An observational search for carbonates on Mars

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Abstract
A search for carbonates and other salts exposed on the surface of Mars was undertaken by observing areas of the surface in the 2.6‐μm to 4.2‐μm spectral region using the University of Hawaii 2.2‐m telescope at the Mauna Kea Observatory, Hawaii. Observations of four Mars areas (Syrtis Major, Hellas, Arabia, and an area northeast of Hellas) were made on the night of September 19, 1986. The reflectance spectra for two partially overlapping areas show a weak absorption band in the 3.8‐μm to 3.9‐μm region, which is in the spectral region where carbonate absorptions appear: 3.8‐μm to 4.0‐μm. The Mars area of overlap contains Syrtis Planum. The spectral feature (from about 3.76 μm to 3.95 μm) is clearly shortward of and broader than the calcium carbonate band (about 3.9 μm to 4.0 μm) and is at a slightly shorter wavelength position than magnesium carbonate (magnesite) bands (about 3.8 μm to 3.96 μm) as measured in this study. Positive identification of this band cannot be made by us at this time. Measurements for two other Mars areas show no evidence of carbonate or other bands. If the band is due to absorptions in the Mars surface material spectrum and if it is due to carbonates, then the results imply about 3 to 5 wt % of a carbonate such as magnesite on average in the upper few millimeters of the Mars surface material over the two areas where the weak spectral feature appears. We can place an upper limit of 3 to 5 wt % calcium carbonate and about 5 wt % magnesite for all areas observed. The interpretations are based on laboratory measurements of the reflectance spectra of calcite and magnesite and their mixtures with palagonite. Effects of thermal emission on the detection of carbonates were also explored for a wide range of temperatures. Thermal effects are not sufficient to mask the presence of calcite in the 3 to 5 wt % range. Theoretical reasoning strongly supporting the presence of carbonates as a component of the Mars surface is reviewed. These and earlier observations do not rule out carbonates below the surface in other locations, as small outcrops, or in concentrations of less than 3 to 5 % of the areas observed.