Space Weathering Affects the Remote Near-IR Identification of Phyllosilicates

Stefano Rubino,Ferenc Borondics,Hugues Leroux,Rosario Brunetto,Cateline Lantz,Donia Baklouti

doi:10.3847/psj/abb94c

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.

Highlights

The study of hydrated silicates in small bodies, as tracers of aqueous alteration, is of utmost importance for understanding the evolution of primitive materials in the early solar system
The peak position of the 2.7 μm feature was estimated by applying different instances of a Savitzky–Golay filter to our spectra, by determining the region of interest centered on our feature, computing the barycenter associated to this region of interest (ROI), and averaging the peak position of each Savitzky– Golay instance
All values are coherent with the peak position associated with the hydroxyl stretching of hydrated silicates in the reference spectra found in the literature for each sample (Rodriguez 1994 for the saponite; Bishop et al 2008; Takir et al 2013 for the serpentines)

Summary

Introduction

The study of hydrated silicates in small bodies, as tracers of aqueous alteration, is of utmost importance for understanding the evolution of primitive materials in the early solar system. The detection of hydration on the asteroids’ surfaces can be achieved by remote sensing in the near-infrared (NIR) spectral range, using the 2.7 μm band of phyllosilicates. This feature is attributed to the stretching vibration of hydroxyl groups (O-H) covalently bonded to metallic atoms (M) of octahedral MO6 sites. The peak position has been well studied in terrestrial phyllosilicates as a function of the Mg/Fe ratio, with wavelengths shorter for Mg-rich than for Fe-rich compositions (Farmer 1974; Besson & Drits 1997; Bishop et al 2008). The 2.7 μm band has been measured in aqueously altered carbonaceous chondrites (Zolensky & McSween 1988; Miyamoto & Zolensky 1994), and detected on asteroids despite telluric

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