Thermal properties of asteroid Ryugu from global, local, and micro-scale and its formation scenario

Thermal properties of C-type asteroid Ryugu have been investigated through remote sensing using the Thermal Infrared Imager (TIR), on the surface using the radiometer MARA on MASCOT lander, and the analysis of return sample. The global average and the local distribution of thermal inertia were mapped by TIR observations, with the lower thermal inertia than that of typical chabonaceous chondrite meteorites. surface boulders and their surroudings have almost the same thermal inertia of 200 to 400 J m^-2 kg^-1 s-^0.5 (tiu, hereafter), indicating that most of boulders are relatively porous and not completely consolidated and the surroundings are covered with boulders and rocks (not sandy regolith) [Okada et al., 2020), which was confirmed during the descent operations for sampling. Boulders have a variety of thermal inertia, with more than 80 % of boulders with 200 to 400 tiu, while some portions have very low or very high thermal inertias. They are identified as Hot Spots and Cold Spors, because they are exceptionally hot or cold compared with their surroundings (Sakatani et al., 2021). The surface experiment by MARA indicated the simiar thermal inertia for a single boulder (Grott et al., 2019), so that the rough boulders should be the representative material on the asdteroid. The thermal properties of return sample do not seem to be the same, although they are not always representative regaring the physical properties since fragile sample might have been broken during the impact sampling process, during the severe shock and shaking in the return capsule when entry to the Earth surface. The return sample dseem to be more consolidated and flatter surface feature, instead of fragile and porous features. The return sample are more like CI chondrite meteorites with darker, more porous and fragile characteristics (Tada et al., 2021).  The difference of thermal inertia between la arger scale (> 1mm)  and a small scale (<0.1ｍｍ）might attribute to the existing cracks and pores inside of boulder materials. A formation scenario of Ryugu will be shown to explain the history of Ryugu formation or planetary formation.