Volatile inventories in clathrate hydrates formed in the primordial nebula

Abstract

Examination of ambient thermodynamic conditions suggest that clathrate hydrates could exist in the martian permafrost, on the surface and in the interior of Titan, as well as in other icy satellites. Clathrate hydrates probably formed in a significant fraction of planetesimals in the solar system. Thus, these crystalline solids may have been accreted in comets, in the forming giant planets and in their surrounding satellite systems. In this work, we use a statistical thermodynamic model to investigate the composition of clathrate hydrates that may have formed in the primordial nebula. In our approach, we consider the formation sequence of the different ices occurring during the cooling of the nebula, a reasonable idealization of the process by which volatiles are trapped in planetesimals. We then determine the fractional occupancies of guests in each clathrate hydrate formed at given temperature. The major ingredient of our model is the description of the guest-clathrate hydrate interaction by a spherically averaged Kihara potential with a nominal set of parameters, most of which being fitted on experimental equilibrium data. Our model allows us to find that Kr, Ar and N 2 can be efficiently encaged in clathrate hydrates formed at temperatures higher than ∼ 48.5 K in the primitive nebula, instead of forming pure condensates below 30 K. However, we find at the same time that the determination of the relative abundances of guest species incorporated in these clathrate hydrates strongly depends on the choice of the parameters of the Kihara potential and also on the adopted size of cages. Indeed, testing different potential parameters, we have noted that even minor dispersions between the different existing sets can lead to non-negligible variations in the determination of the volatiles trapped in clathrate hydrates formed in the primordial nebula. However, these variations are not found to be strong enough to reverse the relative abundances between the different volatiles in the clathrate hydrates themselves. On the other hand, if contraction or expansion of the cages due to temperature variations are imposed in our model, the Ar and Kr mole fractions can be modified up to several orders of magnitude in clathrate hydrates. Moreover, mole fractions of other molecules such as N 2 or CO are also subject to strong changes with the variation of the size of the cages. Our results may affect the predictions of the composition of the planetesimals formed in the outer solar system. In particular, the volatile abundances calculated in the giant planets atmospheres should be altered because these quantities are proportional to the mass of accreted and vaporized icy planetesimals. For similar reasons, the estimates of the volatile budgets accreted by icy satellites and comets may also be altered by our calculations. For instance, under some conditions, our calculations predict that the abundance of argon in the atmosphere of Titan should be higher than the value measured by Huygens. Moreover, the Ar abundance in comets...

Composition-dependent outgassing of comet 67P/Churyumov-Gerasimenko from ROSINA/DFMS

“…The unique CH 4 trend is intriguing, and suggests that additional mechanisms and/or properties of the nucleus need to be considered to fully understand these observations. For instance, CH 4 has been demonstrated to readily form clathrate hydrates under nebular conditions (e.g., Lunine & Stevenson 1985;Mousis et al 2010). Though, CH 4 not following H 2 O closely, and the intercept of the linear fit not passing through the origin (Fig.…”

A cometary nucleus model taking into account all phase changes of water ice: amorphous, crystalline, and clathrate

“…For the crystalline model, the volatile molecules are only in the state "condensed as pure ice". For the clathrate model, the distribution between the states "trapped inside clathrate structure" (up to about 17% in mole) and "condensed as pure ice" come from Mousis et al (2010). CO, CH 4 and H 2 S represent respectively 79%, 10% and 11% of the molecules trapped in the clathrate structure.…”

“…Up to now only these two icy structures are considered by current models of cometary nuclei. However, models of ice formation in the protoplanetary disk (Lewis 1972;Gautier et al 2001a,b;Iro et al 2003;Hersant et al 2004;Marboeuf et al 2008;Mousis et al 2008Mousis et al , 2009Mousis et al , 2010 show that comets formed beyond 15 UA could be fully made up of crystalline water ice and clathrate hydrates. Moreover, as shown by Marboeuf et al (2010Marboeuf et al ( , 2011, this structure of water ice could form within all cometary nuclei, whatever the initial water ice structure considered in comets (amorphous or pure crystalline).…”

Setting the volatile composition of (exo)planet-building material

“…However, these models do not contain any detailed chemistry. Either they simply use the observed chemical abundances in interstellar ices and assume that these abundances are preserved during disk evolution, or they assume that thermodynamic equilibrium is attained so that chemical abundances are controlled by temperature and pressure only (e.g., Mousis et al 2010;Johnson et al 2012;Moses et al 2013;Marboeuf et al 2014;Thiabaud et al 2015). The main observational test is through statistical comparisons with the observed populations of exoplanets and their predicted compositions.…”