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Project Progress

Sulfates have been identified in the Valles Marineris of Mars by various instruments, including CRISM, OMEGA, TES and THEMIS. Sulfates are a group of minerals formed through evaporation of water on or near the surface and are thus important recorders of water-related process, and potentially biological activity, on Mars. Understanding their distribution and geologic context on the surface of Mars is important to astrobiology and to future exploration by rover missions. However, surface dust may obscure their full areal extent from orbital spectral identification.

Thermal inertia is a measure of the how much heat the upper 10-15 cm of a surface can store during the day and re-radiate at night. Thermal inertia primarily is dependent upon physical properties of the surface (e.g., particle size, degree of induration, rock abundance). Generally, fine particulate surfaces have low inertia, and rocky surfaces have high inertia resulting from their greater ability to store heat. Thermal inertia data have been used to map localized mineralogic units identified spectrally into contiguous areas where optically-thick surface dust is present, preventing spectral identification [Hamilton and Christensen, 2005]. We are using Thermal Emission Imaging System (THEMIS) thermal inertia data [Fergason et al., 2006] to characterize the inertias of sulfate outcrops in the Valles Marineris (Figure 1) at 100-meter scales and map their extent into adjacent dusty areas. We are augmenting these data with high resolution (3—18 m/pixel) visible geomorphology data.

We are currently identifying nighttime THEMIS images covering locations where sulfates have been identified within the Valles Marineris. Using this data, we are calculating the nighttime brightness temperature by fitting a Planck curve to THEMIS band 9 (centered at 12.57 μm) calibrated radiance that has been corrected for instrumental effects. For Valles Marineris, special consideration must be made due the drastic changes in elevation and relatively steep slopes. Therefore, we also have to enter slope information manually to account for differential heating/hemispherical effects. We will then convert THEMIS band 9 temperatures to thermal inertia values by interpolation within a look-up table. This process identifies the appropriate thermal inertia value for each THEMIS framelet based on six input parameters, including latitude, season, local solar time, atmospheric dust opacity, elevation (atmospheric pressure) and albedo.

Initial results indicate that the sulfate deposits in Candor Chasma are associated with intermediate thermal inertia values in the uncorrected thermal inertia data sets. Corrections for slope are required to determine the absolute thermal inertia values for the identified sulfates and we are currently applying these corrections to the data set.

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