No abstract

The near-Earth asteroid (162173) Ryugu is thought to be a primitive carbonaceous object that contains hydrated minerals and organic molecules. We report sample collection from Ryugu’s surface by the Hayabusa2 spacecraft on 21 February 2019. Touchdown images and global observations of surface colors are used to investigate the stratigraphy of the surface around the sample location and across Ryugu. Latitudinal color variations suggest the reddening of exposed surface material by solar heating and/or space weathering. Immediately after touchdown, Hayabusa2’s thrusters disturbed dark, fine grains that originate from the redder materials. The stratigraphic relationship between identified craters and the redder material indicates that surface reddening occurred over a short period of time. We suggest that Ryugu previously experienced an orbital excursion near the Sun.

An experimental apparatus measuring free-surface deformation of a centrifuged granular pile is developed. By horizontally rotating a quasi two-dimensional granular pile whose apex is located at the vertical rotation axis, the resultant force of gravity and centrifuge yields the deformation of the granular pile. In this setup, centrifugal force depends on distance from the rotation axis whilst gravitational force is constant everywhere. Therefore, free-surface deformation by various centrifuge degrees can be systematically examined using this apparatus. In the system, a small unit consisting of a camera and computer is rotated with the granular sample to record the rotation-induced deformation. To evaluate the validity of the system, deformation of a rotated water surface is first measured and analyzed. The obtained data are properly explained by the theoretical parabolas without any fitting parameter. Next, we measure the deformation of non-cohesive and cohesive granular piles using the developed apparatus. Both granular samples show the significant deformation of granular pile and finally develop steep granular slopes on the side walls. However, details of the deformation processes depend on the cohesion strength. To quantitatively characterize the difference, the effective strength by cohesion and granular local-slope variations are analyzed based on the experimental results.

No abstract

No abstract