How solar driven mass loss determines the shape of small icy bodies

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When the Rosetta spacecraft arrived at the Jupiter-family comet 67P/Churyumov–Gerasimenko, it found a nucleus with two unexpected peculiar characteristics: its bilobed shape with a steep cliff about one kilometer high, and the striking dichotomy of large-scale morphological features between the north and south hemispheres (Fig. 1). Meanwhile, the data from the mission enabled us to understand at least some of these manifest characteristics in terms of seasonal insolation changes given the comet’s orbit and spin-axis orientation. The Rosetta data alone cannot determine whether the bilobed shape is primordial or evolutionary. However, these findings and related modeling have reignited scientific discussions on the topic of the formation and evolution of nucleus shapes. It was these developments, both in literature and in many discussions among us and our colleagues, that inspired our initial investigation into the role of solar-driven activities in the overall change of the nucleus shape.

Figure 1: left: A view of the northern-hemisphere of the comet 67P/Churyumov-Gerasimenko. This image was captured by Rosetta's OSIRIS narrow-angle camera on 3 August 2014 from a distance of 285 km. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA right: Clear view of on the southern hemisphere of the comet 67P/C-G taken on the 23rd of January, 2016, from a distance of about 76 km. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Because Jupiter-Family comets, and comets in general, are regarded to be as old as our solar system, we first looked at their shape evolution on a long time scale. Therefore,  we started focusing on the mass loss driven shape evolution of Kuiper Belt Objects (KBOs), a subclass of which can be assumed to maintain dynamically stable orbits for the entire evolution. Simultaneously considering orbit, spin state, and sublimation mass loss due to super volatile ices, known to be existing beyond the orbit of Neptune (CO, CH4, N2), we built the first version of a model, MONET (mass-loss-driven shape evolution). With this tool, we have conducted numerical experiments evaluating how solar driven activities affect the shape of small KBOs under different orientation conditions. We found that even low solar-driven mass loss rates play an important role in shape evolution of small icy bodies over long periods of time (Fig. 2).  For example, small obliquity usually leads to elongated shapes, while large obliquity is more probable to produce flattened ones, and highly eccentric orbit is more likely to cause north-south dichotomy.

Fig. 2 The resultant shape changes from an initially spherical object for different configurations of spin axis obliquity (θ) and longitude of spin-axis (ϕ). The circles mark the size of the original (spherical) body. The semi-major axis and eccentricity of orbit are 200 au and 0.3, respectively. Credit: Zhao et al., MNRAS, (2020)

Therefore, different configurations of orbit eccentricity and spin axis orientation would suggest different shape modification processes, including flattening, elongation and north-south dichotomy. Then, when checking the unusually shaped KBO (486958) Arrokoth observed by NASA’s New Horizons spacecraft (Fig. 3), we immediately recognized that some of our modeling results show strong resemblance to the extremely flattened shape of Arrokoth. The prevailing phenomenological argument to explain the flattening was that the shape is primordial. However, based on our modeling results (MONET package), we proposed a hypothesis that sublimation driven mass loss is a natural mechanism for the formation of Arrokoth’s flattened lobes (Fig. 4). This process most likely occurs early in the evolution history of the body, and could proceed rather quickly, on a timescale of about 1-100 Myr, during the period when super volatile ices on the near subsurface layers existed. In addition, we also self-consistently demonstrate that the mass loss induced torque is negligible in changing the planetesimal's spin state.

Fig. 3 The bilobed and flattened shape of (486958) Arrokoth determined by the New Horizons observation data. Credit: PMO/X. Zheng/Y. Zhao;shape model: NASA/Spencer et al., Science, (2020);  background: Courtesy of Southwest Research Institute and Alex H. Parker

In summary, in this paper we suggest that sublimation mass loss may be a ubiquitous process, and it is the dominant factor in shaping the structure of KBOs, on the premise that there is no catastrophic collision reshaping the body in their later history. Furthermore, while cold classical KBOs reserve their shape sculptured by early outgassing, the structures of Centaurs and JFCs would be further modified by the same scenario once they enter their current orbit configuration from the Kuiper Belt, under sublimation of different volatile species.

Fig. 4 Mass loss driven shape evolution of Arrokoth analogues. Credit: PMO/X. Zhang

Yuhui Zhao

Associate Professor, Purple Mountain Observatory, Chinese Academy of Sciences

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