The Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided new data to better understand what comets are made of. The weak tensile strength of the cometary surface materials suggests that the comet is a hierarchical dust aggregate formed through gravitational collapse of a bound clump of small dust aggregates so-called ``pebbles'' in the gaseous solar nebula. Since pebbles are the building blocks of comets, which are the survivors of planetesimals in the solar nebula, estimating the size of pebbles using a combination of thermal observations and numerical calculations is of great importance to understand the planet formation in the outer solar system. In this study, we calculated the thermal inertias and thermal skin depths of the hierarchical aggregates of pebbles, for both diurnal and orbital variations of the temperature. We found that the thermal inertias of the comet 67P/Churyumov-Gerasimenko are consistent with the hierarchical aggregate of cm- to dm-sized pebbles. Our findings indicate that the icy planetesimals may have formed via accretion of cm- to dm-sized pebbles in the solar nebula.

中文翻译：

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卵石堆彗星 67P/Churyumov–Gerasimenko 的热惯性

Rosetta 对 67P/Churyumov-Gerasimenko 彗星的任务提供了新数据，以更好地了解彗星的构成。彗星表面材料的弱抗拉强度表明，彗星是一种分级尘埃聚集体，是由气态太阳星云中所谓的“鹅卵石”的小尘埃聚集体的束缚团块重力坍塌形成的。由于鹅卵石是彗星的组成部分，彗星是太阳星云中小行星的幸存者，因此结合热观测和数值计算来估计鹅卵石的大小对于了解外太阳系行星的形成非常重要。在这项研究中，我们针对温度的昼夜和轨道变化计算了卵石分层聚集体的热惯性和热趋肤深度。我们发现 67P/Churyumov-Gerasimenko 彗星的热惯性与 cm 到 dm 大小的鹅卵石的分层聚合一致。我们的研究结果表明，冰冷的星子可能是通过太阳星云中厘米到分米大小的鹅卵石的吸积形成的。