Daniel Cordier

AN ESTIMATE OF THE CHEMICAL COMPOSITION OF TITAN's LAKES

Hundreds of radar-dark patches interpreted as lakes have been discovered in the north and south polar regions of Titan. We have estimated the composition of these lakes by using the direct abundance measurements from the Gas Chromatograph Mass Spectrometer aboard the Huygens probe and recent photochemical models based on the vertical temperature profile derived by the Huygens Atmospheric Structure Instrument. Thermodynamic equilibrium is assumed between the atmosphere and the lakes, which are also considered nonideal solutions. We find that the main constituents of the lakes are ethane (C2H6) (~76%-79%), propane (C3H8) (~7%-8%), methane (CH4) (~5%-10%), hydrogen cyanide (HCN) (~2%-3%), butene (C4H8) (~1%), butane (C4H10) (~1%), and acetylene (C2H2) (~1%). The calculated composition of lakes is then substantially different from what has been expected from models elaborated prior to the exploration of Titan by the Cassini-Huygens spacecraft.

A Large Stellar Evolution Database for Population Synthesis Studies. III. Inclusion of the Full Asymptotic Giant Branch Phase and Web Tools for Stellar Population Analyses

Stellar evolution tracks and isochrones are key inputs for a wide range of astrophysical studies; in particular, they are essential to the interpretation of photometric and spectroscopic observations of resolved and unresolved stellar populations. We have made available to the astrophysical community a large, homogenous database of up-to-date stellar tracks and isochrones and a set of programs useful in population synthesis studies. In this paper we first summarize the main properties of our stellar model database (BaSTI ) already introduced by Pietrinferni and coworkers. We then discuss an important update of the database, i.e., the extension of all stellar models and isochrones until the end of the thermal pulses along the asymptotic giant branch. This extension of the library is particularly relevant for stellar population analyses in the near-infrared or longer wavelengths, where the contribution to the integrated photometric properties by cool and bright asymptotic giant branch stars is significant. A few comparisons with empirical data are also presented and briefly discussed. We then present three Web tools that allow an interactive access to the database and make it possible to compute user-specified evolutionary tracks, isochrones, stellar luminosity functions, and synthetic color-magnitude diagrams and integrated magnitudes for arbitrary star formation histories. All these Web tools are available at the BaSTI database official Web site.

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.

Titan's lakes chemical composition: sources of uncertainties and variability

Abstract Between 2004 and 2007 the instruments of the Cassini spacecraft, orbiting within the Saturn system, discovered dark patches in the polar regions of Titan. These features are interpreted as hydrocarbon lakes and seas with ethane and methane identified as the main compounds. In this context, we have developed a lake–atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas present on Titan. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte Carlo simulations. Global circulation models (GCM) are also employed in order to investigate the possibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to ∼ 8500 % ) but the distributions of values are narrow. The relative standard deviations remain between 10% and ∼ 300 % depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to ∼ 20 % . Moreover, no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH3CN, our work strongly underlines the need for experimental simulations and the improvement of Titans atmospheric models.

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 � 1300 times the current mass of atmospheric methane.

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 aboard the Huygens probe during its descent to Titans surface in 2005 January. Here we provide an explanation of the mysterious absence or rarity of these noble gases in Titans 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 the 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 the course of 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.

About the Possible Role of Hydrocarbon Lakes in the Origin of Titan's Noble Gas Atmospheric Depletion

An unexpected feature of Titans atmosphere is the strong depletion in primordial noble gases revealed by the Gas Chromatograph Mass Spectrometer aboard the Huygens probe during its descent on 2005 January 14. Although several plausible explanations have already been formulated, no definitive response to this issue has yet been found. Here, we investigate the possible sequestration of these noble gases in the liquid contained in lakes and wet terrains on Titan and the consequences for their atmospheric abundances. Considering the atmosphere and the liquid existing on the soil as a whole system, we compute the abundance of each noble gas relative to nitrogen. To do so, we make the assumption of thermodynamic equilibrium between the liquid and the atmosphere, the abundances of the different constituents being determined via regular solution theory. We find that xenons atmospheric depletion can be explained by its dissolution at ambient temperature in the liquid presumably present on Titans soil. In the cases of argon and krypton, we find that the fractions incorporated in the liquid are negligible, implying that an alternative mechanism must be invoked to explain their atmospheric depletion.

FORMATION CONDITIONS OF ENCELADUS AND ORIGIN OF ITS METHANE RESERVOIR

We describe a formation scenario of Enceladus constrained by the deuterium-to-hydrogen ratio (D/H) in the gas plumes as measured by the Cassini Ion and Neutral Mass Spectrometer. We propose that, similarly to Titan, Enceladus formed from icy planetesimals that were partly devolatilized during their migration within the Kronian subnebula. In our scenario, at least primordial Ar, CO, and N2 were devolatilized from planetesimals during their drift within the subnebula, due to the increasing temperature and pressure conditions of the gas phase. The origin of methane is still uncertain since it might have been either trapped in the planetesimals of Enceladus during their formation in the solar nebula or produced via serpentinization reactions in the satellite’s interior. If the methane of Enceladus originates from the solar nebula, then its D/H ratio should range between ∼4.7 × 10 −5 and 1.5 × 10 −4 . Moreover, Xe/H2O and Kr/H2O ratios are predicted to be equal to ∼7 × 10 −7 and 7 × 10 −6 , respectively, in the satellite’s interior. On the other hand, if the methane of Enceladus results from serpentinization reactions, then its D/H ratio should range between ∼2.1 × 10 −4 and 4.5 × 10 −4 . In this case, Kr/H2O should not exceed ∼10 −10 and Xe/H2O should range between ∼1 × 10 −7 and 7 × 10 −7 in the satellite’s interior. Future spacecraft missions, such as Titan Saturn System Mission, will have the capability to provide new insight into the origin of Enceladus by testing these observational predictions.

ON THE VOLATILE ENRICHMENTS AND HEAVY ELEMENT CONTENT IN HD189733b

Favored theories of giant planet formation center around two main paradigms, namely the core accretion model and the gravitational instability model. These two formation scenarios support the hypothesis that the giant planet metallicities should be higher or equal to that of the parent star. Meanwhile, spectra of the transiting hot Jupiter HD189733b suggest that carbon and oxygen abundances range from depleted to enriched with respect to the star. Here, using a model describing the formation sequence and composition of planetesimals in the protoplanetary disk, we determine the range of volatile abundances in the envelope of HD189733b that is consistent with the 20-80 M ? of heavy elements estimated to be present in the planets envelope. We then compare the inferred carbon and oxygen abundances to those retrieved from spectroscopy, and we find a range of supersolar values that directly fit both spectra and internal structure models. In some cases, we find that the apparent contradiction between the subsolar elemental abundances and the mass of heavy elements predicted in HD189733b by internal structure models can be explained by the presence of large amounts of carbon molecules in the form of polycyclic aromatic hydrocarbons and soots in the upper layers of the envelope, as suggested by recent photochemical models. A diagnostic test that would confirm the presence of these compounds in the envelope is the detection of acetylene. Several alternative hypotheses that could also explain the subsolar metallicity of HD189733b are formulated: the possibility of differential settling in its envelope, the presence of a larger core that did not erode with time, a mass of heavy elements lower than the one predicted by interior models, a heavy element budget resulting from the accretion of volatile-poor planetesimals in specific circumstances, or the combination of all these mechanisms.

PHOTOCHEMICAL ENRICHMENT OF DEUTERIUM IN TITAN'S ATMOSPHERE: NEW INSIGHTS FROM CASSINI-HUYGENS

We reinvestigate a scenario initially proposed by Pinto et al. (1986) and Lunine et al. (1999), dealing with the photochemical enrichment of deuterium in the atmosphere of Titan, which is based on the possibility that the initial methane reservoir accessible to the atmosphere was larger than what is seen today, in light of the recent Cassini-Huygens measurements (Bezard et al. 2007). We show that this photochemical mechanism is not efficient enough in the atmosphere of Titan to explain its current D/H value, even if the current atmospheric reservoir of CH_4 is postulated to exist since 4.5 Gyr.