Some of the interesting aspects of the DJP basin are its benches or terraces, including ones located about 21 meters above the lowermost elevations. These terraces may indicate past, higher water levels within the pond. The geomorphic expression of these benches and their associated topography are important to document if DJP-like signatures are to be detected on Mars via missions such as the MRO. DJP-scale features have been observed on Mars via the Mars Global Surveyor (MGS) at imaging resolutions as high as 2-3 meters/pixel, but terraces and benches haven’t been associated with such features.

Imaging Observations
Initial sub-meter resolution imaging of the DJP region was acquired via the IKONOS satellite thanks to the NASA Scientific Data Purchase program, which supports Mars exploration goals. An initial data acquisition on Jan. 31, 2003, documented the DJP in an essentially dry state, as illustrated in Figure 1 (top). The rectangular-oval pond is clearly demarked in the 1-meter resolution imagery, with a smaller low-albedo inner basin and isolated boulders. The image was registered to an interpolated 2-meter per grid cell digital elevation model (DEM), with 5-centimeter vertical precision, acquired by NASA's airborne LiDAR topographic mapping system known as the ATM.

On Dec. 1, 2006, IKONOS acquired a color image of DJP at similar resolution to the 2003 image. Figures 1 and 2 illustrate both of these datasets and document some of the subtle geomorphic variations that can be observed and measured. Small patches of snow can be seen lingering in the 2006 image (Figure 2), but none were present during late January 2003 (Figures 1 and 3).

When the 2003 and 2006 datasets are co-registered, combined and then differenced (after normalization), key variations across a time interval of nearly three years can be measured. Figure 4 illustrates variations associated with wetting that are manifested as albedo in the two IKONOS images. The extent of shallow wetting in 2006 was far larger and apparently deeper than in 2003, perhaps because evaporation hadn’t yet minimized the wetted zone as it did by late January 2003. The maximum possible variation in wetting can be computed using the IKONOS images when they are tied to the DEM. In this case, the larger low-albedo zone in 2006 represents an additional 49-53 centimeters of hypersaline water depth, which corresponds to a maximum difference in water volume of about 234,000 cubic meters. Figure 4 illustrates these differences in a perspective view, looking down the rock glacier that bounds one side of the DJP basin.

If water depths vary by tens of centimeters as this analysis suggests, then the aquifer system that feeds DJP each year must vary in its groundwater discharge due to factors that haven’t been previously recognized in the literature. This is particularly relevant to the exploration of groundwater discharges on Mars, where freezing and sublimation are much more rapid than on Earth due to environmental conditions, including Mars’ 6 millibar CO2 atmosphere and typical surface temperatures of -20 C to -70 C.

To further resolve the state of DJP wetting using satellite methods, NASA and the Canadian Space Agency have been experimenting with SAR-based detection of liquid water within small, saline Antarctic ponds. A time series of RADARSAT images that were initiated in October 2006 and have continued through January 2007 have been acquired in RADARSAT’s "S2" mode (about 30 degrees incidence) to detect liquid water in the DJP region.

Figure 5 illustrates perspective views of the IKONOS images from 2003 and 2006, together with a Jan. 22, 2007, RADARSAT SAR image at 12.5 meters/pixel. The image clearly reveals the rugged "snout" of the rock glacier that bounds the DJP basin. Due to the likely shallow water depth within the innermost contour of DJP, the SAR backscatter signature isn’t entirely definitive, partly on the basis of exposed boulders and because of a small area of exposed water or mud. Figure 6 provides a more synoptic view of the 2006 IKONOS image and the RADARSAT SAR image, both relative to a topographic perspective view from the NASA LiDAR-based DEM. Continuing analysis of IKONOS and SAR signatures of DJP will be used to develop targets on Mars for MRO imaging in upcoming years.


Authors' Notes: The authors gratefully acknowledge the NASA Scientific Data Purchase program for the acquisition of the 2003 IKONOS image. Mark Brender and the GeoEye team provided the 2006 IKONOS image in support of NASA's exploration goals for Mars. Christine Giguere of the Canadian Space Agency's RADARSAT program provided the RADARSAT SAR image data as part of a joint activity to monitor Mars-like regions on Earth via spaceborne SAR. Special thanks to the efforts of Serdar Manizade in support of these observations (ATM data), the NASA Robotic Lunar Exploration Program (RLEP2), and Dr. Ed Weiler for providing salary support during these efforts. The MRO program is managed by the Jet Propulsion Laboratory for NASA. Jim Garvin acknowledges the encouragement of his wife Cindy and children, Zachary and Danica, in this research.

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