[P-PS03] Small Solar System Bodies: New perspectives on the origin and evolution of the Solar System

[PPS03-05] Thermal evolution model of porous cometary nuclei considering contraction associated with ice sublimation

*Hitoshi Miura¹, Takumi Yasuda² (1.Graduate School of Science, Department of Information and Basic Science, Nagoya City University, 2.MHI Aerospace Systems Corp.)

Keywords:Comet, Asteroid, Comet–asteroid transition object, long-term thermal evolusion, Ice sublimation

There are various small celestial bodies in the solar system, such as comets and asteroids. In the classical classification, comets were thought to have formed outside the orbit of Neptune, were mainly composed of water ice, and when they approached the sun to exhibit a tail and a coma due to the sublimation of the ice that made them up (comet activity). In contrary, asteroids are thought to have formed inside the orbit of Jupiter, mainly composed of rocks, and do not exhibit comet-like activity. However, in recent years, as celestial bodies that possess both characteristics have been discovered, it has become thought that the distinction between the two is extremely vague [1]. One of the reasons is the evolution of comets into asteroids due to solar heating. When a comet approaches the sun and is heated, the ice inside sublimes. Some of the rocks contained in the nucleus are caught up in the vapor flow and released into space together, but rocks of a certain size are held back by the comet's gravity and accumulate on its surface to form a dust mantle. Because the dust mantle is porous, the water vapor permeates the dust mantle and flows out into space. The cometary nucleus loses its support as the internal ice sublimates, and the nucleus contracts towards the center. Eventually, the nucleus loses its internal ice completely to transform an asteroid-like body. It is important to evaluate the timescale for a comet to lose its internal ice (desiccation time) is important for studying the formation and evolution of small solar system bodies. Schörghofer and Hsieh [2] and Yu et al. [3] analytically derived the desiccation time as a function of orbital elements. However, these models did not take into account the contraction of the cometary nucleus due to ice sublimation. Miura et al. [4] examined a theoretical model that took into account the contraction, and showed that Ryugu, an asteroid with a radius of approximately 440 m, could have evolved from a cometary nucleus with a radius of 1.2 km over a period of several tens of thousands of years. However, this model assumed that the internal temperature is uniform and constant, and did not take thermal evolution into account. In this study, we propose a new long-term thermal evolution model that takes into account the contraction of the cometary nucleus due to ice sublimation, and evaluates the desiccation time as a function of orbital elements.

First, numerical calculations of the thermal evolution of cometary nuclei in elliptical orbits were performed, taking into account the seasonal variation in solar heating rates, and numerical solutions for the desiccation time were obtained for various orbital elements. Next, the desiccation time was derived analytically as a function of orbital elements based on an analytical model that takes into account the seasonal-averaged solar heating rate. The numerical and analytical solutions for the desiccation time were compared, and the conditions under which the analytical model is applicable were clarified. Furthermore, based on the assumed amount of ice remaining inside the comet, we derived analytical solutions for estimating the emission rates of water vapor and rocks on the surface of the cometary nucleus, the maximum size of the emitted rocks, and the emission velocity. Using these analytical solutions, we considered the internal structure and evolution process of some comet–asteroid transition objects. Our analytical model was generally consistent with the results of previous observations. Our theoretical model provides a theoretical guideline for discussing the evolution of comet nuclei and the possibility of retaining internal ice in asteroids.

References: [1] H. H. Hsieh (2017), Philos. Trans. R. Soc. A 375, 20160259. [2] N. Schörghofer and H. H. Hsieh (2018), J. Geophys. Res: Planets 123, 2322. [3] L. L. Yu et al. (2019), Mon. Not. R. Astron. Soc. 482, 4243. [4] H. Miura et al. (2022), Astrophys. J. Lett. 925:L15.