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

Cliff collapse on Comet 67P/Churyumov-Gerasimenko exposes relatively pristine nucleus material and offer rare opportunities to characterise ice-rich comet material. Here, Rosetta/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 MKS, having sublimating CO2 ice located 11+/-4 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+/-0.2 cm dust mantle, a thermal inertia of 40+/-20MKS, no CO2 ice to within 1.0m depth, and a mantle bulk density of 340+/-80 kg m^-3 that is higher than the theoretically expected 180+/-10 kg m^-3, suggesting that compression has taken place.