Cliff collapse on Comet 67P/Churyumov–Gerasimenko – I. Aswan

Abstract

The Aswan cliff on Comet 67P/Churyumov–Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of $\sim 12\, \mathrm{m}$ was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/Microwave Instrument for Rosetta Orbiter (MIRO) have been retrieved from 8 months prior to collapse, as well as from 5, 7, and 11 months post-collapse. The MIRO data are analysed with thermophysical and radiative transfer models. The pre-collapse observations are consistent with a 30 MKS thermal inertia dust mantle with a thickness of at least $3\, \mathrm{cm}$. The post-collapse data are consistent with (1) a dust/water–ice mass ratio of 0.9 ± 0.5 and a molar CO2 abundance of ∼30 per cent relative to water; (2) formation of a dust mantle after ∼7 months, having a thickness of a few millimetres or a fraction thereof; (3) a CO2 ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to $20\pm 6\, \mathrm{cm}$; (4) a thermal inertia ranging 10–45 MKS; (5) a gas diffusivity that decreased from $0.1$ to $0.001\, \mathrm{m^2\, s^{-1}}$; and (6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a first empirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating.