed black
arrows in Fig. 2A).
Some impact craters have ejecta that consists of DM mixed with target material in
a subtle radial scour texture (Fig. 3A-B). Dark material in such ejecta typically forms thin
radial rays extending asymmetrically from the crater floor. The asymmetry of the ejecta
could be due to impact on a slope or an oblique impact. These craters also exhibit
irregular concentrations of DM along the crater rim and walls. Other examples of DM,
such as that at the Rubria crater (7.4S, 18.4E), have discontinuous ejecta blankets, usually
as patches.
Other craters have dark interiors and ejecta (e.g. Fig. 3C). In these cases, DM is
commonly distributed radially around the crater rim. A small impact crater (Fig. 3C) is
! )!
9. located on the ejecta blanket of the 60-km diameter impact crater Marcia. Its dark ejecta
suggest either that the impactor was a dark object or that a shallow subsurface low albedo
layer was excavated.
2.2 Dark Spots
Dark spots appear as low albedo pits and can be found on the ejecta blankets of
large impact craters such as the 30-km diameter Numisia crater (Fig. 2A). A majority of
these dark spots are <1 km in diameter with some showing no evidence for a central
craterlet. Fig. 3D shows examples of dark spots surrounding two larger impact craters.
Dark spots are either the result of secondary impacts in cases where they have a
discernable craterlet, or the result of ejecta material originating from nearby impact
craters. Some dark spots appear independent of impact craters. An example is located in
the region centered at 30°S, 120° E where dark spots are ubiquitous and unassociated
with any larger impact craters.
2.3 Topographic Highs
Dark material deposits associated with topographic highs are found at two
locations on Vesta: Aricia Tholus (Fig. 4) with a diameter of 37 km and Lucaria Tholus
(Fig. 5) with a diameter of approximately 22 km. Aricia Tholus has DM in the form of
rays (see white arrows in Figs. 4 A-B) radially emanating from a central crater that could
represent an impact crater on a hill. In the case of Lucaria Tholus, located just south of
the set of giant equatorial grooves, DM is draping part of a short ridge. Dark material
forks out from this ridge crest into the floor of nearby grooves. The downslope movement
has formed a long flow-like feature to the east (indicated by white arrows in Figs. 5A-D).
! *!
10. Similarly, a debris flow has been deposited to the south of this edifice, forming another
branch of DM in the downslope direction (indicated by black arrows in Figs. 5A-D).
3. Quantifying Color Parameters of Dark Material
3.1 Data Processing
Framing camera images are processed in three levels (1a-1c) in which the data
stream received on the ground (level 0) is converted to PDS format images (level 1a) that
contain unprocessed, uncalibrated digital values from 0 to 16383 DN. Level 1a data are
converted to level 1b as radiometrically calibrated PDS-compliant format images. The
level 1b data are converted to reflectance (I/F) by dividing the observed radiance by solar
irradiance from a normally solar-illuminated Lambertian disk. Dawn FC images are
affected by infield stray light that is corrected on the level 1b I/F data. The stray light
reduction algorithm removes this effect in the color data where the in-field stray light can
be up to 15% depending on the filter. The stray-light corrected data is then input to the
ISIS color pipeline. ISIS (Integrated Software for Imagers and Spectrometers) (Anderson
et al. 2004) is a UNIX-based program developed and maintained by USGS. ISIS
performs photometric correction of the color data to standard viewing geometry (30°
incidence and 0° emission angles) using Hapke functions derived from disk-integrated
ground-based telescopic observations of Vesta and Vestoids and spacecraft data acquired
during the approach phase (RC1-3 and Survey). The photometrically corrected data is
map projected and coregistered to align the seven color frames in order to create color
cubes prior to analysis. A detailed description of the FC data processing pipeline is
presented in Reddy et al. (2012a).
3.2 Color Parameters
! "+!
11. The albedo of DM is a key factor in its identification and displays the highest
visual contrast in the 0.75 µm filter. The albedo trend between various surface color units
on Vesta appears to form a continuum, with the DM having the lowest values (0.09-0.15).
A typical spectrum of Vesta is dominated by the mineral pyroxene with characteristic
absorption bands at 0.9 and 2.0 µm. Pyroxene on the surface of Vesta is a product of
igneous activity. Four of the seven Dawn FC color filters (0.44-1.0 µm) cover the 0.9-µm
pyroxene band. The intensity of the 0.9-µm pyroxene absorption is also affected by the
presence of DM (Le Corre et al. 2011; Reddy et al. 2012a). We used R(0.75)/R(0.92),
where R is the reflectance at wavelength !(µm), as a proxy for quantifying the band depth,
which is usually an indicator of iron content, particle size, space weathering, and
abundance of opaques like carbon/metal (Reddy et al. 2012).
The ratio of 0.98/0.92-µm is called the Eucrite-Diogenite (ED) ratio as it helps to
distinguish the two meteorite types. Eucrites have higher iron and calcium contents in
pyroxenes than diogenites. Hence, the 0.90-µm pyroxene band is shifted toward longer
wavelength for eucrites compared to diogenites. This leads to a 0.98/0.92-µm ratio that is
!1 for eucrites, but >1 for diogenites. Given the collisional mixing of the compositional
units on Vesta, the ED ratio at best helps identify eucrite-rich or diogenite-rich polymict
terrains. By using this ED ratio one can determine if the target materials of the impact
craters with DM are rich in eucrite or diogenite.
3.3 Albedo: How dark is dark material?
The albedo of the DM is a key factor in its identification along with surface
temperature. Reflectance of a surface varies with incidence, emission and phase angles
and therefore photometric correction of the data is required prior to any scientific analysis.
! ""!
12. In order to quantify the albedo of DM, it is also important to quantify the albedo of the
brightest material on Vesta. Reddy et al. (2012a) noted that the brightest material (in clear
filter) has a geometric albedo of ~0.67 whereas the darkest material has a geometric
albedo of only ~0.10. This albedo range is the highest among asteroids that have been
observed from the ground or visited by spacecraft.
To quantify the albedo change due to the presence of the DM, we have chosen the
0.75-µm filter as it is one of the least affected by infield stray light. This filter also
provides a good contrast between the DM and the other units. Figure 6A shows an image
of two impact craters with DM ejecta near Divalia Fossa (Rubria and Occia craters). A
profile drawn across (red line) the ejecta of the crater to the lower right is plotted in Fig.
6B. The albedo at 0.75 µm of the gray ejecta adjacent to the DM is ~0.22 but drops
steeply to ~0.12 over the DM ejecta (~45 % decrease in reflectance). As a comparison,
the brightest material on Vesta has an albedo of ~0.45 for the same filter. Figs. 6A-B
provide evidence that the albedo of the DM at 0.750 "m is significantly lower than its
immediate surroundings and is an important parameter for identification.
3.4 Band Depth: How deep is the 0.9- µm pyroxene band?
Reddy et al. (2012a) showed that the average spectrum for DM has a lower band
depth (23%) compared to Vesta’s global average (46%). Figures 7A-C show color-coded
maps of various color parameters for the 15-km diameter impact crater Cornelia with DM.
Figure 7A shows the image of the impact crater in 0.75 µm filter and Fig. 7B shows the
ratio of 0.75/0.92 µm filters (proxy for band depth). The color code is red for areas with
deepest 0.9-µm pyroxene bands and blue for areas with weakest bands. Figure 7B
confirms the spectral properties that we described earlier from the color spectra: the DM
! "#!
13. suppresses the 0.9-µm absorption band and the intensity of the suppression could be a
function of DM in the surface mixture.
3.5 Eucrite or Diogenite
Identification of the target material with which the DM is mixed can help
decipher its origin. As noted earlier, ED ratio helps to identify eucrite-rich or diogenite-
rich polymict terrains. In the ED color-coded image of Cornelia (Figure 7C), yellow to
red areas are those richer in diogenite-like material. Dark material seen in Fig. 7A
corresponds to blue in the ED color-coded map suggesting the target material is a mixture
of eucrite (ED ratio ~1) and diogenite (ED ratio >1), but with more diogenite. However,
it must be noted that the presence of DM might affect the ED ratio leading to false
identification of eucrite-rich terrains.
3.6 Summary of Color parameters
Based on the results inferred from Figs. 6A-B and 7A-C, it is clear that the DM
has lower albedo (~0.08-0.15), and shallower 0.9-µm band depth (~23%) compared to the
average spectrum of Vesta. We have performed similar analysis on other DM terrains (i.e.
for dark spots and for DM deposited on topographic highs) and the observed trends in
color parameters are identical. This suggests that a majority of the DM we see on Vesta
shares common properties (i.e. composition, grain size) and origin. In an effort to
constrain the origin of this DM, we have created a matrix that describes the color
properties (albedo, band depth) of all the possible analog materials we identified (Table
1). This matrix lists processes or analogs that have been observed either in the laboratory
or on other planetary bodies. By solving for this matrix we hope to narrow down the best
source for the DM.
! "$!
14. 4. Endogenous Sources of Dark Material
4.1 Pyroclasts from Vesta
The paucity of pyroclastic products such as glass and opaques in HEDs is well
established (Wilson and Keil, 1997; Keil, 2002). Magma gas content of HED meteorites
(Mittlefehldt 1987; Grady et al. 1996, 1997) is far lower (tens of parts per million) than
what would be required (>3.8 wt.%) for eruption speed to exceed the escape velocity
(~390 m s-1) of Vesta (Wilson and Keil, 1991). However, this low gas content is
sufficient to allow explosive activity to create pyroclastic products.
Wilson and Keil (1997) explained the paucity of pyroclastic products in HED by
modeling eruption speeds and trajectories of pyroclastic magma droplets. They suggested
that pyroclastic magma droplets formed in lava fountains are extremely optically dense
and hence will undergo limited radiative cooling during their trajectory to the surface. As
a consequence, the droplets would merge to form lava ponds rather than deposition of
glass spherules like those found on the Moon, thereby explaining the lack of traditional
pyroclastic products in HEDs. Wilson and Keil (1997) estimated that <1% of the erupted
lava would form pyroclastic fall deposits similar to glass beads. Moreover, they also
predicted that minerals crystallizing from these lava ponds would be indistinguishable
from those formed by extrusive lava flows (i.e. basaltic eucrites). Therefore any lava
flows erupting from a fire fountain would be spectrally identical to basaltic eucrites.
Aside from the low abundance of vestan pyroclastic fall deposits, the
identification of pyroclasts in the HEDs is also challenging (Singerling and McSween,
2012). Compared to lunar pyroclasts, which exhibit distinct increase in magnesium and
decrease in aluminum along with a typical glassy texture, vestan pyroclasts with similar
! "%!
15. properties can easily be mistaken for impact melts. Comparison with lunar pyroclasts
shows that the weakest band depth found in lunar pyroclasts (i.e., Apollo 15 green glass)
is nevertheless still stronger than the band depth of DM.
4.2 Eucrites with Opaques
The presence of opaque phases in eucrites (chromite, ilmenite, troilite, and metal)
(Mayne et al. 2010) reduces both albedo and band depth (Miyamoto et al. 1982). The
intensity of these effects is a function of the grain size and modal abundance of the
opaque phases. A band depth vs. albedo plot (Fig. 8) of three opaque-rich eucrites (MET
01081, BTN 00300, Chervony Kut) (Mayne et al. 2010) and vestan DM reveals that all
three have higher albedo and band depth compared to the DM. Due to this mismatch,
these meteorites are unlikely to be DM analogs. Furthermore, the abundance of opaque-
rich meteorites among unbrecciated eucrites provides additional support for this
conclusion.
Opaque-rich eucrites are rare (only 3 of 29) (Mayne et al. 2010) and the
abundance of opaques (i.e., chromite/ulvospinel, ilmenite, sulfides, and FeNi metal) in
these three eucrites is !2% (typically ~1%). Thus, opaques are rare among the
unbrecciated eucrites and if present, are only observed in trace amounts even for opaque-
rich eucrites. At higher abundances, chromite and ilmenite would exhibit absorption
features in the 0.5-0.6 "m region (Adams, 1974; Cloutis et al. 2004), which are not
observed in FC spectra of the DMs. Abundant troilite or metal (Britt and Pieters 1989)
would lead to progressively redder slopes, again a characteristic that is not observed in
DM FC spectra (Cloutis et al. 2009). Apart from the three opaque-rich eucrites,
ALHA81001 is a quenched basaltic eucrite that has <1% fine-grained ilmenite scattered
! "&!
16. throughout the meteorite. This meteorite’s lower spectral contrast has been attributed to
its fast cooling rate rather than the presence of ilmenite (Mayne et al. 2010). Our
independent study (Reddy et al. 2012b) to detect opaque-rich terrains similar to
ALHA81001 using the chromium absorption band in pyroxene has been fruitless,
suggesting that it is not a dominant assemblage.
4.3 Impact Shock and Impact Melt
Impact shock can lower the albedo and reduce the band depth of the original
target material (Gaffey, 1976; Adams et al. 1979). In shock-blackened ordinary
chondrites, the dispersion of micron-scale nickel iron metal and troilite is the primary
cause of these effects (Britt and Pieters, 1989). The metal and silicate components of
black ordinary chondrites are identical to their unshocked counterparts. However, black
chondrites have albedos typically <0.15 compared to 0.15-0.30 for intact ordinary
chondrites. 40Ar-39Ar measurements of eucrites suggest at least four large impact events
occurred between 4.1 and 3.4 Ga that were capable of resetting Ar ages (Bogard and
Garison, 2003). Those impacts would be strong enough to significantly shock the target
material, as evidenced in HED meteorites, and could potentially produce shock-
blackened eucrites like the observed shock-blackened ordinary chondrites. However, very
few shock-blackened eucrites are known in the terrestrial meteorite collections.
Another product of the impact process is glass. Although a few howardites have
impact melt glass between 15-20 vol% (Desnoyers and Jerome, 1977), in general, glass
represents <1 vol% of known HED meteorites. An impact melt clast from eucritic breccia
Macibini contains 50% devitrified glass and 50% silicates (Buchanan et al. 2000) and the
Padvarninkai meteorite is the most heavily shocked eucrite known with most of the
! "'!
17. plagioclase converted to maskelynite with significant impact melt glass (Hiroi et al.
1995). LEW85303 is a polymict eucrite with an impact melt matrix. Jiddat al Harasis
(JaH) 626 is a polymict eucrite that experienced significant impact shock and melt
quenching (Irving, 2012). To test the affinity of DM to shocked eucrites and impact melt
we obtained spectra of the Macibini clast and the Padvarninkai eucrite from RELAB
database. We also measured the visible-near-IR spectra of JaH 626. All spectral data
were resampled to Dawn FC filter band passes to obtain their color spectra.
The albedo of a <45 "m powder of JaH 626 at 0.75 µm is comparable to other
polymict eucrites (albedo of 0.32) (Fig. 9). The albedo of JaH 626 (and other eucrites)
drops significantly with increasing particle size with the lowest albedo being 0.12 at 250-
500 µm. Band depth also decreases with increasing grain size suggesting that shocked
impact melt-rich material could be a possible analog for some DM units. However,
albedo and band parameters of JaH 626 appear higher than those of DM. The mismatch
in spectral band parameters coupled with the paucity of shock-blackened eucrites in
meteorite collections makes it unlikely for JaH 626-type material to be a possible analog
for the DM on Vesta.
The albedo of the Pavarninkai impact melt sample with 25-45 micron grain size
(albedo is 0.15) is comparable to the DM observed on in Cornelia crater. However, the
band depths of both grain sizes are lower and they plot below the DM trend in the band
depth vs. albedo plot (Fig. 9). Macibini impact melt clast and LEW85303 have albedo
(0.24, 0.36) and band depth that is significantly higher than DM. Like JaH 626 the color
parameters of impact melts are significantly different from DM observed on Vesta
making them unlikely analogs.
! "(!
18. 5. Exogenous Sources of Dark Material
5.1 Carbonaceous Chondrite Impactor
Exogenous sources (e.g., a carbonaceous chondrite impactor) could explain the
DM seen on Vesta. The presence of exogenous carbonaceous chondrite meteorite clasts
in HED meteorites from Vesta is well documented (e.g., Buchanan et al. 1993; Zolensky
et al. 1996). Unshocked CM-like carbonaceous chondrite clasts have also been identified
in other meteorite types, including H chondrite meteorite breccias, where they make up
the most abundant variety of foreign materials in those rocks (e.g., Rubin and Bottke
2009). Carbonaceous material can also occur as microclasts (25-800 µm) similar to
micrometeorites on Earth (Gounelle et al. 2003).
In HEDs, carbonaceous chondrite clasts generally comprise up to 5 vol. %
(Zolensky et al. 1996), but on rare occasions can be 60 vol. % of howardites (Herrin et al.
2011). Antarctic howardites Mt. Pratt (PRA) 04401 and 04402 along with Scott Glacier
(SCO) 06040 have 10-60% carbonaceous chondrite clasts (Herrin et al. 2011). A vast
majority (83%) of these are CM2 carbonaceous chondrites followed by CR2 (15%). Both
of these meteorite types are aqueously altered (Zolensky et al. 1996). Clasts are primarily
composed of a phyllosilicate matrix (mostly serpentine) with Fe-Ni sulfides, carbonates,
hydrated sulfates, organics, and Fe3+-bearing phases, and inclusions of olivine, pyroxene,
relict chondrules and calcium-aluminum-rich inclusions. Although some of these clasts
have been heated and dehydrated during impact (~400°C), a majority are still hydrated
(containing H2O- and/or OH-bearing phases) (Zolensky et al. 1996).
In order to test the possibility of carbonaceous chondrite impactors being the
source of DM on Vesta, we studied color properties of eucrite (Millbillillie) and CM2
! ")!
19. carbonaceous chondrite (Murchison) mixtures (Le Corre et al. 2011) and Antarctic
howardite PRA04401. Figure 10A shows a spectrum of DM overlaid on color spectra of
CM2+eucrite mixtures. Both data sets are normalized to unit at 0.75 µm. Band depth vs.
albedo of DM along with those of PRA04401 and CM2+eucrite mixtures are plotted in
Fig. 10B. Band depth and albedo values for CM2+eucrite mixtures (particle size <45 µm)
are offset from the DM trend. This offset between the two could be a compositional effect
as the CM2+eucrite mixture does not have a diogenitic component like PRA04401
howardite.
Adding a diogenitic component would likely increase the overall band depth of
the mixture as diogenites typically have deeper pyroxene bands than eucrites. This would
shift the overall CM2+eucrite mixture trend upward matching that of DM on Vesta.
However, the band depth and albedo trends of PRA04401 and CM2+eucrite mixtures are
the best possible matches for DM among all the analogs we have investigated so far.
Areal and intimate mixture band depth-albedo trends run parallel on either side of the
DM trend, suggesting that both processes are prevalent on Vesta’s surface, with the
former being similar to carbonaceous chondrite clasts (Zolensky et al. 1996) and the latter,
microclasts (Gounelle et al. 2003). The same color parameters for PRA04401 (Fig. 10B)
plot in the middle of the DM trend, confirming this interpretation.
5.2. Abundance of CM2 in Dark Material
Estimating the abundance of CM2 on the surface of Vesta is challenging because
other types of surface processing and materials can mimic the color characteristics of DM.
Although the effects of these secondary components (space weathering, impact melt,
eucrites with opaques, shocked eucrites, iron abundance) on band depth and albedo are
! "*!
20. limited compared to carbonaceous chondrites, they nevertheless introduce uncertainties.
Abundance was initially estimated using two equations based on laboratory calibration of
CM2 and eucrite mixtures (Le Corre et al. 2011). We independently developed a third
equation based on R(0.75)/R(0.95). All three equations are based on intimate mixing
model and their R2 (regression) values are ~0.99.
CM2 vol% = 31531*#4 - 34378*#3 + 13655*#2 - 2469.3*# + 192.48 (Eq. 1)
# = 0.75 µm reflectance
CM2 vol% = -180.88*$3 + 924.72*$2 - 1604.5*$ + 946.35 (Eq. 2)
$ = R(0.75)/R(0.92)
CM2 vol% = -178.32*%3 + 905.99*%2 - 1563.5*% + 918.23 (Eq. 3)
% = R(0.75)/R(0.95)
Using these equations, we estimated the CM2 abundance range for several sites
on Vesta with DM. The method we employed included the extraction of color spectra
over the region of interest showing DM. Average, minimum (darkest) and maximum
(brightest) spectra were calculated for each site and these values were used to obtain #, $,
and % parameters. CM2 abundance values were calculated for these nine parameters, and
two average CM2 abundances were calculated. The first used all three parameters (#, $,
and %) and the second used just $ and %.
Determination of the exact abundance of carbonaceous material on the surface
depends on the mixture model used (areal vs. intimate). We estimate abundance of
carbonaceous chondrite material for the average surface of Vesta to be between <6 vol%,
consistent with HED meteorites (Zolensky et al. 1996) and GRaND observations
(Prettyman et al. 2012). Concentrations as high as ~50 vol% are noted in DM-rich areas
which also agrees with 10-60 vol.% observed in howardites PRA 04401, PRA 04402 and
SCO 06040 (Herrin et al. 2011). These localized concentrations are small and cover only
! #+!
21. an area of a few 10s of meters. These localized concentrations could be a result of non-
uniform deposition/mixing/excavation of the impactor material and ejecta.
Phyllosilicates in CM2 carbonaceous chondrites contain adsorbed water as well as
structural OH (Beck et al. 2010). Laboratory analysis of PRA04401 indicates that it has
approximately 6 wt.% water, which produces an absorption feature at ~3-µm. If the DM
found on Vesta is equivalent to PRA04401, then we can expect this spectral feature.
Detection of such a feature by Dawn’s VIR spectrometer associated with some DM
strengthens this link (McCord et al. 2012). The independent mapping of hydrogen
distribution provides further evidence by Dawn’s GRaND instrument that show values
consistent with hydrogen abundances in howardites containing CM2 carbonaceous clasts
(Prettyman et al. 2012). The GRaND data shows clear correlation between H abundance
and DM (Prettyman et al. 2012).
6. Delivery of Dark Material
The global distribution of the DM offers some insights into its origin, though
there are a number of ways that material could have reached Vesta. Below we present
several possible delivery mechanisms, and discuss some possible pros and cons to each
one.
6.1. Delivery of DM by a large carbonaceous chondrite impactor
Evidence from HED meteorites suggests that at least two impacts (one each for
CM2 and CR2 clasts) are necessary for the delivery of carbonaceous material. By
mapping the location of morphological features associated with DM, we found a
preferential concentration near the rim and on the floor of the ancient 450 km diameter
Veneneia (52°S, 170°E) impact basin (Figs. 11-12) and a large low-albedo unit
! #"!
22. immediately to the north. In the sequence of events affecting the vestan south pole, we
hypothesize that a low velocity impactor related to the Veneneia impact basin formation
delivered some DM to Vesta. Presumably, this impactor would have been the one that
delivered the CM2-type material, as it constitutes ~83% of all macro clasts in howardites1.
The subsequent Rheasilvia impact (75°S, 301°E) erased about half of the Veneneia
basin’s structure and covered the other half with bright ejecta mantling (Fig. 12). In this
scenario, more recent impacts excavated the sandwiched layers of bright Rheasilvia
ejecta material, DM from Veneneia impactor, and some in situ regolith. These layered
deposits are visible on crater walls (Fig. 1A and 2A), and some smaller impact craters
also expose a possible sub-surface regional dark deposit layer (Fig. 3C). They are less
consistent with DM excavated from craters on the floor of the Veneneia crater.
6.1.1 Impact Velocity
The impact velocity necessary for significant amounts of dark material to be
emplaced on the surface of Vesta varies depending on the model parameters. Numerical
studies indicate that Vesta’s impact velocity distribution has a low-velocity tail where
~6% of impacts can occur at less than 2 km/s (Figs. 13A-B, Table 2). As discussed
further in the following section, a low velocity impact would be advantageous for several
reasons: Less impactor material would be lost to space in a low-velocity impact than in a
high-velocity impact; and since lower velocity impacts form smaller craters, the
remaining projectile material would be diluted in a smaller volume of ejecta and hence
the ejecta would be richer in impactor material. It is also possible that during such an
impact, significant quantities (cm size) of hydrated, unshocked CM2 material are spalled
off the projectile (Rubin and Bottke, 2009) and achieved low relative velocity to land on
! ##!
23. Vesta (Bland et al. 2008). Dynamical simulations (Jutzi & Asphaug, 2011) show that the
fraction of impactor material that can survive is strongly dependent on impact angle with
~90% for a head-on impact and ~50% for 45° impact angle. The fact that most CM2
clasts in howardites are still hydrated and unshocked suggests that they were deposited on
Vesta at relatively low velocities (Zolensky et al. 1996; Buchanan et al. 1993; Gounelle et
al. 2003). The VIR and GRaND data also support extensive hydration of DM relative to
Vesta (Prettyman et al. 2012; McCord et al. 2012). A few CM clasts are shock heated and
dehydrated during impact and probably accreted at higher impact velocities (Rubin and
Bottke 2009) possibly producing pitted terrains observed in Marcia and Cornelia (Denevi
et al. 2012).
On the other hand, this scenario makes use of several low probability events. The
velocity threshold where a projectile leaves behind a substantial fraction of projectile
material is unknown. Our calculations indicate that V < 2 km/s impacts should only
occur once out of every 17 events, yet the Veneneia event also managed to make one of
the largest craters observed on Vesta. If the V < 2 km/s velocity threshold were even
lower, the odds would be significantly worse.
Lower velocities also mean that a larger projectile is needed to make a same-sized
crater, at least when compared to mean values. Using crater-scaling relationships from
Melosh (1989), we find that the ~400 km diameter Veneneia can be made by a roughly
30 km diameter projectile at 4.75 km/s, the mean impact velocity of main belt asteroids
hitting Vesta, or a 50 km diameter projectile hitting at 2 km/s. Using the main belt size
frequency distribution described in Bottke et al. (2005), we find that 30 km diameter
impactors would hit Vesta about twice as often as 50 km diameter bodies.
! #$!
24. Assuming the main belt size distribution has not substantially changed over solar
system history, the mean interval between !30 km diameter impactors striking Vesta is
~4 Gy, whereas that for a 50 km projectile is ~8 Gy. These values are consistent with
Veneneia-sized craters being rare on Vesta. Additional consideration must also be given
to the fact that only some of the D > 30 km bodies in the main belt are carbonaceous
chondrite-like.
Taken together, these calculations imply that either the Veneneia impact was an
exceptional event, the velocity threshold needed to leave behind substantial quantities of
unshocked projectile material is actually higher than 2 km/s, or that large impacts are not
the source of the projectile material. From this point on, we will assume that one or both
of the first two options are valid.
6.1.2 Estimating Impactor Volume Fraction in Crater Ejecta
Crater scaling laws can be used to estimate the fraction of impactor material that
may survive in the ejecta from a crater. For crater volume as a function of projectile mass
and impact velocity, Holsapple (1993, Table I) gives expressions for crater volume (m3)
in a range of materials, including hard rock
! ! !!!!!"!!!!!!"# !
!
!!!"
! ! !!!!"!!!!!"# ! !!!!" !!"#
!
!!!
soft rock
! ! !!!!!"!!!!!!"# !
!
!!!"
! ! !!!!"!!!!!!"# !! !!!!" !!!"# !
!
!!!
dry soil
! ! !!!!"!!!!!!!"# !
!
!!!"
! ! !!!!"!!!!!!" !! !!!!" !!!"# !
!
!!!"
! #%!
25. and sand
! ! ! !!!!!"!!!!!!" !! !!!!" !!!"# !
!
!!!"
where Vs and Vg are the crater volumes in the strength and gravity regimes, m is the
projectile mass in kilograms, vimp is the impact velocity in km/s, and G is the surface
gravity in units of Earth g (for Vesta, G = (0.22 m/s2)/(9.81 m/s2) = 0.22). Vertical
impacts are assumed in these expressions, and the more likely case of non-vertical
impacts (the average impact angle is 45 degrees) will result in somewhat smaller crater
volumes. The final crater volume depends on whether strength or gravity limits the final
size of the crater, and thus will be the smaller of Vs and Vg.
As a first-order assumption of the ejecta volume, we assume that it is equal to the
volume of the crater. This is likely an overestimate, as some of the crater volume is due
to displacement rather than excavation, and some of the ejecta is likely lost to space
rather than retained on the surface, but this upper-limit estimate will yield a lower-limit
value of the volume fraction of projectile material that would be mixed with the crater
ejecta in the following calculations.
For an impactor diameter D, the projectile mass and volume are
!
!!"# ! !!! ! !
!
!!! ! ! !!"# !!"# !
where we assume !imp= 2000 kg/m3 for carbonaceous impactors.
For the mass fraction of the projectile that escapes in sub-10 km/s impacts,
Svetsov (2011, Eq. 8) finds
! #&!
26. !!"#
! !!! ! ! !!!"! ! !!!!!"!!!!"# !!"!!!"# ! !!!!!!!!"# !
!!!!"
!
!
where vesc is 0.35 km/s for Vesta. This yields a retained mass fraction of projectile
material that ranges from ~0.75 for 1 km/s impacts to ~0.5 for 5 km/s impacts. As a
conservative estimate, we use a retained mass fraction of 0.5 for all calculations.
If we assume that the retained projectile material is uniformly mixed in with the
retained ejecta material, then for a given cratering event we can estimate the volume
fraction of projectile material (CC for carbonaceous chondrite) in the ejecta blanket as
!!!!!!"#
!!! !
!"# ! ! !
! !
Figure 14 shows the carbonaceous chondrite volume fraction in the ejecta for a
range of target materials, impactor diameters and velocities. Although there is some
uncertainty about the best scaling law to use for Vesta, it will certainly lie within the
range of materials used here. These are likely conservative estimates for several reasons.
As noted above, we likely overestimate ejecta volume and use a low-end value of 0.5 for
the fraction of impactor material retained on the surface, both of which will serve to give
a lower value of fCC. In addition, the crater volume calculations assume vertical impacts,
and the more likely oblique impacts will eject less material and thus lead to a larger
fraction of impactor material in the ejecta.
6.1.3. Dynamical Modeling of Veneneia Ejecta
To begin to test the hypothesis that DM on Vesta is distributed where ejecta from
the Veneneia basin-forming event would be deposited on the asteroid's surface, we
performed a simple dynamical simulation using the model described in Durda (2004),
! #'!
27. modified for Vesta conditions. Vesta was modeled as a triaxial ellipsoid with major axes
of 578&560&458 km and a rotation rate of 5.342 hours. Ejecta from Veneneia was
modeled as 10000 point masses launched from within the footprint of a 450-km-diameter
crater at latitude -52 degrees and a model longitude with the appropriate relative position
of the crater with respect to the asteroid's major axis. Ejecta speeds were distributed
between 100-400 m/s, with the number of ejecta particles traveling at greater than a given
speed determined by a power law. As the aim here was merely to examine in a qualitative
sense the overall distribution of Veneneia ejecta across Vesta's surface under the
combined effects of the asteroid's shape and rotation, we did not attempt to incorporate
more detailed crater ejecta scaling laws at this stage.
Figure 15 shows the results of the dynamical model of the distribution of DM.
Modeled ejecta are distributed generally south of the equator and to the west of the crater.
The effects of rotation lead to a sharply delineated ejecta blanket boundary to the north of
the crater and in the direction of the asteroid’s spin, caused by the asteroid rotating ‘out
from under’ the reaccreting ejecta. This boundary qualitatively resembles a sharp-edged
boundary in the distribution of DM on Vesta located near latitude 0°, longitude 120°W.
The modeled distribution does not produce reaccreted ejecta within the floor of the crater,
but the dynamical model used here is an idealized representation of the crater ejecta
process and was not intended to model the details of impact processesapplicable to basin-
scale impacts. Still, the qualitative match between our dynamically modeled Veneneia
ejecta distribution and the observed distribution of DM on Vesta suggests that the
hypothesis of DM as Veneneia ejecta warrants further examination.
6.1.4 Summary: Model vs. Observations
! #(!
28. CM2 material abundance estimated for several DM sites on Vesta show a range whose
limits are consistent with our modeling results (Fig. 14). For the large low albedo unit,
the average lower limit of CM2 abundance ranges from 8-11 vol. % consistent with a fCC
range produced by an impactor arriving at 2 km/sec (Fig. 14). Localized concentrations
as high as 53 vol. % are noted. Moderate impact craters like the 15-km diameter Cornelia
and 30-km diameter Numisia have CM2 abundance range of 9-32 vol. % and 11-35 vol.
%, respectively. Smaller impact craters like the one shown in Fig. 3C have localized
CM2 abundances between ~12-33 vol. %. The lower end of these ranges for impactor
material abundance are consistent with fCC estimated using scaling laws for velocities
between 1-2 km/sec (Fig. 14).
Our estimates of CM2 abundance and distribution (Fig. 14) are on the
conservative side, and the actual fractions are likely to be higher than our estimates. As
noted in section 6.1.2, the ejecta volume is not equal to the crater volume as the material
is only ejected from about ~1/3-1/2 of the depth of the transient crater. The rest of the
crater volume is due to displacement of material (i.e. material getting pushed down by the
impactor). This alone could increase the fCC in the ejecta by a factor of two or more
compared to our estimates. A second factor is the distribution of the impactor material in
the ejecta, which is likely to be non-uniform. This would imply that some parts of the
surface within range of the Veneneia ejecta may contain little DM, whereas others may
include larger concentrations. Both these factors, as well as others discussed in Sec.
6.1.2, suggest that very low velocity (<2 km/s) impacts may not be required, and that
! #)!
29. somewhat higher velocity (and hence higher probability) impacts could potentially give a
large enough fCC to match the observations.
6.2 Delivery of dark material by micrometeorites
Another possible source of some DM to Vesta is micrometeorites. This DM layer
would be more akin to a background global layer due to mixing with in-situ howardite
regolith. Consider that most of the micrometeorites reaching Earth today are CI, CM, or
CR-like bodies, with much of their mass being in the form of 100-200 micron diameter
particles (Love and Brownlee 1995, Taylor et al. 2000, Love and Allton 2006).
Numerical modeling work of the Zodiacal cloud indicates the vast majority of this
material is produced by the disruption of Jupiter-family comets, or JFCs, whose ice-free
component is likely to be similar to primitive carbonaceous chondrites (Nesvorny et al.
2009). Those tiny bodies that escape ejection via a close encounter with Jupiter evolve
inward toward the Sun by Poynting-Robertson drag. As they cross the asteroid belt, many
should have an opportunity to hit Vesta and other main belt asteroids.
This process has probably been going on in a similar fashion since the formation
of the trans-Neptunian scattered disk about 4 Gy ago. We know that about 2% of lunar
maria soils are made up of extralunar components that are most consistent with CI/CM-
like chondrites (see Wasson et al. 1975). The current flux of micrometeorites hitting the
Moon, 1600 tons per year, is sufficient to produce this fraction, provided it has been
hitting the Moon at the same rate, more or less, over the last few billion years or so.
Accordingly, we predict that Vesta should have also seen a steady flux of primitive
micrometeorites over the same time interval.
! #*!
30. Using the Nesvorny et al. (2009) simulation results, we find that for every
micrometeorite that hits Vesta, about 260 hit the Moon. Scaling from the lunar flux, these
rates imply that primitive micrometeorites add an amount of DM to Vesta of 2&10-12
meters per year. Thus, even over a timescale of a few Gy, the depth of DM added to
Vesta is less than a cm, not enough to explain the observed dark layer deposits. In
addition, this material is added incrementally, giving impact processes an opportunity to
mix it into Vesta’s regolith. This mixing would further dilute any layer produced, though
it could generally lower Vesta’s albedo.
A potentially larger source of micrometeorites would be those comets that
disrupted early in solar system history. Here we refer to the planetary evolution scenario
known as the Nice Model which suggests that the migration of the giant planets about 4
Gy ago scattered and decimated a primordial disk of comets that was residing just outside
the original orbits of Uranus and Neptune (Tsiganis et al. 2005; Gomes et al. 2005). This
disk may have contained as much as 35 Earth masses of material prior to giant planet
migration, much more than the 0.1 Earth masses of material found in the present-day
trans-Neptunian scattered disk. Moreover, the interval when these comets were crossing
the inner solar system was limited to a mere few tens of My, short enough that
micrometeorites coming from this source might build a substantial dark layer of material
on Vesta.
Assuming that scattered primordial comets disrupt in the same manner as JFCs do
today, Nesvorny et al. (2010) predict that the mass of micrometeorite material added to
the Moon during the Late Heavy Bombardment (LHB) in the form of CI/CM material
was 2&1020 g. This implies the amount of DM added to Vesta was 2&1020 g / 260, or
! $+!
31. 7.6&1017 g. If we assume micrometeorites have a bulk density of 1 g/cm3, and that this
material was added uniformly over Vesta’s surface, we get a layer that is ~1 m thick.
Impacts would gradually mix in and dilute this layer, at least until everything was buried
by ejecta from some sufficiently large impact event.
A ~1 meter thick layer of dark material could potentially lead to observable
effects on Vesta. The problem, however, is that it is unclear whether it is the right order
of magnitude to match observations once regolith mixing and dilution had been carried
out. It is also unclear whether observations are consistent with a global layer of DM
hidden from sight by brighter layers. Thus, as before, we believe these values are
interesting enough to warrant further work, but we lack definitive answers at this point. A
related argument against a micrometeorite source is that a micrometeorite layer deposited
over the course of solar system history would be uniformly distributed while the DM
distribution on Vesta is non-uniform with localized concentrations. Meteoritical evidence
also points to two distinct types of carbonaceous chondrite inclusions (macro clasts and
micrometeorites) with two different mechanisms for their origin (Zolensky et al. 1996;
Gounelle et al. 2003).
We conclude this section by pointing out that asteroid disruption events during the
earliest times of main belt evolution could provide an additional source of primitive
micrometeorites to Vesta (e.g., Bottke et al. 2005; Levison et al. 2009; Rubin and Bottke
2009) apart from impact of dark asteroids. The strength of the source has yet to be
assessed in a quantitative fashion. The hope is that Vesta constraints can be used to rule
this source in or out.
! $"!
32. 7. Conclusion
Dawn FC color images show that DM is ubiquitous on Vesta and preferentially
enriched in and around the rim of the ancient Veneneia basin. The composition and
distribution of DM on Vesta is consistent with Veneneia formation by a low-V
carbonaceous impactor. In respect to geomorphology, the distribution of DM seems to be
mainly linked to impact craters with DM found around the crater rims in ejecta blankets,
forming ejecta rays, on crater rims, in debris flows on crater walls and sometimes
deposited on crater floors, and likely in small secondary impact craters or crater chains.
In addition, Aricia Tholus is an impact crater with dark ejecta rays that looks peculiar
because it is placed on a hill, but the nature of Lucaria Tholus with its dark flow features
remains to be confirmed.
Comparison of color parameters of various meteoritical analogs suggests that
carbonaceous clast-rich howardites (Buchanan et al. 1993; Zolensky et al. 1996) such as
PRA04401 (Herrin et al. 2011) are the best analogs for the DM. This result also confirms
ground-based telescopic observations of Vesta that suggested contamination from
impacting carbonaceous chondrites (Hasegawa et al. 2003; Rivkin et al. 2006). Our
analysis of endogenous sources of DM, such as pyroclastic deposits (Wilson and Keil,
1996), impact shock/melt (Burbine et al. 2001), and eucrites rich in opaque minerals
(Mayne et al. 2010) indicates that these are unlikely sources of DM. Petrologic models
(Wilson and Keil, 1996) and meteorite studies (Mayne et al. 2010) also support our
results. The abundance of CM2 carbonaceous chondrite material on the surface of Vesta
estimated by Dawn FC data (<6 vol.%) is consistent with HED meteorites (Zolensky et al.
1996; Herrin et al. 2011). This value is also consistent with estimates of CM-like material
! $#!
33. (<10%) proposed by Rivkin et al. (2006) to explain the 3-µm absorption feature. The
connection between Vesta, Vestoids and HED meteorites is primarily based on near-IR
spectral observations and dynamical modeling of the delivery mechanism (Binzel and Xu,
1993). The association between dark material on Vesta and carbonaceous chondrite clasts
in howardites provides the first direct and independent evidence that these meteorites
were actually delivered from Vesta.
Acknowledgement
We thank the Dawn team for the development, cruise, orbital insertion, and operations of
the Dawn spacecraft at Vesta. The Framing Camera project is financially supported by
the Max Planck Society and the German Space Agency, DLR. We also thank the Dawn at
Vesta Participating Scientist Program (NNX10AR22G) for funding the research. Dawn
data is archived with the NASA Planetary Data System. EAC thanks CFI, MRIF, CSA,
and the University of Winnipeg for support of this project and the necessary
infrastructure. We thank Paul Buchanan (Kilgore College) and Tom Burbine (Mt
Holyoke College) for their helpful reviews to improve the manuscript. VR would like to
thank Richard Binzel (MIT), Rhiannon Mayne (TCU), and Guneshwar Thangjam (MPS)
for their helpful suggestions to improve the manuscript.
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Figure Captions
Figure 1. (A) Mosaic of 0.750 "m filter HAMO images (60 m/pixel) of the 58-km
diameter Marcia crater showing downslope movement of bright material covering the
underlying DM in periodic mass-wasting events probably trigged by seismic shaking. In
the close-up view showing a LAMO clear filter image at 20 m/pixel (B), discontinuous
clumps of material along the crater wall seem to channel the bright material (dashed
white arrows) around the DM deposits, creating ‘tadpole’ like DM features (white
arrows). North is up in all images. All coordinates are based on the new Claudia
! $)!
39. coordinate system, which is different from the older Olbers system used with the HST
data. Olbers reference longitude is located approximately at 210°E in the Claudia system.
Figure 2. (A) Mosaic of 0.75-"m filter HAMO images (60 m/pixel) of the Numisia
impact crater (7°S, 247°E) showing downslope movement of DM. The northern dark
debris flow (B) is likely similar to the ejecta blanket seen to the north of the crater rim.
Like for Marcia crater, discontinuous clumps of bright material along the crater wall are
channeling the DM around them. (B) is a LAMO clear filter image at 20 m/pixel. (C)
Close-up view (0.75-"m filter LAMO image) of the northwest part of the rim and crater
wall of Numisia showing bright outcrops, dark deposits (white arrows) and bright to
medium albedo debris flows (dashed white arrows). North is up in all images.
Figure 3. Impact craters Occia (A) and Vibidia (B) showing mixture of DM with bright
material. (C) Small impact crater (14.9°N, 179.9°E) on the ejecta blanket of the impact
crater Marcia showing DM ejecta radially distributed. (D) Dark spots associated with two
shallow impact craters (the one on the right is located at 27.8°S, 152.6°E). Some of these
dark spots are likely secondary impact craters. Image (A) was obtained during the LAMO
phase at a resolution of 20 m/pixel and images (B-D) during the HAMO phase at a
resolution of 60 m/pixel. (A-D) are photometrically corrected 0.75 "m-filter images.
North is up in all images.
! $*!
40. Figure 4. (A) Radial ejecta of DM from a crater on Aricia Tholus (12.1°N, 161.8°E), a
37-km diameter hill shown overlaid on topographic map in Fig. 4 (B) and (C). These
clear filter images were obtained during the HAMO phase at a resolution of 60 m/pixel.
Figure 5. (A) Lucaria Tholus, a 22-km diameter ridge (13°S, 104°E) showing DM
deposits along the ridgeline. Figures 5 (B-D) show the image wrapped on a topographic
map to give vertical perspective of the feature. These clear filter images were obtained
during the HAMO phase at a resolution of 60 m/pixel.
Figure 6. (A) Image at 0.75 "m of two impact craters showing DM from RC3 phase at a
resolution of 487 m/pixel. The crater near the top of the image is named Rubria and the
one near the bottom is Occia. (B) Profile of the albedo from the red line drawn in (A) on
the Occia crater. A distinct drop in albedo is seen across the DM ejecta.
Figure 7. (a) 0.75-µm filter image of 15-km diameter impact crater Cornelia (9.3°S,
225.5°E) showing the DM mixed with bright materials in the crater walls. (b) Band depth
color-coded image (R(0.75 µm)/R(0.92 µm)) of the same crater showing weaker band
depth for DM (blue) and deeper band depth for fresh bright material (red). Albedo and
band depth are the only two color parameters affected by the presence of DM on the
surface. (C) Eucrite-Diogenite ratio (0.98/0.92 µm) image showing areas rich in diogenite
(red) and those more eucrite-like (deep blue). Howardite-rich areas appear more yellow-
cyan. The ED ratio is calculated based on R(0.98)/(0.92), where R is the reflectance at
wavelength (.
! %+!
41. Figure 8. Band depth vs. albedo plot with the vestan DM and three opaque-rich eucrites
and quenched eucrite identified by Mayne et al. (2010). The band depth and albedo of all
four eucrites are higher than those of the DM found in our Cornelia crater test site. Error
bars plotted for the DM are one standard deviation.
Figure 9. Band depth vs. albedo plot with the vestan DM and shocked eucrite JaH 626 in
three grain size ranges, shocked eucrite Padvarninkai in two grain size ranges, eucrite
Macibini impact melt clast and impact melt matrix from polymict eucrite LEW85303.
The band depth and albedo of all samples differ from those of the DM found in our
Cornelia crater test site.
Figure 10. (a) Comparison of color spectra of DM (black) and laboratory intimate
mixtures of <45 "m size CM2 carbonaceous chondrite Murchison and eucrite Millbillillie
normalized to unity at 0.75-µm. Average DM spectrum plots between 10-20 wt.% CM2
abundance spectra. (b) Albedo vs. band depth plot showing DM from Cornelia crater
(open circles), areal (solid gray line) and intimate (dotted gray line) mixtures of
CM2+eucrite, and carbonaceous chondrite-rich howardite PRA04401 (yellow circle). The
dark material trend falls in between areal and intimate mixtures of CM2+eucrite
suggesting that both areal and intimate mixing has occurred on the surface of Vesta. The
match between PRA04401 and DM along with similar albedo vs. band depth trends with
CM2+eucrite mixtures suggests that at least some DM on Vesta is due to infall of
carbonaceous chondrite meteorites.
! %"!
42. Figure 11. Distribution of geologic features associated with DM plotted on a 0.75 µm
filter mosaic of Vesta obtained at a resolution of ~480 and ~60 m/pixel. Dark material is
divided in three categories with spots, craters and topographic highs. Craters marked with
the circle symbol exhibit DM either on the crater wall in the form of linear debris flows,
avalanches or V-shaped deposits, on crater rim or in the ejecta blanket (ejecta rays and/or
units draping the surrounding terrain). Some craters have DM both inside the crater and
outside in the ejecta. Many of the dark spots (white diamond) are likely secondary impact
craters, even though not all can be traced back to the primary impact, and that are so
small that the detection of their crater rim is uncertain. The last type is represented here
by the only topographic high visible in the southern hemisphere, Lucaria Tholus (star).
Along with those features, the approximate outline of the rim of the Rheasilvia basin
(black line) and the incomplete rim of the Veneneia basin (red line) are depicted on the
map. Two large dark albedo areas forming a regional mantling and corresponding to the
ejecta of Marcia (10°N, 190°E) and Octavia (3.3°S, 147°E) craters are not specifically
mapped but are included in the crater category (represented by a circle for each crater).
Figure 12. Global distribution of geologic features associated with DM displayed onto a
color-coded and hill shaded topographic map of Vesta in cylindrical projection. Elevation
is computed relative to a 285 x 229 km reference ellipsoid with blue corresponding to the
lowest elevation and red being the highest. Dark material is divided in three categories
with spots (white diamond), craters (circle) and topographic highs (star). Along with
those features, the approximate outline of the rim of the Rheasilvia basin (black line) and
! %#!
43. the incomplete rim of the Veneneia basin (red line) are depicted on the map. Data was
trimmed in illumination angles (diagonal lines area) so DM was not mapped at the
extreme South Pole.
Figure 13. (a) Collisional velocity distributions for Vesta and the entire main belt,
calculated using the Bottke et al. (1994) algorithm and the orbital distribution of all
asteroids larger than 30 km diameter. The velocity distribution of asteroidal impacts on
Vesta has a mean impact velocity of 4.75 km/s (RMS 5.18 km/s, median 4.41 km/s),
which is comparable to but somewhat lower than the calculated mail-belt average value
of 5.14 km/s (O’Brien and Sykes 2011). While the distribution has a mean value, impacts
can occur at significantly higher and lower values. For reference, Vesta’s escape velocity
is approximately 350 m/s. (b) Same as (a) but integrated to show the probability of
impacts occurring at less than a given velocity for Vesta and the entire main belt. Figures
are adopted from O’Brien and Sykes (2011).
Figure 14. Carbonaceous chondrite volume fraction in the crater ejecta for a range of
target materials, impactor diameters and velocities, calculated using the methods
described in Sec. 6.1.2. Note that the scaling laws for hard rock and soft rock overlap for
impactor diameters larger than a few km, as do the laws for dry soil and sand.
Figure 15. Results of a dynamical model of the ejecta distribution from the Veneneia
basin on Vesta using methods described in Sec. 6.1.3 and Durda et al. (2004).
! %$!
44. Table 1. Color parameters of dark material along with possible analogs
Band
Analogs/Effects Albedo Examples
Depth
Dark Material Low Weak Vesta
Pyroclastic deposits
Volcanic (pyroclasts) Low-Moderate Weak
(Moon)
Shock/Impact Melt Moderate Moderate Moon/Vesta
Chervony Kut
Eucrites with Opaques Moderate Moderate
meteorite
Exogenous Material
CM2, CR clasts in
(carbonaceous Low Weak
HEDs
chondrites)
Table 2. Percentages of total impacts occurring below a given velocity (km/sec). For
reference, Vesta's escape velocity is approximately 350 m/sec.
Velocity Fraction of impacts less
(km/sec) than or equal to V (%)
1 0.42
2 5.8
3 19.6
4 40.8
5 60.2
! %%!
45. Figure 1: Dawn at Vesta, Dark Material
Figure 2: Dawn at Vesta, Dark Material
! %&!