Investigation of boulder distribution in (1) Ceres and insight into its surface evolution

Abstract

The surface conditions of terrestrial bodies strongly reflect their geological evolutionary processes and vary among various terrestrial bodies. This diversity is attributed to variations in the timescales of boulder formation through processes such as impact cratering, rockfalls from crater walls, seismic motion, and boulder fragmentation caused by micrometeoroid impacts and thermal stress. In this study, we examined boulders on Ceres using high-resolution images with a resolution of approximately 5 m/px obtained during the Ceres Extended Mission 2 Orbit 7 of the Dawn mission. Almost all boulders were present around impact craters, even at a resolution of 5 m/px, thus indicating that the boulders on Ceres were created by impact cratering alone. The maximum boulder size on Ceres is approximately 200 m, even around large craters, which may indicate the upper size limit determined by the mechanical strength of the boulders, such as the tensile strength, the scale effect, and/or shattering strength. The slope of the size-frequency distribution of boulders on Ceres varied significantly across the range of boulder sizes, thus making it difficult to describe it using a single function of a power-law relationship; in particular, it changed at approximately 100 m, thus indicating that destructive or formation mechanisms may be different for large boulders >100 m and for small boulders <100 m. There may also be a subsurface structure that prevents the formation of small boulders, although this is difficult to argue conclusively. We estimated that the lifetime of boulders larger than 50 m was equivalent to or shorter than 100 Myr. This lifetime is consistent with a theoretical estimation assuming that micrometeoroid impacts are the primary destructive mechanism.