Surface Properties of the Mars Science Laboratory Candidate Landing Sites: Characterization from Orbit and Predictions

Abstract

This work describes the interpretation of THEMIS-derived thermal inertia data at the Eberswalde, Gale, Holden, and Mawrth Vallis Mars Science Laboratory (MSL) candidate landing sites and determines how thermophysical variations correspond to morphology and, when apparent, mineralogical diversity. At Eberswalde, the proportion of likely unconsolidated material relative to exposed bedrock or highly indurated surfaces controls the thermal inertia of a given region. At Gale, the majority of the landing site region has a moderate thermal inertia (250 to 410 J m⁻² K⁻¹ s^−1/2), which is likely an indurated surface mixed with unconsolidated materials. The primary difference between higher and moderate thermal inertia surfaces may be due to the amount of mantling material present. Within the mound of stratified material in Gale, layers are distinguished in the thermal inertia data; the MSL rover could be traversing through materials that are both thermophysically and compositionally diverse. The majority of the Holden ellipse has a thermal inertia of 340 to 475 J m⁻² K⁻¹ s^−1/2 and consists of bed forms with some consolidated material intermixed. Mawrth Vallis has a mean thermal inertia of 310 J m⁻² K⁻¹ s^−1/2 and a wide variety of materials is present contributing to the moderate thermal inertia surfaces, including a mixture of bedrock, indurated surfaces, bed forms, and unconsolidated fines. Phyllosilicates have been identified at all four candidate landing sites, and these clay-bearing units typically have a similar thermal inertia value (400 to 500 J m⁻² K⁻¹ s^−1/2), suggesting physical properties that are also similar.

Appendix: Thermal Inertia Derivation

The Thermal Emission Imaging Spectrometer (THEMIS) infrared (IR) data have an improved spatial resolution (100 m/pixel) over previous datasets, such as Mars Global Surveyor Thermal Emission Spectrometer (TES) or Viking Infrared Thermal Mapper. The THEMIS data set enables the modeling of surface physical characteristics to determine particle size information and identify surface exposures of bedrock, and allows these physical properties to be correlated to morphologic features. This data set can also facilitate an improved understanding of geologic processes that have influenced the Martian surface.

To derive thermal inertia from THEMIS data, we used the method of Fergason et al. (2006a). The brightness temperature of the surface is first determined by fitting a Planck curve to band 9 (centered at 12.57 μm) calibrated radiance that has been corrected for instrumental effects. This wavelength range was chosen because it has the highest signal to noise ratio and is relatively insensitive to atmospheric dust. The THEMIS band 9 temperatures are converted to a thermal inertia by interpolation within a 7-dimensional look-up table using latitude, season, local solar time, atmospheric dust opacity, thermal inertia, elevation (atmospheric pressure), and albedo as input parameters.

The look-up table includes a thermal inertia range of 24 to 3000 J m⁻² K⁻¹ s^−1/2, and values exceeding 1800 have been observed (e.g. Edwards et al. 2009). This thermal inertia range is significantly larger than that used in the TES standard model (maximum of 800), and allows the detection of exposures of consolidated materials or bedrock on the surface. This extended thermal inertia range was required because of: (1) the higher spatial resolution of THEMIS; (2) initial results from THEMIS nighttime temperatures suggesting the presence of bedrock (e.g. Christensen et al. 2003); and (3) the fact that many regions on Mars were saturated at the maximum value of thermal inertia in the TES model (Fergason et al. 2006a).

This look-up table is generated using a thermal model developed by H. H. Kieffer, which was derived from the Viking IRTM thermal model (Kieffer et al. 1977) with several modifications, the most significant being an improved atmospheric component. This improved atmospheric component consists of a one-layer atmosphere that is spectrally gray at solar wavelengths with the direct and diffuse illuminations computed using a 2-stream delta-Eddington model. The effects of 3-dimensional blocks on the surface, condensate clouds, and the latent heat of water ice are not considered (Kieffer 2011). This model can incorporate the effects of a radiatively-coupled sloping surface at any azimuth, but for the nominal thermal inertia calculations, slopes are not considered. Generally, slopes below 10° at all azimuths have a small effect on the nighttime surface temperature, and therefore the thermal inertia. Higher slope angles may be problematic, but this conclusion is dependent on the slope azimuth and the season. Due to the potential for slopes to be a factor, surfaces with slopes greater than ∼10° were interpreted with caution (Fergason et al. 2006a).

Model parameters appropriate for the THEMIS image and the measured band 9 surface temperatures are then used to interpolate the thermal inertia between the calculated look-up table node values. Interpolation is performed on a pixel-by-pixel basis using season, latitude, and local solar time from spacecraft ephemeris. The remaining model input parameters are obtained from external datasets. The albedo of features in the THEMIS image is determined from the TES albedo binned at 8 pixels per degree (Christensen et al. 2001). Elevation information is ascertained from Mars Orbiter Laser Altimeter (MOLA) elevation (Zuber et al. 1992; Smith et al. 1999, 2001a) binned at 128 elements per degree. Finally, the opacity is inferred by using the THEMIS image season to select a TES dust opacity value from data binned at 0.3 pixel per degree in latitude and 0.13 pixel per degree in longitude for every 15° Ls during the first Martian year of MGS mapping (e.g. Smith et al. 2001b).

Uncertainties in the THEMIS derived thermal inertia values are primarily due to (1) instrument calibration; (2) uncertainties in model input parameters, including albedo and opacity, at the resolution of the THEMIS instrument; and (3) thermal model uncertainties. Random and systematic errors in the THEMIS surface measurements result in an absolute calibration accuracy of THEMIS nighttime temperature between 1.8 K and 2.8 K (Fergason et al. 2006a), and a relative precision of 1.2 K (P.R. Christensen, THEMIS calibration report, http://themis-data.asu.edu/pds/calib/calib.pdf, (2005) Accessed 15 August 2011). TES albedo, TES atmospheric dust opacity, and MOLA elevation values are incorporated as model input parameters at a coarser resolution than that of the THEMIS temperature data, and thus may not be adequately taking into account the effects of these parameters on the thermal inertia derivation. TES atmospheric dust opacity is used for the first Martian year of MGS mapping when there were no global dust storms (Smith et al. 2000, 2001b). We assume that the amount of dust in the atmosphere is repeatable from year to year; this has been shown to be a reasonable approximation during seasons devoid of major dust storms (Clancy et al. 2000; Smith 2004). To avoid dusty atmospheric conditions, we do not determine thermal inertia when the atmospheric opacity is greater than 0.40 (visible wavelength). Elevation (used to determine the atmospheric pressure) has a minor effect on the thermal inertia, and the atmospheric dust opacity does not vary significantly over the area of a THEMIS image (Clancy et al. 2000; Smith et al. 2001b), so these two approximations are likely adequate. However, sub-TES-pixel variations in albedo that affect the nighttime surface temperature in THEMIS data likely affect the accuracy at which thermal inertia values can be calculated, and regions where albedo is varying over short distances (less than 3 km) are interpreted with caution. Considering the uncertainties in both the THEMIS instrument calibration and input parameters, the absolute accuracy of the THEMIS thermal inertia is ∼20 % (for additional detail, see Fergason et al. 2006a).