Reflectance spectra diversity of silica-rich materials: Sensitivity to environment and implications for detections on Mars

Hydrated silica-rich materials have recently been discovered on the surface of Mars by the Mars Exploration Rover (MER) Spirit, the Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), and the Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) in several locations. Having been interpreted as hydrothermal deposits and aqueous alteration products, these materials have important implications for the history of water on the martian surface. Spectral detections of these materials in visible to near infrared (Vis-NIR) wavelengths have been based on a H₂O absorption feature in the 934-1009nm region seen with Spirit's Pancam instrument, and on SiOH absorption features in the 2.21-2.26μm range seen with CRISM. Our work aims to determine how the spectral reflectance properties of silica-rich materials in Vis-NIR wavelengths vary as a function of environmental conditions and formation. Here we present laboratory reflectance spectra of a diverse suite of silica-rich materials (chert, opal, quartz, natural sinters and synthetic silica) under a range of grain sizes and temperature, pressure, and humidity conditions. We find that the H₂O content and form of H₂O/OH present in silica-rich materials can have significant effects on their Vis-NIR spectra. Our main findings are that the position of the ~1.4μm OH feature and the symmetry of the ~1.9μm feature can be used to discern between various forms of silica-rich materials, and that the ratio of the ~2.2μm (SiOH) and ~1.9μm (H₂O) band depths can aid in distinguishing between silica phases (opal-A vs. opal-CT) and formation conditions (low vs. high temperature). In a case study of hydrated silica outcrops in Valles Marineris, we show that careful application of a modified version of these spectral parameters to orbital near-infrared spectra (e.g., from CRISM and OMEGA) can aid in characterizing the compositional diversity of silica-bearing deposits on Mars. We also discuss how these results can aid in the interpretation of silica detections on Mars made by the MER Panoramic Camera (Pancam) and Mars Science Laboratory (MSL) Mast-mounted Camera (Mastcam) instruments.