[P-PS04] 火星と火星衛星

[PPS04-P02] Slope Distributions of Asteroids and Satellites: Implications for Surface Materials of Phobos and Deimos

*清水 俊輔¹、逸見 良道¹、宮本 英昭¹ (1.東京大学)

キーワード：フォボス、ダイモス、傾斜、形状モデル

The Martian Moons eXploration (MMX) mission, scheduled to be launched in 2024, will explore the two Martian satellites, Phobos and Deimos, whose origins and evolutional histories remain unclear even after observations through previous Mars missions. The MMX spacecraft will perform proximity observations and return samples to obtain crucial information about its history [1]. The surface conditions of Phobos (e.g., crater distribution, boulder distribution, surface roughness, regolith scattering properties, etc.) should be appropriately estimated for landing safety and landing-site selection before acquiring samples. This work tries to obtain such kind of information from the slope distributions of Phobos and Deimos. While determining a slope at a specific location of a celestial body is challenging without high-resolution images and gravity measurements, we assume that their gravitational acceleration can be roughly estimated from numerical shape models [e.g., 2] with a constant-density approximation. The derived slope distributions at a global scale could be expected to reflect the bodies’ geophysical properties, which may relate to their evolutional histories [3]. We use the currently-available highest-resolution shape models of fifteen bodies, including asteroids, comets, and satellites. A slope is defined as the angle between a normal vector of a facet of a shape model and the sum of the gravitational and centrifugal accelerations calculated from the model with an assumed constant density [e.g., 4, 5]. As a result, we find that the frequency distributions of slopes of Phobos and Deimos are similar to, respectively, those of Phoebe and Vesta and that of Janus. In addition, a significant difference is observed between the red and blue units of Phobos, which may come from either a compositional contrast or geological histories. We also find that the frequency distribution of slopes of Deimos’s northern hemisphere significantly differs from that of the southern hemisphere, where the largest depression exists. This could imply heterogeneous distributions of surface materials or inhomogeneous evolutional history.

References
[1] Kuramoto et al., Earth, Planets and Space 74, 12 (2022). [2] Ernst et al. (2018), LPSC, 2018. [3] Murdoch et al., Asteroid IV, 767-792 (2015). [4] Watanabe et al., Science 364, 268-272 (2019). [5] Scheeres et al., Nature Astronomy 3, 352-361 (2019).