Fig. 8. Conceptual view of surface and sub-surface physico-chemical features along with regolith depth.

Conceptual diagram of the effects of thermal cycling in the diurnal (i.e., variation in daytime and nighttime for max. and min. temperatures; ~100 °C)⁴ and seasonal illumination cycles, which are constant exposure processes by photon and cosmic ray irradiation on the surface and subsurface regolith at Ryugu. The purple and orange arrows indicate ultraviolet and cosmic rays, respectively, and the length of each line is a qualitative model of the penetration depth into the regolith. Kitazato et al.¹⁹ estimated that the current seasonal thermal skin depth of Ryugu is up to ~1.1 metre by the model simulation, whereas the total accumulated heat storage within the daily thermal skin depth is estimated to be ~ few centimeter scale below the surface layer⁵⁷.