On the stability of clathrate hydrates in comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen

Abstract

Context. For several years, Jupiter-family comets have been the targets of spacecraft missions whose aims are to determine the comets' composition, structure, and physical properties. The Rosetta mission is currently flying towards comet 67P/Churyumov Gerasimenko for a rendezvous in August 2014 and comet 46P/Wirtanen is considered for a rendezvous in 2021 with the PriME (Primitive Material Explorer) mission, which is currently proposed to NASA. Aims. Here we investigate the stability conditions of clathrate hydrates within the comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen by considering an initial mixture of amorphous H 2 O with CO, CO 2 , CH 4 , and H 2 S in the nuclei. Methods. We use a one-dimensional nucleus model, which considers an initially homogeneous sphere composed of a predefined porous mixture of ices and dust in specified proportions and describes heat transmission, gas diffusion, sublimation/recondensation of volatiles within the nucleus, water ice phase transition, dust release, and mantle formation. Results. We show that stability conditions of multiple guest clathrates are permanently reached in the subsurface of both comets, and in a broader manner in the subsurface of all short period comets. The thickness of the stability zone of the clathrate slightly oscillates with time as a function of the heliocentric distance, but never vanishes. When comets approach perihelion, our calculations suggest that clathrate layers, which are located closer to the nucleus surface, may destabilize before amorphous ice is tranformed into crystalline ice.

Low CO/CO2ratios of comet 67P measured at the Abydos landing site by thePtolemymass spectrometer

“…A very homogeneous D/H in water would imply that the building blocks of 67P were formed at close distances in the PSN and argue in favor of a homogeneous accretion. Under these circumstances, the comet's heterogeneity would result from its thermal evolution after accretion, as suggested by thermal evolution models (Marboeuf et al 2011;De Sanctis et al 2005;Prialnik et al 2008). In contrast, the measurement of D/H variations would indicate that the comet's building blocks formed with different compositions at various locations of the PSN (Bonev et al 2008).…”

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

“…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).…”

“…As a result, their formation and decomposition inside the nucleus can generate an outgassing profile of volatile molecules at the surface that differ markedly from that expected from models without formation of clathrate since this structure selectively retains and releases the gases trapped inside. The existence of clathrate inside comets has been discussed since a long time (Delsemme & Swings 1952;Klinger et al 1986;Schmitt & Klinger 1987;Smoluchowski 1988;Mousis et al 2000;Prialnik et al 2004;Huebner 2008;Marboeuf et al 2010Marboeuf et al , 2011. The authors proposed that the presence of clathrate in extraterrestrial ices might account for their anomalous volatile molecules retention and release (Blake et al 1991;Iro et al 2003).…”

In situmeasurement of gas-solid interactions in astrophysical dust & planetary analogues

“…SO 2 , CO 2 , CH 4 and H 2 S clathrates should be stable on the surface/oceans of Europa (Prieto-Ballesteros et al 2005) and CO, CO 2 , CH 4 and H 2 S clathrates may form in certain comets (Marboeuf et al 2011). Demineralised water was frozen at 200 K in the HPC and Fig.…”