VISIBLE AND NEAR-INFRARED SPECTRAL PROPERTIES OF MARTIAN SLOPE STREAKS

Martian slope streaks are large (20–200m wide, 100m–few km long), relatively low-albedo streaks that are one of the only active geologic processes occurring on Mars today. Our goal is to analyze the mineralogy of slope streaks across Mars using visible – near-infrared spectroscopy as one means of constraining likely formation mechanisms of the streaks. We investigated slope streaks’ spectral properties using images from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) at 8 sites, including 3 sites with observations at multiple times to enable time-series analysis; this is the first large scale survey of slope streak spectra. For each individual slope streak within each image, we determined both the average spectra of each streak and that of the slope immediately adjacent to it in order to account for changes in geology across a CRISM image or within slopes containing streaks. We find a trend where the visible spectral slope (0.4 to 0.6 µm) is strongly negative or “blue” for the darkest slope streaks and the spectral slope increases (reddens) as slope streaks fade. There is no similar reddening in the infrared (0.6 to 1.0 µm). This trend of visible reddening and fading is consistent across all sites and over multiple observations of the same site. At the same time, we do not find absorptions at 1.4 or 1.9 µm associated with hydration. In some cases, we see evidence of Fe-bearing minerals in some slope streaks (e.g., near Archeron Fossae, Olympus Mons) but these mineral signatures are also found in the streak-free slope directly adjacent, suggesting that the streaks have similar mineralogy to their surroundings. The spectral changes that we see at the sites are most consistent with changes in dust abundance. This finding could support a dry flow mechanism whereby slope streaks are formed by processes that trigger dust removal from the slope in particular locations, and the streaks slowly fade as dust returns to that surface.