46 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
Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends
Magnesian andesite and dacite lavas from Mt. Shasta, northern California: products of fractional crystallization of H2O-rich mantle melts
The Psyche Gravity Investigation
The objective of the NASA Psyche mission gravity science investigation is to map the mass distribution within asteroid (16) Psyche to elucidate interior structure and to resolve the question of whether this metal-rich asteroid represents a remnant metal core or whether it is a primordial body that never melted. Measurements of gravity will be obtained via the X-band telecommunication system on the Psyche spacecraft, collected from progressively lower mapping altitudes. Orbital gravity will allow an estimate of GM to better than 0.001 km3âsâ2. A spherical harmonic model of gravity to degree and order 10 will be achievable and, in concert with spherical harmonic data sets from topography and magnetometry, as well as surface composition data, will provide information regarding the spatial and radial distribution of mass that will be used to constrain the origin and evolution of (16) Psyche
Exoplanet Science Priorities from the Perspective of Internal and Surface Processes for Silicate and Ice Dominated Worlds
The geophysics of extrasolar planets is a scientific topic often regarded as
standing largely beyond the reach of near-term observations. This reality in no
way diminishes the central role of geophysical phenomena in shaping planetary
outcomes, from formation, to thermal and chemical evolution, to numerous issues
of surface and near-surface habitability. We emphasize that for a balanced
understanding of extrasolar planets, it is important to look beyond the natural
biases of current observing tools, and actively seek unique pathways to
understand exoplanet interiors as best as possible during the long interim
prior to a time when internal components are more directly accessible. Such
pathways include but are not limited to: (a) enhanced theoretical and numerical
modeling, (b) laboratory research on critical material properties, (c)
measurement of geophysical properties by indirect inference from imprints left
on atmospheric and orbital properties, and (d) the purpose-driven use of Solar
System object exploration expressly for its value in comparative planetology
toward exoplanet-analogs. Breaking down barriers that envision local Solar
System exploration, including the study of Earth's own deep interior, as
separate from and in financial competition with extrasolar planet research, may
greatly improve the rate of needed scientific progress for exoplanet
geophysics. As the number of known rocky and icy exoplanets grows in the years
ahead, we expect demand for expertise in 'exogeoscience' will expand at a
commensurately intense pace. We highlight key topics, including: how water
oceans below ice shells may dominate the total habitability of our galaxy by
volume, how free-floating nomad planets may often attain habitable subsurface
oceans supported by radionuclide decay, and how deep interiors may critically
interact with atmospheric mass loss via dynamo-driven magnetic fields