Abstract
Context.The JAXA asteroid sample return mission Hayabusa2 acquired a huge quantity of data from the asteroid (162173) Ryugu during its 1.5 years in asteroid proximity orbit. On December 5, 2020 (Japan time), Hayabusa brought back to Earth a 5.4 g sample from Ryugu’s surface.Aims.We analyzed the near-infrared spectra of Ryugu, in particular the band at 2.72 μm, with the aim to investigate the hydrogen content of the H2O and OH−groups in hydrated phyllosilicates on Ryugu’s surface.Aims.We applied two different methods, normalized optical path length (NOPL) and effective single-particle absorption thickness (ESPAT), to the 3 μm region absorption band, and we compared the obtained spectral parameters with those obtained from carbonaceous chondrite meteorites whose H content was determined in the laboratory.Methods.We derived an exponential correlation between the selected meteorite H content and its respective ESPAT and NOPL parameters. The average value of the H content obtained on Ryugu’s surface with its relative variations, combining the results obtained with the two methods, is 0.52−0.21+0.16wt.%. These methods can be applied to other asteroids that exhibit a 3 μm region absorption band to estimate the mean average of H content.Results.The results of the ESPAT and NOPL methods used on the Ryugu spectral data present small variations across Ryugu’s surface and do not show any evident relation with the surface geomorphological structures. Our estimation of the global average H content of Ryugu is in agreement with those of several aqueously altered carbonaceous chondrites measured in the laboratory and is most similar to the H content of heated CM. The study of phyllosilicate H2O and OH−group hydrogen content on Ryugu and the derived method may be applied to other observed primitive asteroids. The obtained results will allow Solar System evolution models to be constrained and will allow the formation and evolution of the Solar System to be better understood.
Highlights
In late June 2018, Hayabusa2, the Japanese Aerospace Exploration Agency (JAXA) spacecraft for the asteroid sample return mission, arrived at home position (HP), located at an altitude of about 20 km from Ryugu’s surface, in a hovering position orbit. It acquired a large quantity of data of the near-Earth asteroid (162173) Ryugu using several instruments: the Optical Navigation Camera Telescope (ONC-T), a thermal infrared imager (TIR), a near-infrared spectrometer (NIRS3), and a laser light detection and ranging (LIDAR) system (Watanabe et al 2019)
The absorption band indicates the presence of OH-bearing minerals, associated with Mg-rich phyllosilicates (Kitazato et al 2019) that are ubiquitously present on the surface of Ryugu
Lower H content meteorites are characterized by low normalized optical path length (NOPL) as well as low effective single-particle absorption thickness (ESPAT), and vice versa
Summary
In late June 2018, Hayabusa, the Japanese Aerospace Exploration Agency (JAXA) spacecraft for the asteroid sample return mission, arrived at home position (HP), located at an altitude of about 20 km from Ryugu’s surface, in a hovering position orbit. It acquired a large quantity of data of the near-Earth asteroid (162173) Ryugu using several instruments: the Optical Navigation Camera Telescope (ONC-T), a thermal infrared imager (TIR), a near-infrared spectrometer (NIRS3), and a laser light detection and ranging (LIDAR) system (Watanabe et al 2019). No meteoritic samples with spectra available at NIRS3 wavelengths seem to fully match those of Ryugu, but there are similarities
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