939 research outputs found
Modeling the Jovian subnebula: II - Composition of regular satellites ices
We use the evolutionary turbulent model of Jupiter's subnebula described by
Alibert et al. (2005a) to constrain the composition of ices incorporated in its
regular icy satellites. We consider CO2, CO, CH4, N2, NH3, H2S, Ar, Kr, and Xe
as the major volatile species existing in the gas-phase of the solar nebula.
All these volatile species, except CO2 which crystallized as a pure condensate,
are assumed to be trapped by H2O to form hydrates or clathrate hydrates in the
solar nebula. Once condensed, these ices were incorporated into the growing
planetesimals produced in the feeding zone of proto-Jupiter. Some of these
solids then flowed from the solar nebula to the subnebula, and may have been
accreted by the forming Jovian regular satellites. We show that ices embedded
in solids entering at early epochs into the Jovian subdisk were all vaporized.
This leads us to consider two different scenarios of regular icy satellites
formation in order to estimate the composition of the ices they contain. In the
first scenario, icy satellites were accreted from planetesimals that have been
produced in Jupiter's feeding zone without further vaporization, whereas, in
the second scenario, icy satellites were accreted from planetesimals produced
in the Jovian subnebula. In this latter case, we study the evolution of carbon
and nitrogen gas-phase chemistries in the Jovian subnebula and we show that the
conversions of N2 to NH3, of CO to CO2, and of CO to CH4 were all inhibited in
the major part of the subdisk. Finally, we assess the mass abundances of the
major volatile species with respect to H2O in the interiors of the Jovian
regular icy satellites. Our results are then compatible with the detection of
CO2 on the surfaces of Callisto and Ganymede and with the presence of NH3
envisaged in subsurface oceans within Ganymede and Callisto.Comment: 9 pages, A&A, in pres
Modeling the Jovian subnebula: I - Thermodynamical conditions and migration of proto-satellites
We have developed an evolutionary turbulent model of the Jovian subnebula
consistent with the extended core accretion formation models of Jupiter
described by Alibert et al. (2005b) and derived from Alibert et al.
(2004,2005a). This model takes into account the vertical structure of the
subnebula, as well as the evolution of the surface density as given by an
-disk model and is used to calculate the thermodynamical conditions in
the subdisk, for different values of the viscosity parameter. We show that the
Jovian subnebula evolves in two different phases during its lifetime. In the
first phase, the subnebula is fed through its outer edge by the solar nebula as
long as it has not been dissipated. In the second phase, the solar nebula has
disappeared and the Jovian subdisk expands and gradually clears with time as
Jupiter accretes the remaining material. We also demonstrate that early
generations of satellites formed during the beginning of the first phase of the
subnebula cannot survive in this environment and fall onto the proto-Jupiter.
As a result, these bodies may contribute to the enrichment of Jupiter in heavy
elements. Moreover, migration calculations in the Jovian subnebula allow us to
follow the evolution of the ices/rocks ratios in the proto-satellites as a
function of their migration pathways. By a tempting to reproduce the distance
distribution of the Galilean satellites, as well as their ices/rocks ratios, we
obtain some constraints on the viscosity parameter of the Jovian subnebula.Comment: Accepted in Astronomy and Astrohpysic
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
Pebble accretion at the origin of water in Europa
Despite the fact that the observed gradient in water content among the
Galilean satellites is globally consistent with a formation in a circum-Jovian
disk on both sides of the snowline, the mechanisms that led to a low water mass
fraction in Europa () are not yet understood. Here, we present new
modeling results of solids transport in the circum-Jovian disk accounting for
aerodynamic drag, turbulent diffusion, surface temperature evolution and
sublimation of water ice. We find that the water mass fraction of pebbles
(e.g., solids with sizes of 10 -- 1 m) as they drift inward is globally
consistent with the current water content of the Galilean system. This opens
the possibility that each satellite could have formed through pebble accretion
within a delimited region whose boundaries were defined by the position of the
snowline. This further implies that the migration of the forming satellites was
tied to the evolution of the snowline so that Europa fully accreted from
partially dehydrated material in the region just inside of the snowline.Comment: Accepted for publication in Ap
On the volatile enrichments and composition of Jupiter
Using the clathrate hydrates trapping theory, we discuss the enrichments in
volatiles in the atmosphere of Jupiter measured by the \textit{Galileo} probe
in the framework of new extended core-accretion planet formation models
including migration and disk evolution. We construct a self-consistent model in
which the volatile content of planetesimals accreted during the formation of
Jupiter is calculated from the thermodynamical evolution of the disk. Assuming
CO2:CO:CH4 = 30:10:1 (ratios compatible with ISM measurements), we show that we
can explain the enrichments in volatiles in a way compatible with the recent
constraints set from internal structure modeling on the total amount of heavy
elements present in the planet.Comment: Accepted in ApJLetter
Volatile inventories in clathrate hydrates formed in the primordial nebula
Examination of ambient thermodynamic conditions suggest that clathrate
hydrates could exist in the martian permafrost, on the surface and in the
interior of Titan, as well as in other icy satellites. Clathrate hydrates
probably formed in a significant fraction of planetesimals in the solar system.
Thus, these crystalline solids may have been accreted in comets, in the forming
giant planets and in their surrounding satellite systems. In this work, we use
a statistical thermodynamic model to investigate the composition of clathrate
hydrates that may have formed in the primordial nebula. In our approach, we
consider the formation sequence of the different ices occurring during the
cooling of the nebula, a reasonable idealization of the process by which
volatiles are trapped in planetesimals. We then determine the fractional
occupancies of guests in each clathrate hydrate formed at given temperature.
The major ingredient of our model is the description of the guest-clathrate
hydrate interaction by a spherically averaged Kihara potential with a nominal
set of parameters, most of which being fitted on experimental equilibrium data.
Our model allows us to find that Kr, Ar and N can be efficiently encaged in
clathrate hydrates formed at temperatures higher than 48.5 K in the
primitive nebula, instead of forming pure condensates below 30 K. However, we
find at the same time that the determination of the relative abundances of
guest species incorporated in these clathrate hydrates strongly depends on the
choice of the parameters of the Kihara potential and also on the adopted size
of cages. Indeed, testing different potential parameters, we have noted that
even minor dispersions between the different existing sets can lead to
non-negligible variations in the determination of the volatiles trapped in
clathrate hydrates formed in the primordial nebula.Comment: Accepted for publication in Faraday Discussion
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
Equilibrium composition between liquid and clathrate reservoirs on Titan
Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by
the Cassini spacecraft to cover the polar regions of Titan. A significant
fraction of these lakes or seas could possibly be interconnected with
subsurface liquid reservoirs of alkanes. In this paper, we investigate the
interplay that would happen between a reservoir of liquid hydrocarbons located
in Titan's subsurface and a hypothetical clathrate reservoir that progressively
forms if the liquid mixture diffuses throughout a preexisting porous icy layer.
To do so, we use a statistical-thermodynamic model in order to compute the
composition of the clathrate reservoir that forms as a result of the
progressive entrapping of the liquid mixture. This study shows that clathrate
formation strongly fractionates the molecules between the liquid and the solid
phases. Depending on whether the structure I or structure II clathrate forms,
the present model predicts that the liquid reservoirs would be mainly composed
of either propane or ethane, respectively. The other molecules present in the
liquid are trapped in clathrates. Any river or lake emanating from subsurface
liquid reservoirs that significantly interacted with clathrate reservoirs
should present such composition. On the other hand, lakes and rivers sourced by
precipitation should contain higher fractions of methane and nitrogen, as well
as minor traces of argon and carbon monoxide.Comment: Accepted for publication in Icaru
Removal of Titan's Atmospheric Noble Gases by their Sequestration in Surface Clathrates
A striking feature of the atmosphere of Titan is that no heavy noble gases
other than argon were detected by the Gas Chromatograph Mass Spectrometer
(GCMS) aboard the Huygens probe during its descent to Titan's surface in
January 2005. Here we provide an explanation of the mysterious absence or
rarity of these noble gases in Titan's atmosphere: the thermodynamic conditions
prevailing at the surface-atmosphere interface of the satellite allow the
formation of multiple guest clathrates that preferentially store some species,
including all heavy noble gases, over others. The clean water ice needed for
formation of these clathrates could be delivered by successive episodes of
cryovolcanic lavas that have been hypothesized to regularly cover the surface
of Titan. The formation of clathrates in the porous lavas and their propensity
for trapping Ar, Kr and Xe would progressively remove these species from the
atmosphere of Titan over its history. In some circumstances, a global clathrate
crust with an average thickness not exceeding a few meters could be sufficient
on Titan for a complete removal of the heavy noble gases from the atmosphere.Comment: Accepted for publication in The Astrophysical Journal Letter
Expectations for the Deep Impact collision from cometary nuclei modelling
Using the cometary nucleus model developed by Espinasse et al. (1991), we
calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of
360 years. Starting from an initially amorphous cometary nucleus which
incorporates an icy mixture of H2O and CO, we show that, at the time of Deep
Impact collision, the crater is expected to form at depths where ice is in its
crystalline form. Hence, the subsurface exposed to space should not be
primordial. We also attempt an order-of-magnitude estimate of the heating and
material ablation effects on the crater activity caused by the 370 Kg
projectile released by the DI spacecraft. We thus show that heating effects
play no role in the evolution of crater activity. We calculate that the CO
production rate from the impacted region should be about 300-400 times higher
from the crater resulting from the impact with a 35 m ablation than over the
unperturbed nucleus in the immediate post-impact period. We also show that the
H2O production rate is decreased by several orders of magnitude at the crater
base just after ablation
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