Estimation of the particle-size distributions of regolith on the surface of (25143) Itokawa based on near-infrared reflectance spectra

Aims . We used near-infrared reflectance spectra to infer the particle-size distributions of regolith on asteroid Itokawa to provide a potential alternative to the traditional thermal infrared method. Methods . We calculated the model reflectance spectra of LL5 and LL6 chondrite particles with varying sizes and took space weathering effects into account. The model spectra were then compared with the Itokawa near-infrared spectra to estimate the particlesize distributions of its regolith. In addition, we performed a spectral feature analysis and retrieved the mineral content to validate the results of the size retrieval. Results . The average regolith size we retrieved based on all near-infrared spectrometer data is 25-310 μm, and the results reveal a clear dichotomy: The particle sizes in flat and rough terrains are approximately 10-100 μm and 50-500 μm, respectively. The spectral features agree with the topography features of Itokawa, and the olivine contents of different regions are quite uniform. These results reveal that the spectral variations in different regions of Itokawa are mainly caused by variations in the grain size of the regolith and not by its mineral compositions. Conclusions . Near-infrared reflectance spectroscopy may be a promising tool for estimating the particle-size distributions of regolith on planetary bodies such as asteroid Itokawa.