TITAN'S BULK COMPOSITION CONSTRAINED BYCASSINI-HUYGENS: IMPLICATION FOR INTERNAL OUTGASSING

Abstract

In the present report, by using a series of data gathered by the Cassini-Huygens mission, we constrain the bulk content of Titan's interior for various gas species (CH 4 , CO 2 , CO, NH 3 , H 2 S, Ar, Ne, Xe), and we show that most of the gas compounds (except H 2 S and Xe) initially incorporated within Titan are likely stored dissolved in the subsurface water ocean. CO 2 is likely to be the most abundant gas species (up to 3% of Titan's total mass), while ammonia should not exceed 1.5 wt%. We predict that only a moderate fraction of CH 4 , CO 2 , and CO should be incorporated in the crust in the form of clathrate hydrates. By contrast, most of the H 2 S and Xe should be incorporated at the base of the subsurface ocean, in the form of heavy clathrate hydrates within the high-pressure ice layer. Moreover, we show that the rocky phase of Titan, assuming a composition similar to CI carbonaceous chondrites, is a likely source for the noble gas isotopes ( 40 Ar, 36 Ar, 22 Ne) that have been detected in the atmosphere. A chondritic core may also potentially contribute to the methane inventory. Our calculations show that a moderate outgassing of methane containing traces of neon and argon from the subsurface ocean would be sufficient to explain the abundance estimated by the Gas Chromatograph Mass Spectrometer. The extraction process, implying partial clathration in the ice layers and exsolvation from the water ocean, may explain why the 22 Ne/ 36 Ar ratio in Titan's atmosphere appears higher than the ratio in carbonaceous chondrites.

Temporal variations of Titan’s surface with Cassini/VIMS

“…A liquid water layer in addition to the presence of an internal reservoir of methane, trapped in methane clathrates (Tobie et al, 2005(Tobie et al, , 2010(Tobie et al, , 2012Fortes et al, 2007) or formed through serpentinization if the liquid water ocean is in contact with the rocky core, has also been predicted in several Titan interior models (Atreya et al, 2009; Mousis et al, 2009). Considering the presence of a subsurface liquid water body and of methane clathrates in the ice shell in Titan's interior, current models taking into account heat transfer, convection, tidal dissipation, clathrate dissociation and cooling of the subsurface ocean suggest cryovolcanic and tectonic phenomena as a possible 'pathway' for methane to resupply the atmosphere through outgassing.…”

Evolution of Titan’s atmosphere during the Late Heavy Bombardment

“…In our study we consider only the methane supplied by the impactor, no crustal CH 4 degassing is estimated. Nevertheless, methane could be present in a significant amount in the form of clathrates hydrates (Osegovic and Max, 2005;Tobie et al, 2006Tobie et al, , 2012. The pressures and temperatures reached during the high-velocity impacts are high enough to destabilize the methane clathrate and, hence, to release CH 4 to Titan's atmosphere.…”

“…Large impactors might also be more numerous than assumed here, and have more complex effects than what we considered in the present model. As shown by Zahnle et al (2014), impactors larger than d i > 13 km with an impact velocity of 11 km s À1 may completely break the ice crust, resulting in a major water flooding event and strong degassing from volatiles contained in the ocean (Tobie et al, 2012). Such catastrophic events would strongly affect the atmospheric balance, possibly leading to the formation of the atmosphere.…”

“…If Titan was undifferentiated at the time of the LHB, NH 3 would be rather uniformly distributed in the interior and the concentration of the outer layer would be equivalent to the primordial value. If Titan was differentiated, NH 3 would be mostly contained in the subsurface ocean owing to the high solubility of ammonia into liquid water (Grasset et al, 2000;Tobie et al, 2012). However, even in this condition, some NH 3 might have been incorporated into the crust by intrusion of ammonia-enriched liquids (Kargel, 1991;Fortes et al, 2007;Choukroun and Grasset, 2010).…”

“…Moreover, Mandt et al (2014) showed that the 14 N/ 15 N ratio measured in Titan's N 2 (Niemann et al, 2010) is consistent with isotopic ratio recently inferred from NH 2 radicals produced by the photodissociation of NH 3 in comets (Rousselot et al, 2014;Shinnaka et al, 2014), providing additional evidence for ammonia as the main source of nitrogen on Titan. Several mechanisms have been proposed to explain the conversion of NH 3 into N 2 at Titan's conditions: photochemical conversion (Atreya et al, 1978), impact-induced conversion in the atmosphere (McKay et al, 1988;Ishimaru et al, 2011) or in a NH 3 -enriched icy crust (Sekine et al, 2011), as well as endogenic processes (Glein http Tobie et al, 2012). Here we focus on the conversion proposed by Sekine et al (2011).…”