VISIBLE/NEAR-INFRARED REFLECTANCE SPECTROSCOPY OF LONGITUDINAL RIPPLE SANDS OBSERVED DURING PHASE 2 OF THE BAGNOLD DUNE CAMPAIGN, GALE CRATER, MARS

Phase 2 of the Bagnold Dune campaign conducted by the Mars Science Laboratory Curiosity rover included in situ relative reflectance observations of disturbed and undisturbed sands during Sols 1601-1653. The ChemCam instrument was used to acquire point spectra (400-840 nm) and the Mastcam stereo camera obtained multispectral images (445-1012 nm). The intent for the Phase 2 campaign was to compare longitudinal dunes during an active wind season to the barchan dunes that were observed during the more quiescent Phase 1 [1,2]. Phase 2 observations were acquired at 3 areas on the eastern edge of Nathan Bridges Dune (Mapleton, Sandy Point Beach, Southern Cove), and at Ogunquit (where the rover scooped samples for onboard analyses). Spectral differences among disturbed, undisturbed, and ripple crest/trough sands were marked by changes in the visible spectral slope, maximum reflectance position, and the steepness of the near-infrared dropoff. These were due to variations in the relative abundance (and/or grain size) of mafic minerals (olivines, pyroxenes) versus finer-grained, more ferric materials. In the Mapleton and Sandy Point Beach (Towow) areas, spectra were typical of reddish sands observed elsewhere throughout the mission. At Southern Cove, the crest of the Ripogenus ripple exhibited reflectance maxima at shorter wavelengths than the flank, consistent with more ferrous materials. At Ogunquit, the Baxter Peak ripple crest showed the strongest near-infrared downturn, consistent with more ferrous sands compared to the trough. Overall, Phase 2 investigations of the Bagnold Dune sands showed similar overall spectral trends to the Phase 1 campaign, but most sands were redder in the visible wavelengths. The majority of locations exhibited lower red/infrared ratios, higher ~530 nm band depths, and higher red/blue ratios than Phase 1 samples, suggesting a greater proportion of redder, fine-grained/ferric surfaces in Phase 2 samples. This is consistent with Phase 2 samples’ greater proportion of hematite determined from CheMin analyses of the Ogunquit sample, which is interpreted to reflect contamination from the surrounding hematite-bearing Murray formation bedrock. [1] Johnson, J., et al., JGR, doi:10.1002/2016JE005187, 2017; Johnson, J., et al., GRL, doi:10.1002/2016JE005187, 2018.