Laboratory studies on the 3 μm spectral features of Mg-rich phyllosilicates and insights for the interpretation of asteroid Ryugu surface spectra

Abstract

The Near Infrared Spectrometer (NIRS3) on Hayabusa 2 mission detected a weak and narrow absorption feature centered at 2.72 µm across the entire observed surface of the asteroid 162173 Ryugu [1]. The presence of an absorption feature at 2.72 µm is indicative of the presence of (OH)-bearing minerals. Furthermore the position of the observed peak can be likely associated to that of the Mg-OH features, as in Mg-rich phyllosilicates. [1] However, the collected spectra from the Ryugu surface show no other absorption features in the 3 µm region. It has been suggested that thermal alteration processes, can darken the surfaces of carbonaceous chondrites, thus decreasing the reflectance values around 3 µm. In addition, thermal alteration processes, have been taken into account to explain the formation of Ryugu asteroid [2]. To investigate on this point and to check the behavior of the spectral features around 3 µm with thermal alteration, we performed laboratory experiments on two Mg-rich phyllosilicates (serpentine and saponite). In particular, we studied two different situations: 1) thermal alteration at increasing T - the samples were heated at different steps of 100°C, starting from 100°C up to 700°C, for 4 hours each; 2) long time heating at constant T - samples were heated at constant T~250°C for 1 month (1th step) and then for 2 months (2nd step).