Thin regolith layer anticipated on the surface of the Tianwen-2 target asteroid (469219) Kamo‘oalewa

Context. Near-Earth asteroid (469219) Kamo‘oalewa is the first target of the ongoing Tianwen-2 sample-return mission. Ground-based observations measured its rotation period at only about 28 minutes, while the low thermal inertia value (Γ = 150 or 181 J m−2 K−1 s−1/2) estimated from the Yarkovsky effect determination implies a thermal insulating layer on the surface. Aims. The first goal of this paper is to facilitate the sample collection of the Tianwen-2 mission with a prediction of the current status of the possible regolith layer on Kamo‘oalewa. The second goal is to build a framework of regolith evolution on small fast-rotating asteroids as a reference for future observations and studies. Methods. We used finite element analysis software, Abaqus, to simulate the thermal stress field of Kamo‘oalewa under seasonal and diurnal temperature cycles, with different thicknesses (H) of the regolith layer. At each latitude, the stress excursion at the top of the bedrock as a function of H was incorporated into the ordinary differential equations for regolith total mass and grain size distribution evolutions. The negative feedback between H and thermal fragmentation rate was considered together with regolith removal processes of micro-impact ejecta escape and electrostatic dust lofting in the ordinary differential equations. Results. The competition between the regolith production and removal processes results in an equilibrium regolith thickness, which increases with latitude due to the seasonal temperature and stress cycles. Under different choices of materials and obliquities and a range of possible fragmentation rates, the equilibrium thickness, H, is estimated to be 0.4–71 mm, equivalent to 0.3–53.5 diurnal regolith thermal skin depth. Our prediction of the equilibrium H satisfies both constraints from thermal inertia observations and cohesive strength estimations on airless bodies.