252 research outputs found
Elemental ratios in stars vs planets
Context. The chemical composition of planets is an important constraint for
planet formation and subsequent differentiation. While theoretical studies try
to derive the compositions of planets from planet formation models in order to
link the composition and formation process of planets, other studies assume
that the elemental ratios in the formed planet and in the host star are the
same.
Aims. Using a chemical model combined with a planet formation model, we aim
to link the composition of stars with solar mass and luminosity with the
composition of the hosted planets. For this purpose, we study the three most
important elemental ratios that control the internal structure of a planet:
Fe/Si, Mg/Si, and C/O.
Methods. A set of 18 different observed stellar compositions was used to
cover a wide range of these elemental ratios. The Gibbs energy minimization
assumption was used to derive the composition of planets, taking stellar
abundances as proxies for nebular abundances, and to generate planets in a
self-consistent planet formation model. We computed the elemental ratios Fe/Si,
Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed
in different protoplanetary discs, and compared them to stellar abundances.
Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are
essentially identical to those in the star. Some deviations are shown for
planets that formed in specific regions of the disc, but the relationship
remains valid within the ranges encompassed in our study. The C/O ratio shows
only a very weak dependence on the stellar value.Comment: 8 pages, 5 figures, 3 tables. Accepted for publication in A&
Core Formation and Mantle Differentiation on Mars
Geochemical investigation of Martian meteorites (SNC meteorites) yields important constraints on the chemical and geodynamical evolution of Mars. These samples may not be representative of the whole of Mars; however, they provide constraints on the early differentiation processes on Mars. The bulk composition of Martian samples implies the presence of a metallic core that formed concurrently as the planet accreted. The strong depletion of highly siderophile elements in the Martian mantle is only possible if Mars had a large scale magma ocean early in its history allowing efficient separation of a metallic melt from molten silicate. The solidification of the magma ocean created chemical heterogeneities whose ancient origin is manifested in the heterogeneous 142Nd and 182W abundances observed in different meteorite groups derived from Mars. The isotope anomalies measured in SNC meteorites imply major chemical fractionation within the Martian mantle during the life time of the short-lived isotopes 146Sm and 182Hf. The Hf-W data are consistent with very rapid accretion of Mars within a few million years or, alternatively, a more protracted accretion history involving several large impacts and incomplete metal-silicate equilibration during core formation. In contrast to Earth early-formed chemical heterogeneities are still preserved on Mars, albeit slightly modified by mixing processes. The preservation of such ancient chemical differences is only possible if Mars did not undergo efficient whole mantle convection or vigorous plate tectonic style processes after the first few tens of millions of years of its histor
From stellar nebula to planets: the refractory components
We computed the abundance of refractory elements in planetary bodies formed
in stellar systems with solar chemical composition by combining models for
chemical composition and planet formation. We also consider the formation of
refractory organic compounds, which have been ignored in previous studies on
this topic. We used the commercial software package HSC Chemistry in order to
compute the condensation sequence and chemical composition of refractory
minerals incorporated into planets. The problem of refractory organic material
is approached with two distinct model calculations: the first considers that
the fraction of atoms used in the formation of organic compounds is removed
from the system (i.e. organic compounds are formed in the gas phase and are
nonreactive); and the second assumes that organic compounds are formed by the
reaction between different compounds that had previously condensed from the gas
phase. Results show that refractory material represents more than 50 wt % of
the mass of solids accreted by the simulated planets, with up to 30 wt % of the
total mass composed of refractory organic compounds. Carbide and silicate
abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02
for the Sun. Less than 1 wt % of carbides; pyroxene and olivine in similar
quantities are formed. The model predicts planets that are similar in
composition to those of the Solar system. It also shows that, starting from a
common initial nebula composition, a wide variety of chemically different
planets can form, which means that the differences in planetary compositions
are due to differences in the planetary formation process. We show that a model
in which refractory organic material is absent from the system is more
compatible with observations. The use of a planet formation model is essential
to form a wide diversity of planets in a consistent way.Comment: 18 pages, 29 figures. Accepted for publication in A&
Possible Chemical Composition And Interior Structure Models Of Venus Inferred From Numerical Modelling
Venus’ mass and radius are similar to those of Earth. However, dissimilarities in atmospheric properties, geophysical activity, and magnetic field generation could hint toward significant differences in the chemical composition and interior evolution of the two planets. Although various explanations for the differences between Venus and Earth have been proposed, the currently available data are insufficient to discriminate among the different solutions. Here we investigate the possible range of models for Venus’ structure. We assume that core segregation happened as a single-stage event. The mantle composition is inferred from the core composition using a prescription for metal-silicate partitioning. We consider three different cases for the composition of Venus defined via the bulk Si and Mg content, and the core’s S content. Permissible ranges for the core size, mantle, and core composition as well as the normalized moment of inertia (MoI) are presented for these compositions. A solid inner core could exist for all compositions. We estimate that Venus’ MoI is 0.317–0.351 and its core size 2930–4350 km for all assumed compositions. Higher MoI values correspond to more oxidizing conditions during core segregation. A determination of the abundance of FeO in Venus’ mantle by future missions could further constrain its composition and internal structure. This can reveal important information on Venus’ formation and evolution, and, possibly, the reasons for the differences between Venus and our home planet
Internal water storage capacity of terrestrial planets and the effect of hydration on the M-R relation
Understanding the chemical interactions between water and Mg-silicates or
iron is essential to constrain the interiors of water-rich planets. Hydration
effects have, however, been mostly neglected by the astrophysics community so
far. As such effects are unlikely to have major impacts on theoretical
mass-radius relations this is justified as long as the measurement
uncertainties are large. However, upcoming missions, such as the PLATO mission
(scheduled launch 2026), are envisaged to reach a precision of up to and for radii and masses, respectively. As a result, we may
soon enter an area in exoplanetary research where various physical and chemical
effects such as hydration can no longer be ignored. Our goal is to construct
interior models for planets that include reliable prescriptions for hydration
of the cores and the mantles. These models can be used to refine previous
results for which hydration has been neglected and to guide future
characterization of observed exoplanets. We have developed numerical tools to
solve for the structure of multi-layered planets with variable boundary
conditions and compositions. Here we consider three types of planets: dry
interiors, hydrated interiors and dry interiors + surface ocean where the ocean
mass fraction corresponds to the mass fraction of equivalent in the
hydrated case. We find H/OH storage capacities in the hydrated planets
equivalent to corresponding to up to deep ocean layers. In the mass range the
effect of hydration on the total radius is found to be whereas the
effect of differentiation into an isolated surface ocean is .
Furthermore, we find that our results are very sensitive to the bulk
composition.Comment: 34 page
Genesis of acid/basic rock associations: a case study The Kallithea intrusive complex, Samos, Greece
The isotope composition of selenium in chondrites constrains the depletion mechanism of volatile elements in solar system materials
Solar nebula processes led to a depletion of volatile elements in different chondrite groups when compared to the bulk chemical composition of the solar system deduced from the Sun's photosphere. For moderately-volatile elements, this depletion primarily correlates with the element condensation temperature and is possibly caused by incomplete condensation from a hot solar nebula, evaporative loss from the precursor dust, and/or inherited from the interstellar medium. Element concentrations and interelement ratios of volatile elements do not provide a clear picture about responsible mechanisms. Here, the abundance and stable isotope composition of the moderately- to highly-volatile element Se are investigated in carbonaceous, ordinary, and enstatite chondrites to constrain the mechanism responsible for the depletion of volatile elements in planetary bodies of the inner solar system and to define a δ(82/78)Se value for the bulk solar system. The δ(82/78)Se of the studied chondrite falls are identical within their measurement uncertainties with a mean of −0.20±0.26‰ (2 s.d., n=14n=14, relative to NIST SRM 3149) despite Se abundance depletions of up to a factor of 2.5 with respect to the CI group. The absence of resolvable Se isotope fractionation rules out a kinetic Rayleigh-type incomplete condensation of Se from the hot solar nebula or partial kinetic evaporative loss on the precursor material and/or the parent bodies. The Se depletion, if acquired during partial condensation or evaporative loss, therefore must have occurred under near equilibrium conditions to prevent measurable isotope fractionation. Alternatively, the depletion and cooling of the nebula could have occurred simultaneously due to the continuous removal of gas and fine particles by the solar wind accompanied by the quantitative condensation of elements from the pre-depleted gas. In this scenario the condensation of elements does not require equilibrium conditions to avoid isotope fractionation. The results further suggest that the processes causing the high variability of Se concentrations and depletions in ordinary and enstatite chondrites did not involve any measurable isotope fractionation. Different degrees of element depletions and isotope fractionations of the moderately-volatile elements Zn, S, and Se in ordinary and enstatite chondrites indicate that their volatility is controlled by the thermal stabilities of their host phases and not by the condensation temperature under canonical nebular conditions
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