Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS08] Planetary Sciences

Mon. May 21, 2018 9:00 AM - 10:30 AM International Conference Room (IC) (2F International Conference Hall, Makuhari Messe)

convener:Takaya Okamoto(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency), Kenji Kurosaki(Department of Physics, Nagoya University), Chairperson:Morota Tomokatsu(名古屋大学大学院 環境学研究科), Hong Peng(千葉工業大学 惑星探査研究センター)

10:00 AM - 10:15 AM

[PPS08-11] Topographic degradation of craters on Phobos and the comparison with other small bodies

Seiya Morita1, *Tomokatsu Morota1, Sei-ichiro WATANABE1 (1.Graduate School of Environmental Studies, Nagoya University)

Keywords:Phobos, crater, Topographic degradation

The origin of the two Mars satellites Phobos and Deimos is controversial and evolution thereafter is also unclear. A previous numerical study showed that nearly all impact fragments ejected from the inner moon Phobos remain trapped in Mars orbits until re-impact with Phobos. Such ejecta re-accumulation may accelerate topographic degradation of Phobos craters because this process increases the number of small impacts on Phobos. It is expected that, therefore, the degree of crater degradation on Phobos is higher than other small bodies if Phobos has been in the Mars orbit throughout the geologic history of Phobos. In this study, we performed a crater shape analysis based on a topographic diffusion model and estimated the degradation model ages κt (where κ is the diffusivity and t is the crater age) of craters larger than 1 km in radius on Phobos. In addition, similar analysis was performed on craters of the near Earth asteroid Eros, which has the mean diameter and the surface gravity comparable to Phobos.

We find that the distributions of κt are different between these small bodies. The medians of κt are estimated to be (1.4±0.1) ×105 m2 for Phobos, and (4.4±1.0) ×104 m2 for Eros, suggesting that the Phobos craters are more degraded than the Eros craters.
The observed high crater degradation rate of Phobos, which is about 3 times higher than that of Eros and the corrected that of Lutetia, implies that the ejecta re-accumulation has occurred throughout the geologic history of Phobos. To evaluate the effect of ejecta re-accumulation, we calculated the total number of craters generated by re-impacts of ejecta from all Phobos craters larger than 1 km in radius. The result shows that the total number of re-impact craters is 2–10 times of the primary impacts, suggests that the high crater degradation rate of Phobos can be explained by the ejecta re-accumulation.