Phoenix Mars
Photo credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University

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Picking up Clues from the Discard Pile

As NASA's Phoenix Mars Lander excavates trenches, it also builds piles with most of the material scooped from the holes. The piles, like this one called "Caterpillar," provide researchers some information about the soil.

On Aug. 24, 2008, during the late afternoon of the 88th Martian day after landing, Phoenix's Surface Stereo Imager took separate exposures through red, green and blue filters that have been combined into this approximately true-color image.

This conical pile of soil is about 10 centimeters (4 inches) tall. The sources of material that the robotic arm has dropped onto the Caterpillar pile have included the "Dodo" and ""Upper Cupboard" trenches and, more recently, the deeper "Stone Soup" trench.

Observations of the pile provide information, such as the slope of the cone and the textures of the soil, that helps scientists understand properties of material excavated from the trenches.

For the Stone Soup trench in particular, which is about 18 centimeters (7 inches) deep, the bottom of the trench is in shadow and more difficult to observe than other trenches that Phoenix has dug. The Phoenix team obtained spectral clues about the composition of material from the bottom of Stone Soup by photographing Caterpillar through 15 different filters of the Surface Stereo Imager when the pile was covered in freshly excavated material from the trench.

The spectral observation did not produce any sign of water-ice, just typical soil for the site. However, the bigger clumps do show a platy texture that could be consistent with elevated concentration of salts in the soil from deep in Stone Soup. The team chose that location as the source for a soil sample to be analyzed in the lander's wet chemistry laboratory, which can identify soluble salts in the soil.

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