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

The Aswan cliff on Comet 67P/Churyumov-Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of ~12m was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/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 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 ~6 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+/-6 cm; 4) a thermal inertia ranging 10-45 MKS; 5) a gas diffusivity that decreased from 0.1 m^2 s^-1 to 0.001 m^2 s^-1; 6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a firstempirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating.

67P/楚留莫夫-格拉希门克彗星（Comet 67P/Churyumov-Gerasimenko）上的阿斯旺悬崖于2015年7月10日发生坍塌，由此使得约12米深度处近乎原始的彗星物质暴露于彗星表面。罗塞塔/MIRO微波仪器对该坍塌位点的观测数据已完成采集，覆盖时段包括坍塌前8个月，以及坍塌后5、7和11个月。研究团队采用热物理模型与辐射传输模型对上述数据开展分析：坍塌前的观测结果与厚度至少3厘米的30 MKS热惯量尘埃幔层模型相吻合；坍塌后的数据则与以下结论一致：1）尘埃/水冰质量比为0.9±0.5，且相对于水的二氧化碳摩尔丰度约为30%；2）约6个月后形成了厚度为数毫米或其分数级厚度的尘埃幔层；3）二氧化碳冰升华锋面从0.4厘米处逐步后退至2.0厘米，后续又后退至20±6厘米处；4）热惯量范围为10~45 MKS；5）气体扩散率从0.1 m² s⁻¹降至0.001 m² s⁻¹；6）部分时段存在固态温室效应。本次观测数据与分析结果首次提供了实证视角，揭示了富冰彗星物质暴露于太阳加热环境时的老化与演化过程。