title: Visible-to-Near-Infrared Spectral Variability of Hydrated Sulfates and Candidate Mars Landing Sites: Implications for the Mastcam-Z Investigation on NASA’s Mars-2020 Rover Mission
Creators: name: Dixon, Darian
Publisher: name: Western Washington University
Year: 2018

description: <p>The Mars-2020 rover mission will explore an astrobiologically relevant ancient environment on Mars, establish geologic context of the region, assess past habitability, and cache rocks for a future sample return. The Mastcam-Z instrument is a stereoscopic, zoom-enabled multispectral imager that will be critical to these objectives. As one of the mission’s primary reconnaissance tools, Mastcam-Z’s two cameras will acquire red-green-blue (RGB) true-color images and visible-to-near-infrared (VNIR) images with 11 narrowband filters from ~400-1100 nm. Mastcam-Z’s new ~975 nm spectral filter will help characterize the ~950-1000 nm absorption band in hydrated minerals, which has not been resolvable by previous rover multispectral imagers. We hypothesize that this filter will allow Mastcam-Z to better characterize VNIR hydration bands in hydrated sulfates, although mineral mixtures, iron-oxide dust contamination, and varying grain size will affect band depths. At the time of this writing, three candidate landing sites are being considered for the Mars-2020 mission, each bearing mineralogical units extensively investigated by high-resolution orbital SWIR (short-wave-infrared) imagery. We also hypothesize that Mastcam-Z VNIR spectral parameters can distinguish the prominent geologic units characterized by orbital SWIR at each landing site, although iron-oxide dust distribution is the suspected primary control on VNIR spectral variability on Mars. Synthetic magnesium and calcium sulfate samples were measured with a laboratory spectrometer and acquired spectra were convolved to expected Mastcam-Z resolution to identify the spectral filter combinations most sensitive to signatures of hydration. Sulfate samples were subject to bimodal mineral mixing, grain size separation, and contamination with a martian dust simulant to quantify the spectral effects these properties have on the ~950-1000 nm hydration band. SWIR imagery from the Mars 2020 candidate landing sites were also convolved to expected Mastcam-Z resolution. Spectral parameters were then developed at expected Mastcam-Z resolution that would corroborate mineral detections made by extensively-tested SWIR spectral parameters and SWIR spectra of prominent units. Mastcam-Z-simulated hydrated sulfate spectra reveal that Mastcam-Z can detect hydration in bimodal mixtures of hydrated Mg-sulfates but bimodal mixtures Ca-sulfates may present challenge unless significantly gypsum-rich. Iron-oxide dust contamination significantly shallows the ~950-1000 nm hydration band in hydrated sulfates, whereas band depth generally increases with increasing grain size. Mastcam-Z-simulated orbital imagery reveals distinct Mastcam-Z VNIR spectral parameters distinguishing prominent geologic units for each landing site, although modified versions of Pancam parameters are recommended for the Columbia Hills. Seasonal changes in spectral variability at the Columbia Hills and regional variation in spectral parameter effectiveness at NE Syrtis indicate VNIR spectral properties on Mars are likely controlled by surface dust distribution. These results provide specific operational recommendations for Mastcam-Z and insights into the nature of VNIR spectra on Mars.</p>