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This HiRISE image is located near an area under consideration as a landing site for the Mars Phoenix lander mission. A network of shallow surface troughs and fissures coalesce into polygonal patterns. Polygonal patterned ground of this nature is common in permafrost regions of Earth, where seasonal thermal contraction of ice-cemented soil produces a honeycomb network of subsurface cracks. Cracks of this nature can also be produced by desiccation (mud cracks) or lava cooling (columnar joints), though typically on a smaller scale. The diameter of these martian polygons are dominantly 10-20 meters, analogous to terrestrial permafrost. The individual troughs are frequently only a couple of meters or less wide. Other characteristics, such as small ridges on either side of the troughs and the distribution of rocks in and around each polygon, are readily apparent. Small rocks and occasional larger boulders are also seen scattered throughout the image. Rocks protruding above the surface soil can be seen to cast shadows (solar illumination is from the lower left), which can aid in the determination of the rock's size and height. The image scale is 32 cm/pixel (with 1 x 1 binning) so objects ~96 cm across are resolved. The image shown here has been map-projected to 75 cm/pixel and north is up. The image was taken at a local Mars time of 3:01 PM and the scene is illuminated from the west with a solar incidence angle of 53.4 degrees, thus the sun was about 35.1 degrees above the horizon.


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