2,017 research outputs found
A primordial origin for the atmospheric methane of Saturn's moon Titan
The origin of Titan's atmospheric methane is a key issue for understanding
the origin of the Saturnian satellite system. It has been proposed that
serpentinization reactions in Titan's interior could lead to the formation of
the observed methane. Meanwhile, alternative scenarios suggest that methane was
incorporated in Titan's planetesimals before its formation. Here, we point out
that serpentinization reactions in Titan's interior are not able to reproduce
the deuterium over hydrogen (D/H) ratio observed at present in methane in its
atmosphere, and would require a maximum D/H ratio in Titan's water ice 30%
lower than the value likely acquired by the satellite during its formation,
based on Cassini observations at Enceladus. Alternatively, production of
methane in Titan's interior via radiolytic reactions with water can be
envisaged but the associated production rates remain uncertain. On the other
hand, a mechanism that easily explains the presence of large amounts of methane
trapped in Titan in a way consistent with its measured atmospheric D/H ratio is
its direct capture in the satellite's planetesimals at the time of their
formation in the solar nebula. In this case, the mass of methane trapped in
Titan's interior can be up to 1,300 times the current mass of atmospheric
methane.Comment: Accepted for publication in Icaru
Constraints from deuterium on the formation of icy bodies in the Jovian system and beyond
We consider the role of deuterium as a potential marker of location and
ambient conditions during the formation of small bodies in our Solar system. We
concentrate in particular on the formation of the regular icy satellites of
Jupiter and the other giant planets, but include a discussion of the
implications for the Trojan asteroids and the irregular satellites. We examine
in detail the formation of regular planetary satellites within the paradigm of
a circum-Jovian subnebula. Particular attention is paid to the two extreme
potential subnebulae - "hot" and "cold". In particular, we show that, for the
case of the "hot" subnebula model, the D:H ratio in water ice measured from the
regular satellites would be expected to be near-Solar. In contrast, satellites
which formed in a "cold" subnebula would be expected to display a D:H ratio
that is distinctly over-Solar. We then compare the results obtained with the
enrichment regimes which could be expected for other families of icy small
bodies in the outer Solar system - the Trojan asteroids and the irregular
satellites. In doing so, we demonstrate how measurements by Laplace, the James
Webb Space Telescope, HERSCHEL and ALMA will play an important role in
determining the true formation locations and mechanisms of these objects.Comment: Accepted and shortly to appear in Planetary and Space Science; 11
pages with 5 figure
Composition of Ices in Low-Mass Extrasolar Planets
We study the formation conditions of icy planetesimals in protoplanetary
disks in order to determine the composition of ices in small and cold
extrasolar planets. Assuming that ices are formed from hydrates, clathrates,
and pure condensates, we calculate their mass fractions with respect to the
total quantity of ices included in planetesimals, for a grid of disk models. We
find that the composition of ices weakly depends on the adopted disk
thermodynamic conditions, and is rather influenced by the initial composition
of the gas phase. The use of a plausible range of molecular abundance ratios
and the variation of the relative elemental carbon over oxygen ratio in the gas
phase of protoplanetary disks, allow us to apply our model to a wide range of
planetary systems. Our results can thus be used to constrain the icy/volatile
phase composition of cold planets evidenced by microlensing surveys,
hypothetical ocean-planets and carbon planets, which could be detected by Corot
or Kepler.Comment: Accepted for publication in The Astrophysical Journa
Planetesimal Compositions in Exoplanet Systems
We have used recent surveys of the composition of exoplanet host stars to
investigate the expected composition of condensed material in planetesimals
formed beyond the snow line in the circumstellar nebulae of these systems. Of
the major solid forming elements, we find that, as for the Sun, the C and O
abundances (and particularly the C/O abundance ratio) have the most significant
effect on the composition of icy planetesimals formed in these systems. The
calculations use a self-consistent model for the condensation sequence of
volatile ices from the nebula gas after refractory (silicate and metal) phases
have condensed. The resultant proportions of refractory phases and ices were
calculated for a range of nebular temperature structure and redox conditions.
Planetesimals in systems with sub-solar C/O should be water ice-rich, with
lower than solar mass fractions of refractory materials, while in super-solar
C/O systems planetesimals should have significantly higher fractions of
refractories, in some cases having little or no water ice. C-bearing volatile
ices and clathrates also become increasingly important with increasing C/O
depending on the assumed nebular temperatures. These compositional variations
in early condensates in the outer portions of the nebula will be significant
for the equivalent of the Kuiper Belt in these systems, icy satellites of giant
planets and the enrichment (over stellar values) of volatiles and heavy
elements in giant planet atmospheres.Comment: Accepted for publication in The Astrophysical Journa
The Formation of Uranus and Neptune in Solid-Rich Feeding Zones: Connecting Chemistry and Dynamics
The core accretion theory of planet formation has at least two fundamental
problems explaining the origins of Uranus and Neptune: (1) dynamical times in
the trans-Saturnian solar nebula are so long that core growth can take > 15
Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10
Earth masses necessitates a sudden gas accretion cutoff just as the ice giant
cores reach critical mass. Both problems may be resolved by allowing the ice
giants to migrate outward after their formation in solid-rich feeding zones
with planetesimal surface densities well above the minimum-mass solar nebula.
We present new simulations of the formation of Uranus and Neptune in the
solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor
axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005;
Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15
AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the
outermost after 5.3-6 Myr, a considerable time decrease from previous
one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay
subcritical, eliminating the need for a sudden gas accretion cutoff. Our
calculated carbon mass fractions of 22% are in excellent agreement with the ice
giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based
on the requirement that the ice giant-forming planetesimals contain >10% mass
fractions of methane ice, we can reject any solar system formation model that
initially places Uranus and Neptune inside the orbit of Saturn. We also
demonstrate that a large population of planetesimals must be present in both
ice giant feeding zones throughout the lifetime of the gaseous nebula.Comment: Accepted for publication in Icarus. 9 pages, including 3 figure
Planetary nebulae, tracers of stellar nucleosynthesis
We review the information that planetary nebulae and their immediate
progenitors, the post-AGB objects, can provide to probe the nucleosynthesis and
mixing in low and intermediate mass stars. We emphasize new approaches based on
high signal-to-noise spectroscopy of planetary nebulae and of their central
stars. We mention some of the problems still to overcome. We emphasize that, as
found by several authors, planetary nebulae in low metallicity environments
cannot be used to probe the oxygen abundance in the interstellar medium out of
which their progenitors were formed, because of abundance modification during
stellar evolution.Comment: 13 pages, to appear in "StellarNucleosynthesis: 50yearsafterB2FH",
eds. C. Charbonnel and J.-P. Zahn, EAS PublicationsSerie
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&
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