Irradiation-energy Dependence on the Spectral Changes of Hydrous C-Type Asteroids Based on 4kev and 20kev He Exposure Experiments of Murchison Cm Chondrite

Abstract

C-type asteroid 162173 Ryugu was observed by remote sensing apparatus onboard Hayabusa2 spacecraft and found to be very dark object whose reflectance is (1.60 0.15) % at 0.55m and showed a small 2.7m absorption band indicative of phyllosilicates. The optical navigation camera detected color variations of Ryugus surface in the wavelength range from 0.4 to 0.95m: Bluer spectra are ob-served at both poles and on the equatorial ridge, both of which are topographic highs and thus may be fresh material exposed by gradual erosion. On the other hand, many locations at middle-latitude areas exhibit redder and darker colors. Similar color variations are also detected in the near-infrared wavelength range. These observations suggest that a surface-correlated process is responsible for the color variation, most prob-ably from blue to red, but the mechanism for the change is not yet identified. Space weathering is one possible mechanism responsible for the color variation, but the spectral changes of C-type asteroids from space weathering are far from being fully understood. Past experimental studies using hydrous carbonaceous chondrites such as Murchison and Tagish Lake show that He exposure (simulating solar wind irradiation) changes spectra to bluer and brighter. Recently our He exposure experiments indicate that spectral changes depend on physical properties such as porosity of exposed material. In this study, we per-formed further He exposure experiments using Murchison CM chondrite in order to understand energy dependence on the spectral changes. We found that He energy is a critical parameter, as well as physical properties of the samples, that affects spectral changes of space weathering of hydrated C-type asteroids