Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..668t&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Astronomy and Astrophysics
Astrophysics
Scientific paper
ABSTRACT The GCMS instrument aboard the Huygens probe has measured the composition of Titan's atmosphere [1] and detected for the first time 36Ar and 40Ar, but no Xe and Kr. Assuming that planetesimals which formed the satellite originated from the cold solar nebula around 10 AU, we predict, on the basis of our interpretation of the CNS enrichments in Saturn [2], that they must have contained silicates, H2O ice, CO2, CH4, H2S, NH3 and some amount of noble gases. Using the evolution model of Tobie et al. [3], we have determined the fate of the different volatile species present in Titan's interior and in the atmosphere from the accretion to present time. At the end of accretion, most of the region outward of this proto-corewas warmliquid water (T > 300K), in which gas compound has very low solubility, and so potentially very large amounts of volatiles, notably methane, ended up in the primitive atmosphere and on the surface. During that early epoch, the composition of the hot-proto atmosphere should have reflected the composition of the planetesimals. The atmosphere at that time was probablymainly composed of H2O, NH3, CO2, CH4, H2S, which strongly contrasts with the nitrogen dominating atmosphere we have on Titan today. Early escape, photolysis, impact-driven chemistry and progressive condensation to the surface of the different species initially present in the primitive atmosphere gradually change the composition of the atmosphere, so that most of the primordial gas compound disappeared fromthe atmosphere. After that catastrophic early epoch, only the inner undifferentiated portion of Titans interior was able to hold primordial volatiles. These volatile species were released fromthe deep interior when internal differentiation occured, roughly 0.5 Gyr after accretion. Depending on their ability to interact with water molecules, each species follow a different evolutionnary pathway. For pressure conditions occurringwithin Titan, we show thatmost of the volatile species combinewith watermolecules to form clathrate hydrate structure. However, the temperature at which clathration can occur depends on the properties of each molecule. Among the different species potentially present in Titan's interior, Xe and H2S are the most stable species in the clahrate phase (Figure 1), and they are the two first species to be enclathrated when the satellite cools down. Our calculations reveal that clathrates of a mixture of Xe and H2S should be sequestered at the bottom of the H2O-NH3 subsurface ocean owing to their high stability and their high density compared to that of liquid water. The preferential sequestration of xenon in Titan's interior would explain why its abundance remains below the detection limit of the Huygens GCMS [1]. On the contrary, the least stable species in the clathrate phase are argon and carbon monoxide. Therefore even if they were present in small amounts at the time of accretion, they are easily released from the interior. Furthermore, we show that only clathrates containing a significant fraction of methane have a density lower than ammonia-water mixtures. As a consequence, methane-rich clathrates released during the core overturn accumulate at the surface of the water ocean, and form a thermally insulating layer [3]. Owing to the low thermal conductivity of clathrate hydrate, the efficiency of heat transport through the icy crust is reduced, leading to an increase of the subsurface ocean temperature up to the dissociation point of methane clathrate. This lead to outgassing of methane, which occurs in three main epochs [3]. Argon and carbon monoxide, dissolved in the water ocean and contained in small amounts in the methane-rich clathrates, should also participate to this massive release of methane. A significant fraction of carbon dioxide should also be released during the outgassing episode, but it rapidly condenses onto the surface owing to the very cold surface temperature. A small amount of krypton might also be released, but as its primordial abundance is small, it remains below the detection limit of the GCMS. The detection of the 40K decay daughter 40Ar is a strong indicator of past and recent internal activities, thus confirming the scenario proposed here. While most of the detected 40Ar comes from the silicate phase, which contains a significant fraction of potassium, we show that only a small fraction of the detected 36Ar can originate from the silicate phase. This strongly suggests that most of the primordial 36Ar has been brought by the ice phase, and that a fraction of argon, even if it is small, has been incorporated at low temperature in the planetesimals that built Titan in the form of clathrate hydrate. This favors the scenario where today's methane mainly originate from the solar nebula, was stored in the interior and later released; and thus was not chemically produced H2O and CO2 in the satellite interior. In situ measurements to be done by a future mission on Titan [4] will permit to test the different ideas present here. In particular, a precise determination of D/H and O16/O18 ratios in H2O, CO2 and CO will provide pertinent tests on the origin of different volatile species. IR spectroscopy and direct sampling of the surface materials will allow to determine the amount of carbon dioxide present in the crust. Detection of 38Ar, Kr and possibly Xe, and estimation of isotopic ratios will also give key informations on the origin and evolution of Titan's atmosphere and interior, in particular on the trapping mechanismes of volatile in Saturn's environnement and on the differentiation processes of Titan's interior. References [1] Niemann, H. B., and 17 colleagues 2005. Nature 438, 779-784. [2] Hersant, F., Gautier, D., Tobie, G., Lunine, J. I. 2008. Planet Space Sci., in press. [3] Tobie, G., Lunine, J. I., Sotin, C. 2006. Nature, 440, 61-64. [4] Coustenis A. and the TANDEM consortium, 2008. Experimental Astron., in press.
Gautier Daniel
Hersant Franck
Lunine Jonathan I.
Tobie Gabriel
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