Cliff collapse on Comet 67P/Churyumov–Gerasimenko – II. Imhotep and Hathor

ABSTRACT Cliff collapses on Comet 67P/Churyumov–Gerasimenko expose relatively pristine nucleus matter and offer rare opportunities to characterize ice-rich comet material. Here, Microwave Instrument for Rosetta Orbiter (MIRO) observations of two collapsed or crumbling cliffs in the Imhotep and Hathor regions have been assembled. The empirical diurnal antenna temperature curves are analysed with thermophysical and radiative transfer models in order to place constraints on the physical properties and degrees of stratification in the near-surface material. The Imhotep site consists of an exposed dust/water-ice mixture with thermal inertia 100–$160\, \mathrm{J\, m^{-2}\, K^{-1}\, s^{-1/2}}$, having sublimating CO2 ice located $11\pm 4\, \mathrm{cm}$ below the surface. Its estimated age is consistent with an outburst observed on 2014 April 27–30. The Hathor site has a $0.8\pm 0.2\, \mathrm{cm}$ dust mantle, a thermal inertia of $40\pm 20\, \mathrm{J\, m^{-2}\, K^{-1}\, s^{-1/2}}$, no CO2 ice to within $1.0\, \mathrm{m}$ depth, and a mantle bulk density of $340\pm 80\, \mathrm{kg\, m^{-3}}$ that is higher than the theoretically expected $180\pm 10\, \mathrm{kg\, m^{-3}}$, suggesting that compression has taken place.