Small‐scale polygonal features on Mars: Seasonal thermal contraction cracks in permafrost

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Abstract
A time‐dependent viscoelastic model of thermal stress in Martian ice‐rich permafrost is developed to test the hypothesis that small‐scale polygonal features observed from orbit and by the Viking Lander 2 are the result of thermal contraction cracking, as commonly occurs in terrestrial permafrost. Results indicate that significant tensile stress occurs in Martian ice‐rich permafrost as a result of seasonal cycles in the ground temperature. Using conservative rheological parameters appropriate for ice at low temperatures, tensile stresses poleward of about 20° to 30° latitude easily exceed the tensile strength (assumed to be 2 to 3 MPa) and fractures should readily form. In the equatorial regions, special conditions may allow tensile stresses to approach the tensile strength. These results support a thermal contraction origin of observed small‐scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice.

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