Astrogeology Science Center

The objective of this work is to improve our understanding of the physical characteristics of the surface at the Mars Science Laboratory (MSL) candidate landing sites and to aid in the selection of a final landing location by generating THEMIS-derived daytime infrared (IR), nighttime IR, visible, and thermal inertia mosaics for each MSL candidate landing site. These mosaics are used to address the task of providing maps of surface thermal inertia over candidate landing sites and surrounding terrain at the highest resolution possible. These mosaics are generated in collaboration with Arizona State University (Philip R. Christensen).

MSL's mission is to collect Martian soil and rock samples, and analyze them for organic compounds, looking for environmental conditions that could have supported microbial life now or in the past. At a workshop in October 2007, scientists chose 4 sites as possible landing site locations. Further analysis of these regions will be performed, and the final landing site will be selected in 2010.

For questions regarding these data products, contact:
Robin L. Fergason;

The ISIS cube file can be loaded into JMars as a custom map. You can download JMars at this site.
http://jmars.asu.edu/wiki/index.php/Download_JMARS

For instructions on how to load a custom map, see this site.
http://jmars.asu.edu/wiki/index.php/Loading_a_Custom_Map

The PNG file is an 8-bit data file that can be viewed using most image viewer software (such as XV, Adobe Photoshop, or Adobe Illustrator).

Geo-TIFF and PNG/Worldfile/PNG Projection Auxiliary data products can be imported into the Arc GIS software package. The projection information for these images is:

Simple cylindrical
Ocentric
Clon: 180.0
Clat: 0.0
Lonsys: 360.0

Individual THEMIS images, both infrared (IR) and visible, are geometrically projected using ISIS software, and then mosaiced together. These mosaics are centered on the latitude and longitude value provided by the proposer and are 3° latitude x 3° longitude (~180 km²). To remove appearance of differences due to seasonal, time of day, and atmospheric variation during acquisition, these images are normalized to one another. Thus, these mosaics do not provide quantitative information, but are helpful for understanding relative differences between geologic materials and morphologic surface features, and for mapping surface units. For both daytime and nighttime IR mosaics, band 9 radiance values (centered at 12.57 µm) are used because this wavelength range has the highest signal to noise value and is relatively transparent to atmospheric dust.

Daytime surface temperatures in a single THEMIS image are primarily affected by albedo and local topography [e.g. Kieffer et al., 1973; Palluconi and Kieffer, 1981; Christensen, 1982]. Because of the dependence of topography on daytime temperature, day IR mosaics are useful for understanding the morphology of a region, and provide context for THEMIS, MOC NA, and CTX visible images. Brighter regions have a relatively high temperature and are typically caused by either sun-facing or low albedo surfaces. Darker regions have relatively low temperatures and are generally surfaces with a high albedo or are in shadow. The images used to make this mosaic are normalized to one another. Thus, these mosaics do not provide quantitative information, but are helpful for understanding relative differences between geologic materials and morphologic surface features, and for mapping surface units.

At night the effects of topography and albedo have largely dissipated, and differences in surface temperature are primarily due to its thermal inertia, which is a function of the physical nature of the surface [e.g. Kieffer et al., 1973; Palluconi and Kieffer, 1981; Christensen, 1982]. Because of this relationship, relative differences in nighttime temperature can be used to understand qualitative differences in thermal inertia. Bright regions have a higher temperature, corresponding to a relatively high thermal inertia (such as exposed bedrock, rockier, more indurated, or coarser particle sizes on the surface). Dark regions have a lower temperature and have a relatively low thermal inertia (such as dustier, finer particle sized material, or unconsolidated grains). The images used to make this mosaic are normalized to one another. Thus, these mosaics do not provide quantitative information, but are helpful for understanding relative differences between geologic materials and morphologic surface features, and for mapping surface units. A delta-temperature (K) is provided and characterizes the difference between the 2σ minimum and maximum nighttime temperature values, and provides a sense of the range in temperature within the mosaic.

For visible mosaics, band 3 images (centered at 654 nm) are mosaiced together. Black space in the mosaic is due to incomplete THEMIS coverage. Complete coverage of the candidate MSL landing sites is a high priority, and gaps in visible image coverage are currently being targeted by the THEMIS mission planners. As increased coverage becomes available, these mosaics will be improved and updated.

THEMIS infrared data have an improved spatial resolution (100 m per pixel) over previous datasets, such as MGS-TES or Viking IRTM. This dataset enables the quantification 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 dataset can also facilitate an improved understanding of geologic processes that have influenced the Martian surface.

The thermal model used to calculate THEMIS thermal inertia values is derived from the Viking IRTM thermal model [Appendix 1 of Kieffer et al., 1977], with the primary modification being the replacement of a constant atmospheric thermal radiation with a one-layer atmosphere that is spectrally gray at solar wavelengths, and the direct and diffuse illuminations are 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. 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° should be interpreted with caution [Fergason et al., 2006]. Work is currently being done to incorporate slopes into these mosaics, and the thermal inertia mosaics will be improved and updated as these data become available.

The THEMIS band 9 nighttime temperatures are converted to a thermal inertia by interpolation within a 7-dimensional look-up table. This table was created by first selecting a range of values appropriate for the Martian surface (nodes) for seven input parameters: latitude, season, local solar time, atmospheric dust opacity, thermal inertia, elevation (atmospheric pressure), and albedo. These nodes were then used to calculate the model-derived surface kinetic temperature for the specified conditions using the thermal model described above. Model parameters appropriate for an individual THEMIS image and the measured band 9 nighttime brightness temperatures are then used to interpolate the thermal inertia between these calculated node values [Fergason et al., 2006].

The look-up table includes a thermal inertia range of 24 to 3000, and values exceeding 1800 have been observed thus far in the mission [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 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., 2006].

To generate the qualitative thermal inertia mosaics, thermal inertia images have been normalized to one another to remove any image to image variations caused by atmosphere, seasonal, or time of day variations that are not adequately accounted for by the thermal model. The scale for thermal inertia mosaics, however, is determined from the quantitative thermal inertia mosaic, and is representative of the mosaiced region.

To maximize the quantitative nature of the information present in the mosaic, no normalization or blending is used to produce quantitative thermal inertia mosaics. Although we have improved the THEMIS instrument calibration (see below discussion), image-to-image differences in thermal inertia are still apparent. These differences may be a result of minor THEMIS calibration uncertainties, or may be caused by real physical phenomenon such as subsurface layering and atmosphere, seasonal, or time of day variations that are not adequately accounted for by the thermal model.

Thermal inertia is independent of season and local time, and thus image-to-image differences are typically less apparent in thermal inertia than temperature data. However, image-to-image differences can still be prominent in derived thermal inertia. Some of the variations between images are due to uncertainties in the input parameters and uncertainties in the thermal models, but the majority of discrepancies are likely due to uncertainties in the calibration of the THEMIS instrument. Much of this uncertainty is present because THEMIS calibration requires an image of a calibration flag that is typically acquired 60-90 seconds after the end of each THEMIS infrared image. During this 90-90 second gap, instrument conditions can change slightly, including slight changes in the temperature of the focal plane array. This causes a radiance offset of the entire image that results in a random (between images) error with a standard deviation of ~4 K at 180 K [Fergason et al., 2006].

The uncertainty in the THEMIS calibration can be reduced by comparing the measured atmospheric radiance between THEMIS (band 10, centered at 14.88 µm) and TES. The radiometric accuracy of TES (~1 to 2 K at 180 K [Christensen et al., 2001]) is better than THEMIS (~1.8 to 2.8 K at 180 K [Fergason et al., in press] and, outside periods of high dust activity, Martian atmospheric properties are repeatable from year to year [Clancy et al., 2000; Smith., 2004]. By binning TES data as a function of location and season (MGS and Mars Odyssey local time differences in measured radiance intergrated over band 10 are not significant) and convolving it to THEMIS band 10 radiance, it is possible to predict the THEMIS measured radiance. Any difference between the TES predicted and THEMIS measured band 10 radiance is assumed to be due to calibration error and is converted to a raw DN equivalent radiance correction for each of the THEMIS channels. We avoid periods of high dust opacity (9 micron opacities >0.30) present in either the THEMIS or TES datasets because of the inherent variability in the Martian atmosphere present during these periods. Although this technique ties THEMIS data to TES, the thermal inertia values derived from surface temperatures remain largely independent. As a result, TES and THEMIS derived thermal inertia values can be compared to add confidence in both maps where there is agreement and to identify any potential discrepancies.

Acknowledgement of the THEMIS site and its resources will help ensure the continuing support necessary for the validation, archiving, and distribution of THEMIS images.

For news media or educational purposes, please credit THEMIS images as: NASA/JPL/Arizona State University.

If you use THEMIS images or mosaics in a research work, please cite them by image ID or mosaic name in the caption and put the following in your references:

If you mention the THEMIS instrument in a research work, please cite it as follows in your references:

If you mention the THEMIS-derived thermal inertia in a research work, please cite it as follows in your references:

Additional information about the MSL mission is available at:

• MSL Mission Overview (website)
  
• MSL Mission Overview (PDF)
• MSL Mission Engineering Constraints (PDF)
• NASA's Mars Exploration Strategy (website)

Data from instruments orbiting Mars that are targeting these sites can be viewed at the following websites:

• THEMIS MSL Support
• MRO-CRISM MSL Support
  Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars
• MRO-HiRISE MSL Image Catalog
  Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment
• Mars Express HRSC MSL Support
  Mars Express High Resolution Stereo Camera
• MGS-MOC Landing Site Support (MSL and others)
  Mars Global Surveyor Mars Orbiter Camera
• MARSOWEB
• USGS MSL Support