Polygonal Terrain in the Northern Plains

A network of shallow surface troughs and fissures coalesce into polygonal
patterns that are ubiquitous throughout this image.  Polygonal patterned
ground of this nature is quite 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, and easily resolved at
HiRISE resolution. Other characteristics, such as small ridges on either side
of the troughs and the distribution of rocks in and around each polygon is
also 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 rocks size and height. This image
is located near an area under consideration as a landing site for the Mars
Scout mission, Phoenix, planned for 2008. Examination of many factors
including surface texture (roughness and morphology) and the size
distribution of rocks will aid in final landing site selection.  

Images from the High Resolution Imaging Science Experiment and additional
information about the Mars Reconnaissance Orbiter are available online at: 
http://www.nasa.gov/mro or http://HiRISE.lpl.arizona.edu.

For information about NASA and agency programs on the Web, visit:
http://www.nasa.gov.

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate, Washington. Lockheed Martin Space Systems is the prime
contractor for the project and built the spacecraft. The HiRISE camera was
built by Ball Aerospace Corporation and is operated by the University of
Arizona.