Yves Ellinger

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.

Sequestration of Noble Gases by H+3 in Protoplanetary Disks and Outer Solar System Composition

Westudytheefficiencyof thenoblegassequestrationbytheionH þ intheformof XH þ complexes(withX ¼ argon, krypton,orxenon)in gas-phase conditions similar to those encountered duringthe cooling of protoplanetarydisks. We show that XH þ complexes form very stable structures in the gas phase and that their binding energies are much higher than those involved in the structures of X-H2O hydrates or pure X-X condensates. This implies that in the presence of H þ ions, argon, krypton, or xenon are likely to remain sequestrated in the form of XH þ complexes embedded in thegasphaseratherthanformingicesduringthecoolingof protoplanetarydisks.Theamountofthedeficiencydepends onhowmuchH þ isavailableandefficientincapturingnoblegases.Inthedensegasof themid-planeofsolarnebula H þ is formed by the ionization of H2 from energetic particles such as those in cosmic rays or those ejected by the young Sun. Even using the largest estimate of the cosmic-ray ionization rate, we compute that the H þ abundance is 2 and 3 orders of magnitude lower than the xenon and krypton abundance, respectively. Estimating the ionization induced by the young Sun, on the other hand, is very uncertain but leaves the possibility of having enough H þ to make kryptonandxenontrappingefficient.Thismaycauseadeficiencyof Kr,Xe,andtoalowerextentof Ar,intheforming icy planetesimals. We then suggest that this sequestration mechanism may explain the deficiency of Titan in noble gases revealed by the Huygens probe measurements. Similarly, comets formed from crystalline water ice in the outer nebula should be also deficient in krypton and xenon, and to a lower extent in argon, in agreement with some recent observations. Subject headingg astrochemistry — comets: general — planetary systems: protoplanetary disks — planets and satellites: formation — solar system: formation