378 research outputs found
Formation of Early Water Oceans on Rocky Planets
Terrestrial planets, with silicate mantles and metallic cores, are likely to
obtain water and carbon compounds during accretion. Here I examine the
conditions that allow early formation of a surface water ocean (simultaneous
with cooling to clement surface conditions), and the timeline of degassing the
planetary interior into the atmosphere. The greatest fraction of a planet's
initial volatile budget is degassed into the atmosphere during the end of magma
ocean solidification, leaving only a small fraction of the original volatiles
to be released into the atmosphere through later volcanism. Rocky planets that
accrete with water in their bulk mantle have two mechanisms for producing an
early water ocean: First, if they accrete with at least 1 to 3 mass% of water
in their bulk composition, liquid water may be extruded onto the planetary
surface at the end of magma ocean solidification. Second, at initial water
contents as low as 0.01 mass% or lower, during solidification a massive
supercritical fluid and steam atmosphere is produced that collapses into a
water ocean upon cooling. The low water contents required for this process
indicate that rocky super-Earth exoplanets may be expected to commonly produce
water oceans within tens to hundreds of millions of years of their last major
accretionary impact, through collapse of their atmosphere.Comment: 8 text pages with 5 figures following; Accepted at Astrophysics and
Space Scienc
Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets
Terrestrial-like exoplanets may obtain atmospheres from three primary
sources: Capture of nebular gases, degassing during accretion, and degassing
from subsequent tectonic activity. Here we model degassing during accretion to
estimate the range of atmospheric mass and composition on exoplanets ranging
from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and
differentiated meteorite compositions. Degassing alone can create a wide range
of masses of planetary atmospheres, ranging from less than a percent of the
planet's total mass up to ~6 mass% of hydrogen, ~20 mass% of water, and/or ~5
mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a
result of oxidizing metallic iron with water, and excess water and carbon can
produce atmospheres through simple degassing. As a byproduct of our atmospheric
outgassing models we find that modest initial water contents (10 mass% of the
planet and above) create planets with deep surface liquid water oceans soon
after accretion is complete.Comment: ApJ, in press. 32 pages, 6 figure
Coreless Terrestrial Exoplanets
Differentiation in terrestrial planets is expected to include the formation
of a metallic iron core. We predict the existence of terrestrial planets that
have differentiated but have no metallic core--planets that are effectively a
giant silicate mantle. We discuss two paths to forming a coreless terrestrial
planet, whereby the oxidation state during planetary accretion and
solidification will determine the size or existence of any metallic core. Under
this hypothesis, any metallic iron in the bulk accreting material is oxidized
by water, binding the iron in the form of iron oxide into the silicate minerals
of the planetary mantle. The existence of such silicate planets has
consequences for interpreting the compositions and interior density structures
of exoplanets based on their mass and radius measurements.Comment: ApJ, in press. 22 pages, 5 figure
Chondrites as samples of differentiated planetesimals
Chondritic meteorites are unmelted and variably metamorphosed aggregates of the earliest solids of the solar system. The variety of metamorphic textures in chondrites motivated the âonion shellâ model in which chondrites originated at varying depths within a parent body heated primarily by the short-lived radioisotope 26Al, with the highest metamorphic grade originating nearest the center. Allende and a few other chondrites possess a unidirectional magnetization that can be best explained by a core dynamo on their parent body, indicating internal melting and differentiation. Here we show that a parent body that accreted to >~200 km in radius by ~ 1.5 Ma after the formation of calciumâaluminum-rich inclusions (CAIs) would have a differentiated interior, and ongoing accretion would add a solid undifferentiated crust overlying a differentiated interior, consistent with formational and evolutionary constraints inferred for the CV parent body. This body could have produced a magnetic field lasting more than 10 Ma. This hypothesis represents a new model for the origin of some chondrites, presenting them as the unprocessed crusts of internally differentiated early planetesimals. Such bodies may exist in the asteroid belt today; the shapes and masses of the two largest asteroids, 1 Ceres and 2 Pallas, can be consistent with differentiated interiors, conceivably with small iron cores with hydrated silicate or iceâsilicate mantles, covered with undifferentiated crusts.National Science Foundation (U.S.) (NSF Astronomy CAREER grant)Mitsui & Co. (U.S.A.), Inc. ( Mitsui Career Development Professorship)United States. National Aeronautics and Space Administration (NASA Origins grant)Massachusetts Institute of Technology (Victor P. Starr Career Development Professorship)United States. National Aeronautics and Space Administration (NASA/Dawn co-investigator grant
A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars
The deuterium-to-hydrogen (D/H or 2H/1H) ratio of Martian atmospheric water
(~6x standard mean ocean water, SMOW) is higher than that of known sources,
requiring planetary enrichment. A recent measurement by NASA's Mars Science
Laboratory rover Curiosity of >3 Gyr clays yields a D/H ratio ~3x SMOW,
demonstrating that most enrichment occurs early in Mars's history. As on Venus,
Mars's D/H enrichment is thought to reflect preferential loss to space of 1H
(protium) relative to 2H (deuterium), but the global environmental context of
large and early hydrogen losses remain to be determined. Here, we apply a
recent model of primordial atmosphere evolution to Mars, link the magma ocean
of the accretion epoch with a subsequent water-ocean epoch, and calculate the
behavior of deuterium for comparison with the observed record. We find that a
~2-3x hydrospheric deuterium-enrichment is produced if the Martian magma ocean
is chemically reducing at last equilibration with the primordial atmosphere,
making H2-CO the initially dominant species, with minor abundances of H2O-CO2.
Reducing gases - in particular H2 - can cause greenhouse warming and prevent a
water ocean from freezing immediately after the magma ocean epoch. Moreover,
the pressure-temperature conditions are high enough to produce ocean-atmosphere
H2O-H2 isotopic equilibrium such that surface H2O strongly concentrates
deuterium relative to H2, which preferentially takes up protium and escapes
from the primordial atmosphere. The proposed scenario of primordial H2-rich
outgassing and escape suggests significant durations (>Myr) of chemical
conditions on the Martian surface conducive to prebiotic chemistry immediately
following Martian accretion.Comment: 5 figure
Martian Igneous Geochemistry: The Nature of the Martian Mantle
Mafic igneous rocks probe the interiors of their parent objects, reflecting the compositions and mineralogies of their source regions, and the magmatic processes that engendered them. Incompatible trace element contents of mafic igneous rocks are widely used to constrain the petrologic evolution of planets. We focus on incompatible element ratios of martian meteorites to constrain the petrologic evolution of Mars in the context of magma ocean/cumulate overturn models [1]. Most martian meteorites contain some cumulus grains, but regardless, their incompatible element ratios are close to those of their parent magmas. Martian meteorites form two main petrologic/ age groupings; a 1.3 Ga group composed of clinopyroxenites (nakhlites) and dunites (chassignites), and a <1 Ga group composed of basalts and lherzolites (shergottites)
A Self-Consistent Model of the Circumstellar Debris Created by a Giant Hypervelocity Impact in the HD172555 System
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of
HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the
system. Using physical arguments and constraints from observations, we rule out
the possibility of the infrared excess being created by a magma ocean planet or
a circumplanetary disk or torus. We show that the infrared excess is consistent
with a circumstellar debris disk or torus, located at approximately 6 AU, that
was created by a planetary scale hypervelocity impact. We find that radiation
pressure should remove submicron dust from the debris disk in less than one
year. However, the system's mid-infrared photometric flux, dominated by
submicron grains, has been stable within 4 percent over the last 27 years, from
IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of
the radiation pressure on fine dust in HD 172555 provide a self-consistent
explanation for this apparent contradiction. We also explore the unconfirmed
claim that 10^47 molecules of SiO vapor are needed to explain an emission
feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless
there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the
system, SiO vapor should be destroyed by photo-dissociation in less than 0.2
years. We argue that a second plausible explanation for the 8 um feature can be
emission from solid SiO, which naturally occurs in submicron silicate "smokes"
created by quickly condensing vaporized silicate.Comment: Accepted to the Astrophysical Journa
Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends
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