Water Content of the Martian Surface Material

Information about the water content on Mars is available from the Viking experiments, spectral measurements, and H abundance. Combining data from multiple sources and multiple locations provides the most complete estimate of water content on Mars; however, at this time, these are still only estimates. Part of the uncertainty lies in that the water and OH determinations depend on which minerals are present and this is not yet well constrained. Water determined from the Viking experiments and spectral observations are described here, while the H detected by the Gamma-Ray Spectrometer (GRS) on board Mars Odyssey attributed to water ice, plus water and OH in minerals is described in detail in Chap. 5. Briefly, Mitrofanov, Feldman, Boynton and co-workers have identified a layer of OH/H2O-rich material buried beneath the OH/H2O-poor surface material on Mars and global measurements show some spatial variations in the density of these surface ice, water and/or OH components [16-18].

A gas chromatograph mass spectrometer (GC-MS) instrument was present on the Viking landers (VL) and measured weight loss as the samples were heated [19]. Although these GC-MS instruments were not designed to measure water on

Mars, experiments performed on VL-2 indicate that less than 2 wt. % water was lost from the surface soil when heated to 500°C [19]. This was interpreted as a minimum value because of uncertainties in the procedure that may have lost water before and/or during the experiment. H2O removed from the Martian soil up to 500°C could include both adsorbed water on mineral or grain surfaces and bound water in minerals, but not structural OH in minerals.

Early spectral analyses identified the presence of water on Mars through a strong absorption near 3 pm [e.g. 20, 21]. Houck et al. [20] analyzed Kuiper Airborne Observatory spectra of Mars in comparison with laboratory spectra of minerals and determined that the 3 pm band observed in the Martian spectra is similar to water features present for most minerals and estimated the water content at 1-2 % on the surface of Mars. Pimentel et al. [21] noted latitudinal variations in the 3 pm band in data from the Infrared Spectrometer (IRS) on Mariner 6 and 7 and suggested that this band may be due to water ice or surface frost as well as water in minerals. Near-infrared spectra measured by the Imaging Spectrometer for Mars (ISM) on Phobos 2 showed confirmation of the strong 3 pm band at higher spatial resolution (~20 km) in several equatorial regions that was attributed to water in surface minerals and not to water ice or frost [22-24]. The water content in the upper Martian surface soil was estimated at 0.5 to 4 wt. % from laboratory experiments and comparison with ISM spectra [25]. This value is comparable to earlier estimates [20] and to the GC-MS data measured by Viking [19]. Regional differences in the depth of the 3 pm band, and hence the water content, on Mars have been observed in telescopic spectra [26] and in Mariner 6 IRS data [27]. More recent analyses of the ISM data suggest differences in the water content that Murchie et al. [28] attribute to variations in ferric minerals or sulfates. They found that about 20 % of the surface measured by ISM exhibited deeper than normal 3 pm bands. Noe Dobrea et al. [29] analyzed Hubble Space Telescope (HST) spectra and found evidence for three distinct (non-icy) surface compositions that contain hydrated minerals, although specific hydration minerals could not be identified.

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