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

The European SpaceAgency 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 throughoutthe 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 mu=1, that airfall material has mu=2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically 2 cm. The average CO2 sublimation front depths near aphelion were 3.8 m and 1.9 m on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverisation due to thermal fatigue during the aphelion passage.We find further evidence that the CO2 vapour pressure is responsible for an observed compaction of near–surface nucleus material, that leads to the formation of consolidated terrain.

欧洲空间局（European Space Agency, ESA）2014至2016年执行的罗塞塔/菲莱（Rosetta/Philae）彗星探测任务，是迄今为止对67P/楚留莫夫-格拉希门克彗星（Comet 67P/Churyumov-Gerasimenko）开展的最全面、最多样化的彗星探测研究。然而，该彗星的诸多物理与化学性质仍存在不确定性或尚未被探明，彗星活动也仍是一个尚未被充分理解的现象。本文尝试对彗核中水冰（H₂O）与二氧化碳冰（CO₂）的丰度以及升华锋面深度进行约束，并重建彗核在近日点飞越期间的演化过程。我们采用热物理建模程序“冰冷小天体演化数值模拟器”（Numerical Icy Minor Body evolUtion Simulator，简称NIMBUS），搜寻能够同时复现近日点前后观测到的H₂O与CO₂产生率的参数条件。研究发现，相对原始的彗核物质的难熔组分与水冰的质量比μ=1，沉降物质的μ=2，且二氧化碳相对于水的摩尔丰度接近30%。尘埃幔的厚度通常为2厘米。远日点附近的平均CO₂升华锋面深度，北半球为3.8米，南半球为1.9米，但该深度随时间发生显著变化。我们提出，沉降物质在远日点飞越期间因热疲劳发生了显著的碎裂与粉化。我们还发现了更多证据，表明CO₂蒸气压是观测到的近地表彗核物质压实现象的诱因，该压实过程最终形成了固结地形。