Space Weathering Affects the Remote Near-IR Identification of Phyllosilicates

Abstract

Abstract Near-infrared (NIR) spectrometers on board current sample return missions Hayabusa2 and the Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer (OSIRIS-REx) from primitive bodies detected the presence of hydrated silicates on the surface of asteroids Ryugu and Bennu, respectively. These detections relied upon the study of the 2.7 μm OH-stretching spectral feature, whose peak position is related to the composition and structure of minerals. However, space weathering might alter the band profile, depth and position, thus complicating the interpretation of remote sensing data. In order to better understand these processes and provide support to space missions, we performed ion bombardment experiments on serpentine and saponite analogs. These two phyllosilicates are among the dominant mineral phases found in hydrated carbonaceous chondrites, which are possible analogs to surface materials observed on these primitive asteroids. We studied the behavior of the 2.7 μm band as a function of ion fluence and found that the evolution of the phyllosilicate depends on its nature. For the saponite sample, the band is only slightly affected by ion bombardment, while for both serpentine samples it shifts toward longer wavelengths. For both samples, peak intensity and width is not strongly affected. The band shift for serpentine indicates that space weathering introduces a bias in the interpretation of NIR remote sensing observations of phyllosilicates. The shift observed in our experiments can be detected by instruments on board Hayabusa2 and OSIRIS-REx, depending on the geometry of observation. Our findings provide support to the interpretation of such data.