Impact Experiment on Asteroid (162173) Ryugu: Structure beneath the Impact Point Revealed by In Situ Observations of the Ejecta Curtain

An impact experiment was performed on the surface of the C-type asteroid (162173) Ryugu using an instrument called the Small Carry-on Impactor (SCI) carried by the Japan Aerospace Exploration Agency spacecraft Hayabusa2. Using a small camera separated from the spacecraft, we observed the development of the ejecta curtain formed by the SCI impact in situ. Based on the patterns appearing in the ejecta curtain, we evaluated the physical properties of large boulders and regolith grains on Ryugu. We found that the large boulders on the surface near the SCI impact point had a compressive strength consistent with the previous estimation of a tensile strength based on low thermal inertia observed for boulders on Ryugu. Furthermore, the ejecta curtain consisted of the boulders with a maximum size of several decimeters. Moreover, the filament structure was formed in the SCI ejecta curtain, strongly suggesting that the SCI impact ejecta curtain particles had a size range greater than one order of magnitude; a characteristic size of the regolith grains in the curtain was estimated to be several centimeters. These regolith grains primarily originated from the subsurface layer. We propose three different hypotheses for the geological process that formed the subsurface structure at the SCI impact point consistent with the above observations. Because these hypotheses have different predictions for materials in SCI ejecta, the samples obtained from the second touchdown will be able to constrain which hypothesis is the most likely.