Letter to the Editor

Hydrogen abundance estimation model and application to (162173) Ryugu

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, UPMC Univ. Paris 06, Sorbonne Universités, 5 place Jules Janssen, 92195 Meudon, France
e-mail: alice.praet@obspm.fr
² The University of Aizu, Aizu-Wakamatsu 965-8580, Japan
³ ISAS, JAXA, Sagamihara 252-5210, Japan
⁴ Planetary Science Institute, Tucson, AZ 85710, USA
⁵ Department of Physics, Ithaca College, Ithaca, NY 14850, USA

Received: 26 March 2021
Accepted: 16 April 2021

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^+0.16 wt.%. 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.