Low thermal inertia of (162173) Ryugu a result of horizontal cracks in boulders

Abstract

Observations of the diurnal variations of the surface temperature of asteroid (162173) Ryugu from orbit and on the surface were performed by the Haybusa2 spacecraft and MASCOT lander. A low thermal inertia of the boulders on Ryugu was derived from these temperature variations and interpreted as the consequence of high porosity. Samples of Ryugu returned to Earth by Hayabusa2 showed higher thermal inertia when investigated by microscopic thermography. Here, we apply a simple thermal model, considering a horizontal fracture interrupting the heat flow into the surface, and investigate whether the low thermal inertia of Ryugu's boulders might be caused by fractures rather than high porosity. We find that the diurnal temperature variations on Ryugu observed by MARA can be partially explained by introducing a single horizontal crack at 9 mm depth below the surface observed by MARA.