Geometry induced bias in the remote near-IR identification of phyllosilicates on space weathered bodies

Abstract

Sample return missions Hayabusa2 (JAXA) and OSIRIS-REx (NASA) found evidence of hydrated silicates on the surface of C and B-type asteroids Ryugu and Bennu. This detection relied on the study of the 2.7 μm OH-stretching spectral feature revealed from remote sensing observations of the asteroids' surfaces. Laboratory studies simulating the effects of space weathering (SpWe) on primitive bodies have shown that the feature's position, considered as the wavelength of the band minimum, can vary under ion implantation, shifting towards longer wavelengths for implanted surfaces. Since SpWe is a surface process, we investigated how the geometry of observation can affect the hydration feature on space weathered surfaces. Here, we report new laboratory Reflectance Factors (REFF) measurements on pristine and ion-bombarded phyllosilicate pellets, to monitor the evolution of the 2.7 μm feature with varying observation geometry. We found that, as we approach specular reflection, the feature's position for He+ bombarded surfaces shifts towards longer wavelengths. We interpret that the spectral shift is due to chemical and physical changes induced by ion implantation in the first hundreds of nanometers of our phyllosilicate pellets. The diversity in the observed amplitude of the shift means that different competing effects are dominating at different optical configurations, mainly volume and surface scattering. The effects of the ion-implanted matter are especially visible when measuring in near-specular conditions, where the specular component (more sensible to the very top surface (implanted layer) of the sample)dominates - hence the larger shift measured. Our results indicate that the geometry of observation can induce a certain bias in the interpretation of remote sensing data from space-weathered bodies.