How to link the relative abundances of gas species in coma of comets to their initial chemical composition?

Abstract

a b s t r a c tComets are expected to be the most primitive objects in the Solar System. The chemical composition of these objects is frequently assumed to be directly provided by the observations of the abundances of volatile molecules in the coma. The present work aims to determine the relationship between the chemical composition of the coma, the outgassing profile of volatile molecules and the internal chemical composition, and water ice structure of the nucleus, and physical assumptions on comets. To do this, we have developed a quasi 3D model of a cometary nucleus which takes into account all phase changes and water ice structures (amorphous, crystalline, clathrate, and a mixture of them); we have applied this model to the Comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission. We find that the outgassing profile of volatile molecules is a strong indicator of the physical and thermal properties (water ice structure, thermal inertia, abundances, distribution, physical differentiation) of the solid nucleus. Day/night variations of the rate of production of species helps to distinguish the clathrate structure from other water ice structures in nuclei, implying different thermodynamic conditions of cometary ice formation in the protoplanetary disc. The relative abundance (to H 2 O) of volatile molecules released from the nucleus interior varies by some orders of magnitude as a function of the distance to the Sun, the volatility of species, their abundance and distribution between the ''trapped'' and ''condensed'' states, the structure of water ice, and the thermal inertia and other physical assumptions (dust mantle, . . .) on the nucleus. For the less volatile molecules such as CO 2 and H 2 S, the relative (to H 2 O) abundance of species in coma remain similar to the primitive composition of the nucleus (relative deviation less than 25%) only around the perihelion passage (in the range À3 to À2 to +2-3 AU), whatever is the water ice structure and chemical composition, and under the conditions that the nucleus is not fully covered by a dust mantle. The relative (to H 2 O) abundance of highly volatile molecules such as CO and CH 4 in the coma remain approximately equal to the primitive nucleus composition only for nuclei made of clathrates. The nucleus releases systematically lower relative abundances of highly volatile species (up to one order of magnitude) around perihelion (in the range À3 to À2 to +2-3 AU) in the cases of the crystalline and amorphous water ice structures in the nuclei. The rate of production, the outgassing profile and the relative abundances (to H 2 O) of volatile molecules are the key parameters allowing one to retrieve the chemical composition and thermodynamic conditions of cometary ice formation in the early Solar System. The coming observations of the coma and nucleus by the Rosetta mission instruments (VIRTIS, MIRO, . . .) should provide the necessary constraints to the model to allow it to infer the primordial ice structure and composition of the comet.

First observations of H2O and CO2vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

“…This indicates that CO 2 sublimates from ice sources inside the nucleus at a depth that is below the diurnal skin depth. CO 2 ice sources very close to the surface would imply a dependence of the flux on the illumination condition, as predicted by comet evolution models (de Sanctis et al 2005;Marboeuf & Schmitt 2014). The diurnal skin depth has been estimated to ∼1 cm from MIRO continuum measurements of the subsurface temperature (Schloerb et al 2015).…”

“…The diurnal skin depth has been estimated to ∼1 cm from MIRO continuum measurements of the subsurface temperature (Schloerb et al 2015). CO 2 might be present beneath as pure ice or trapped in amorphous water ice or clathrate hydrates (de Sanctis et al 2005;Marboeuf & Schmitt 2014).…”

“…However, questions arise as to the extent to which relative abundances measured in cometary atmospheres can be used to constrain chemical models of star-forming regions and protoplanetary disks, since they may not be representative of the nucleus' initial composition. Outgassing from cometary nuclei involves complex surface and subsurface processes that depend on the physico-chemical properties of the cometary material and ice structure, as well as on current and past illumination conditions (Huebner et al 2006;De Sanctis et al 2010;Marboeuf & Schmitt 2014). In addition, compositional inhomogeneities and sublimation from icy grains may affect our understanding of coma composition.…”

A mini outburst from the nightside of comet 67P/Churyumov-Gerasimenko observed by the OSIRIS camera on Rosetta

“…The latter scenario was favored by Belton (2008) to explain mini-outbursts observed in 9P/Tempel-1 but since we found a non-explosive outburst the former mechanism might apply in our case. Recent work on thermal comet evolution models using lower thermal conductivity values as usually employed in the past find that the crystallization front may be located at rather shallow depths of the order of 0.5 to several meters (Marboeuf & Schmitt 2014) and also might be locally inhomogeneous dependent on the initial composition and structure of the nucleus (Rosenberg & Prialnik 2010). These findings make this explanation more attractive in our case because it is difficult to imagine that such a small event has its origin deep inside the nucleus as would be required by older calculations of the depth of the crystallization front.…”

Activity of Comet C/2006 W3 Christensen

“…Some CO molecules could be also trapped in the amorphous H 2 O ice. However, the relative number of molecules possibly stored this way is only about 10% (Schmitt et al, 1989;Marboeuf et al, 2011;Marboeuf and Schmitt, 2014). -We perform simulations for a layer of the nucleus, up to 600 m thick.…”