Crack Orientation of Boulders on Ryugu: Meridional Preference and Exfoliation

Abstract

Hayabusa 2 spacecraft revealed that a small carbonaceous asteroid 162173 Ryugu is a rubble pile and that its surface is covered with various sizes of boulders. A few percent of surface boulders have cracks. They are classified into straight cracks, sinuous cracks, arrested (incomplete) cracks, and complex (branched) cracks. We analyzed 538 boulders (777 cracks) and found 60% of their cracks have the meridional direction (±15deg from N-S) except complex cracks. Impacts or surface mass movement cannot have produced this preferred crack orientation. Thermal stress from solar heating would have grown cracks to meridional direction. Another feature of thermal stress – exfoliation – is also observed on the surface of small boulders of Ryugu as of Bennu.