S. Picaud

CLATHRATION OF VOLATILES IN THE SOLAR NEBULA AND IMPLICATIONS FOR THE ORIGIN OF TITAN'S ATMOSPHERE

We describe a scenario of Titans formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titans atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding ~50 K during their migration within Saturns subnebula. The observed deficiencies of Titans atmosphere in krypton and xenon could result from other processes that may have occurred both prior to or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH+ 3 complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellites surface or in its atmospheric haze. We finally discuss the subsequent observations that would allow us to determine which of these processes is the most likely.

Adsorption of acetone molecules on proton ordered ice. A molecular dynamics study

The adsorption of acetone molecules on a proton ordered ice Ih(0001) surface was studied using classical molecular dynamics simulations between 50 and 150 K. At low coverage, we show that acetone molecules form an ordered monolayer on this ice surface, which is stable for T⩽100 K. At higher temperature, it exhibits orientational disordering, though local translational order remains. Preliminary simulations at higher coverage indicates the formation of additional ordered layers above the first monolayer, which are also stable below 100 K. These results support previous conclusions on the acetone/ice interactions based on the interpretation of experimental data.

Theoretical study of the monolayer structures of CO adsorbed on NaCl(100)

Abstract Interaction potential calculations are performed to interpret recent experimental results on the geometry of CO monolayers adsorbed on clean NaCl(100) substrate deduced from polarized Fourier transform infrared spectroscopy and helium beam scattering. Two isoenergetical structures are calculated: the (1 × 1) geometry with the molecular axes standing upright or inclined with respect to the surface normal and the (2 × 1) geometry with the molecular axes inclined and tilted anti-parallel. A statistical analysis of the energy minima for the CO monolayer is performed and it is shown that the transition from the (2 × 1) to the (1 × 1) structure can proceed in a very small energy range ( ~ a few meV) leading to a rearrangement of the molecular centers of mass above the Na sites with a concomitant redistribution of the molecular axis orientations. These results are in close agreement with experiments indicating a phase transition at 35 K from the (2 × 1) to the (1 × 1) phase.

Geometry and dynamics of formic and acetic acids adsorbed on ice

Energy optimization at 0 K and constrained molecular dynamics simulations at 250 K have been carried out to study adsorption and incorporation of formic and acetic acids on/in ice. The results show that the adsorption and incorporation processes are highly influenced by the formation of two H-bonds between the carboxyl function and two water molecules. The free energy profiles indicate that the two acid molecules are strongly trapped at the ice surface and that the incorporation of formic acid is favored when compared to acetic acid. These data are discussed within the context of tropospheric conditions.

A molecular dynamics study of the structure of water layers adsorbed on MgO(100)

Molecular dynamics simulations are performed at various temperatures (150-300 K) and coverages (1-3 layers) on the adsorption of water on a clean MgO(100) surface using semiempirical potentials. At the monolayer coverage, a number of very stable (m×n) structures are obtained which differ only by the mutual orientations of the molecules. The p(3×2) phase observed above 180 K in low-energy electron diffraction (LEED) and helium atom scattering (HAS) experiments is shown to be the most stable at 200 K and above this temperature. It contains six inequivalently oriented molecules which lie flat above the cation sites with the hydrogens pointing approximately along the Mg rows. When the water coverage increases, a layer of icelike hexagonal structure within which the water molecules are hydrogen bonded is formed above the stable monolayer. This overlayer, which is stable at 150 K, is not hydrogen bonded to the stable monolayer. At 300 K it tends to break up and to aggregate into a 3D ice structure with strong h...

Adsorption of polar molecules on substrates with strong electric surface fields: from aggregates to monolayers

Abstract The adsorption of ammonia and water molecules on MgO (100) and NaCl (100) substrates, which generate strong electric surface fields, is studied for the first steps of aggregation (M) N N=2, …, 5 and for the monolayer. Potential calculations show that water molecules form planar aggregates on both substrates and, at higher coverage, commensurate (2x2) monolayers on MgO or (4x2) bilayers on NaCl. In contrast, the competition between the electrostratic interactions due to the substrate and the lateral molecular interactions leads to a specific behavior of ammonia admolecules: on MgO they do not aggregate and can only arrange in incomplete monolayers, while on NaCl they form 3D aggregates with a tendency to 3D crystallization at higher coverage.

Adsorption of acetic acid on ice: experiments and molecular dynamics simulations.

Adsorption study of acetic acid on ice surfaces was performed by combining experimental and theoretical approaches. The experiments were conducted between 193 and 223 K using a coated wall flow tube coupled to a mass spectrometric detection. Under our experimental conditions, acetic acid was mainly dimerized in the gas phase. The surface coverage increases with decreasing temperature and with increasing concentrations of acetic acid dimers. The obtained experimental surface coverages were fitted according to the BET theory in order to determine the enthalpy of adsorption deltaH(ads) and the mololayer capacity N(M(dimers)) of the acetic acid dimers on ice: deltaH(ads) = (-33.5 +/- 4.2) kJ mol(-1), N(M(dimers)) = (l1.27 +/- 0.25) x 10(14) dimers cm(-2). The adsorption characteristics of acetic acid on an ideal ice I(n)(0001) surface were also studied by means of classical molecular dynamics simulations in the same temperature range. The monolayer capacity, the configurations of the molecules in their adsorption sites, and the corresponding adsorption energies have been determined for both acetic acid monomers and dimers, and compared to the corresponding data obtained from the experiments. In addition, the theoretical results show that the interaction with the ice surface could be strong enough to break the acetic acid dimers that exist in the gas phase and leads to the stabilization of acetic acid monomers on ice.

Variability of the methane trapping in martian subsurface clathrate hydrates

Abstract Recent observations have evidenced traces of methane ( CH 4 ) heterogeneously distributed in the martian atmosphere. However, because the lifetime of CH 4 in the atmosphere of Mars is estimated to be around 300–600 years on the basis of photochemistry, its release from a subsurface reservoir or an active primary source of methane have been invoked in the recent literature. Among the existing scenarios, it has been proposed that clathrate hydrates located in the near subsurface of Mars could be at the origin of the small quantities of the detected CH 4 . Here, we accurately determine the composition of these clathrate hydrates, as a function of temperature and gas phase composition, by using a hybrid statistical thermodynamic model based on experimental data. Compared to the other recent works, our model allows us to calculate the composition of clathrate hydrates formed from a more plausible composition of the martian atmosphere by considering its main compounds, i.e. carbon dioxide, nitrogen and argon, together with methane. Besides, because there is no low temperature restriction in our model, we are able to determine the composition of clathrate hydrates formed at temperatures corresponding to the extreme ones measured in the polar caps. Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH 4 -rich atmosphere has existed or if a subsurface source of CH 4 has been (or is still) present.

Clathrate hydrates as a sink of noble gases in Titan's atmosphere

We use a statistical thermodynamic approach to determine the composition of clathrate hydrates which may form from a multiple compound gas whose composition is similar to that of Titan’s atmosphere. Assuming that noble gases are initially present in this gas phase, we calculate the ratios of xenon, krypton and argon to species trapped in clathrate hydrates. We find that these ratios calculated for xenon and krypton are several orders of magnitude higher than in the coexisting gas at temperature and pressure conditions close to those of Titan’s present atmosphere at ground level. Furthermore we show that, by contrast, argon is poorly trapped in these ices. This trapping mechanism implies that the gas-phase is progressively depleted in xenon and krypton when the coexisting clathrate hydrates form whereas the initial abundance of argon remains almost constant. Our results are thus compatible with the deficiency of Titan’s atmosphere in xenon and krypton measured by the Huygens probe during its descent on January 14, 2005. However, in order to interpret the subsolar abundance of primordial Ar also revealed by Huygens, other processes that occurred either during the formation of Titan or during its evolution must be also invoked.

A theoretical investigation into the trapping of noble gases by clathrates on Titan

In this paper, we use a statistical thermodynamic approach to quantify the efficiency with which clathrates on the surface of Titan trap noble gases. We consider different values of the Ar, Kr, Xe, CH4, C2H6 and N2 abundances in the gas phase that may be representative of Titans early atmosphere. We discuss the effect of the various parameters that are chosen to represent the interactions between the guest species and the ice cage in our calculations. We also discuss the results of varying the size of the clathrate cages. We show that the trapping efficiency of clathrates is high enough to significantly decrease the atmospheric concentrations of Xe and, to a lesser extent, of Kr, irrespective of the initial gas phase composition, provided that these clathrates are abundant enough on the surface of Titan. In contrast, we find that Ar is poorly trapped in clathrates and, as a consequence, that the atmospheric abundance of argon should remain almost constant. We conclude that the mechanism of trapping noble gases via clathration can explain the deficiency in primordial Xe and Kr observed in Titans atmosphere by Huygens, but that this mechanism is not sufficient to explain the deficiency in Ar.