James F. Bell

Mars surface mineralogy from Hubble Space Telescope imaging during 1994–1995: Observations, calibration, and initial results

Visible to near-infrared observations of Mars were made with the Hubble Space Telescope (HST) during 1994–1995 with the goals of monitoring seasonal variability of the surface and atmosphere and mapping specific spectral units to constrain the planets surface mineralogy. This paper presents the details of the collection and calibration of the data, concentrating specifically on the near-IR data that were obtained exclusively for the surface mineralogy aspect of our HST Mars observing program. We also present some initial results from the calibrated data set. Our calibration procedures included the standard “pipeline” processing steps, supplemented by special procedures required for use with the linear ramp filters on the Wide Field/Planetary Camera 2 instrument, and an additional point spread function deconvolution procedure applied in order to realize the full potential spatial resolution of the images (23 to 64 km/pixel between August 1994 and August 1995). The calibration results in a set of images projected onto a standard map grid and presented in radiance factor (I/F) units, having an estimated ≈5% photometric accuracy based on the performance of HST and comparisons with previous ground-based and spacecraft Mars spectra. Initial scientific analyses of these data reveal (1) distinct red/blue color units within the classical bright regions, similar to those seen in Viking Orbiter images and possibly related to variations in nanophase and/or crystalline ferric mineral abundance; (2) near-IR spectral slope variations correlated with albedo on a large scale (darker is “bluer” near-IR slope) but exhibiting wider variations among many of the small-scale features visible in the data; (3) an absorption at 860 nm that occurs in all regions but which is 3 to 5% stronger in many of the classical dark regions than in the bright regions, possibly because of a greater abundance of a well-crystalline ferric phase like hematite or a very low Ca pyroxene or opx/cpx mixture; and (4) an absorption from pyroxene at 953 nm with a band depth that is inversely correlated with albedo (bright regions 0 to 5% deep; dark regions 7 to 15% deep) and which shows the highest band depth values in individual craters, calderas, and other small geologic units that are resolved in the images.