Modelling the water and carbon dioxide production rates of Comet 67P/Churyumov–Gerasimenko

Abstract

The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov–Gerasimenko in 2014–2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well-understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code ‘Numerical Icy Minor Body evolUtion Simulator’, or nimbus, to search for conditions under which the observed H2O and CO2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water–ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically $\lesssim 2\, \mathrm{cm}$. The average CO2 sublimation front depths near aphelion were $\sim 3.8\, \mathrm{m}$ and $\sim 1.9\, \mathrm{m}$ on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverization due to thermal fatigue during the aphelion passage. Sub-surface compaction of material due to CO2 activity near perihelion seems to have reduced the diffusivity in a measurable way.

Airfall on Comet 67P/Churyumov-Gerasimenko

“…We now consider the presence and survival of CO 2 ice in coma chunks. Davidsson et al (2020) reproduced the observed pre-perihelion CO 2 production rate curve of 67P from 3.5 AU to 1.24 AU with a NIMBUS model that had the perihelion CO 2 sublimation front located ∼ 1.9 m below the surface near the equator, and where that depth diminished with latitude to ∼ 0.2 m at the south pole. If the suggested presence of CO 2 ice at shallow depths is correct, it is conceivable that regular sublimation-driven activity occasionally can expel extremely cold chunks containing supervolatiles at perihelion.…”

“…Alternatively, they may have originated from underneath the airfall layer. NIMBUS simulations by Davidsson et al (2020) show that the measured pre-perihelion CO 2 production rate curve is reproduced when the depth of the CO 2 sublimation front is located 3.8 m below the surface on the northern hemisphere, and at 1.9 m below the surface on the southern hemisphere at aphelion (these depths are modified in the model over time because of dust mantle erosion as well as CO 2 sublimation, see Sec. 3.2).…”

A SUBLIME 3D Model for Cometary Coma Emission: the Hypervolatile-Rich Comet C/2016 R2 (PanSTARRS)

“…Migliorini, A. et al 2016;Gasc et al 2017) discovered spatial and temporal variations in the coma H 2 O/CO 2 ratios, implying non-uniform mixing ratios for these two volatiles within the nucleus. As shown by Davidsson et al (2021Davidsson et al ( , 2022 for 67P, this could be caused by physical/chemical evolution of the comet's surface layers due to anisotropic illumination of the comet, but primordial variations in the H 2 O/CO 2 ratio intrinsic to the nucleus are also possible. It is therefore plausible that CH 3 OH in C/2016 R2 exists as a component within an apolar ice matrix dominated by CO and CO 2 , and some of this CH 3 OH is released into the coma when CO sublimates (as observed).…”

Comets beyond 4 au: How pristine are Oort nuclei?

“…Most thermophysical models of nuclei rely on the assumption of dry mantles (where water-ice sublimation is assumed to be negligible) enveloping nuclei rich in ices (Keller et al 2015 ;Davidsson et al 2022 ). Ho we ver, a dry crust or mantle where water-ice sublimation is assumed to be negligible is inconsistent with the definition itself of a comet, i.e.…”

“…Also the observed evolution of the nucleus colour of Comet 67P/Churyumov-Gerasimenko excludes the presence of any ices-depleted crust (Ciarniello et al 2022 ). Models based on a crust where water-ice sublimation is assumed to be negligible (Davidsson et al 2022 ) are inconsistent with the measured 67P Hapi's erosion (Cambianica et al 2020 ), so that they cannot constrain the nucleus refractory-to-water-ice mass ratio δ w . They also require ad-hoc changes of the thickness of the dry crust enveloping the entire nucleus (Skorov et al 2020 ) to fit the observed time evolution of the gas loss rates (L äuter et al 2020 ).…”