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

Abstract

We describe a scenario of Titan's formation matching the constraints imposed\nby its current atmospheric composition. Assuming that the abundances of all\nelements, including oxygen, are solar in the outer nebula, we show that the icy\nplanetesimals were agglomerated in the feeding zone of Saturn from a mixture of\nclathrates with multiple guest species, so-called stochiometric hydrates such\nas ammonia hydrate, and pure condensates. We also use a statistical\nthermodynamic approach to constrain the composition of multiple guest\nclathrates formed in the solar nebula. We then infer that krypton and xenon,\nthat are expected to condense in the 20-30 K temperature range in the solar\nnebula, are trapped in clathrates at higher temperatures than 50 K. Once\nformed, these ices either were accreted by Saturn or remained embedded in its\nsurrounding subnebula until they found their way into the regular satellites\ngrowing around Saturn. In order to explain the carbon monoxide and primordial\nargon deficiencies of Titan's atmosphere, we suggest that the satellite was\nformed from icy planetesimals initially produced in the solar nebula and that\nwere partially devolatilized at a temperature not exceeding 50 K during their\nmigration within Saturn's subnebula. The observed deficiencies of Titan's\natmosphere in krypton and xenon could result from other processes that may have\noccurred both prior or after the completion of Titan. Thus, krypton and xenon\nmay have been sequestrated in the form of XH3+ complexes in the solar nebula\ngas phase, causing the formation of noble gas-poor planetesimals ultimately\naccreted by Titan. Alternatively, krypton and xenon may have also been trapped\nefficiently in clathrates located on the satellite's surface or in its\natmospheric haze.\n