57 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
Expanding the application of the Eu-oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior
Experimentally rehomogenized melt inclusions from the nakhlite Miller Range 03346 (MIL 03346) and the lherzolitic shergottite Allan Hills 77005 (ALH 77005) have been analyzed for their rare earth element (REE) concentrations in order to characterize the early melt compositions of these Martian meteorites and to calculate the oxygen fugacity conditions they crystallized under. D(Eu/Sm)pyroxene/melt values were measured at 0.77 and 1.05 for ALH 77005 and MIL 03346,
respectively. These melts and their associated whole rock compositions have similar REE patterns, suggesting that whole rock REE values are representative of those of the early melts and can be used as input into the pyroxene Eu-oxybarometer for the nakhlites and lherzolitic shergottites. Crystallization fO_2 values of IW + 1.1 (ALH 77005) and IW + 3.2 (MIL 03346) were calculated. Whole rock data from other nakhlites and lherzolitic shergottites was input into the Eu-oxybarometer to determine their crystallization fO_2 values. The lherzolitic shergottites and nakhlites have fO_2 values that range from IW + 0.4 to 1.6 and from IW + 1.1 to 3.2, respectively. These values are consistent
with some previously determined fO_2 estimates and expand the known range of fO_2 values of the Martian interior to four orders of magnitude. The origins of this range are not well constrained. Possible mechanisms for producing this spread in fO_2 values include mineral/melt fractionation, assimilation, shock effects, and magma ocean crystallization processes. Mineral/melt partitioning can result in changes in fO_2 from the start to the finish of crystallization of 2 orders of magnitude. In addition, crystallization of a Martian magma ocean with reasonable initial water content results in oxidized, water-rich, late-stage cumulates. Sampling of these oxidized cumulates or interactions between reduced melts and the oxidized material can potentially account for the range of fO_2 values observed in the Martian meteorites
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
Petrological and rheological controls on volcanism to terrestrial planets
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.Includes bibliographical references.Through experimental petrology and geodynamic modeling, processes of melting under thick lithospheres on the Earth and the moon are investigated. Phase equilibrium experiments were carried out on Apollo 14B and 15C picritic glasses (Chapters 5 and 6) and on a Sierran high-potassium lava (Chapter 1). These, along with petrologic modeling of Cascades high alumina olivine tholeiites (Chapter 4), yield information on depths and pressures of melt generation and constraints on source composition. Geodynamic modeling of lithospheric thinning processes, including delamination under the Siberian flood basalts (Chapter 2), gravitational instabilities in the lunar magma ocean cumulates (Chapter 7), and thinning and convection due to giant meteorite impacts (Chapters 3 and 8), has lead to new models for melt production. These studies together show how lithospheric thinning and unusual mantle compositions can lead to melting without calling on unusual mantle potential temperatures, and can explain the volumes and durations of continental flood basalts and lunar mare basalts.by Linda Tarbox Elkins Tanton.Ph.D
On the Emergent Spectra of Hot Protoplanet Collision Afterglows
We explore the appearance of terrestrial planets in formation by studying the
emergent spectra of hot molten protoplanets during their collisional formation.
While such collisions are rare, the surfaces of these bodies may remain hot at
temperatures of 1000-3000 K for up to millions of years during the epoch of
their formation. These object are luminous enough in the thermal infrared to be
observable with current and next generation optical/IR telescopes, provided
that the atmosphere of the forming planet permits astronomers to observe
brightness temperatures approaching that of the molten surface. Detectability
of a collisional afterglow depends on properties of the planet's atmosphere --
primarily on the mass of the atmosphere. A planet with a thin atmosphere is
more readily detected, because there is little atmosphere to obscure the hot
surface. Paradoxically, a more massive atmosphere prevents one from easily
seeing the hot surface, but also keeps the planet hot for a longer time. In
terms of planetary mass, more massive planets are also easier to detect than
smaller ones because of their larger emitting surface areas. We present
preliminary calculations assuming a range of protoplanet masses (1-10
M_\earth), surface pressures (1-1000 bar), and atmospheric compositions, for
molten planets with surface temperatures ranging from 1000 to 1800 K, in order
to explore the diversity of emergent spectra that are detectable. While current
8- to 10-m class ground-based telescopes may detect hot protoplanets at wide
orbital separations beyond 30 AU (if they exist), we will likely have to wait
for next-generation extremely large telescopes or improved diffraction
suppression techniques to find terrestrial planets in formation within several
AU of their host stars.Comment: 28 pages, 6 figures, ApJ manuscript format, accepted into the Ap
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 “onion shell” 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 calcium–aluminum-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 ice–silicate 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
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