Post by M. R. El Maarry, Max‐Planck Institut für Sonnensystemforschung, Katlenburg‐Lindau, Germany
Polygons are some of the most common features at high latitudes on Mars and have been observed by both lander and orbiting spacecraft. They range in size from 2 m all the way up to 10 km and different formation mechanisms have been proposed that include thermal contraction, desiccation, volcanic, and tectonic processes (Buczkowski and McGill, 2002; Levy et al., 2009; Mangold, 2005; Marchant and Head, 2007; McGill and Hills, 1992; Yoshikawa, 2003).
Crater floor polygons have diameters ranging from 15 to 350 m (Image 1). Although, morphologically they resemble both terrestrial thermal contraction polygons and desiccation cracks, their size distribution is significantly larger than thermal contraction polygons that are ubiquitous in the Martian high latitudes.
An analytical model based on fracture mechanics (El Maarry et al., 2010) reveals that under current climatic conditions, the maximum fracture spacing attainable by thermal stresses alone is 75 m at the most. More reasonable values fall within 18 and 22 m, which is the size range of thermal contraction polygons on Mars. As a result, desiccation of formerly wet sediments is considered to be the likely mechanism for the formation of crater floor polygons (without ruling out thermal contraction processes as a possible contributor in some cases). This implies that lakes or water‐rich sediments occupied the craters in the past.
Many such aqueous environments have no apparent external source of water, and thus, hydrothermal processes occurring shortly after the impact event (Osinski et al., 2005) may be a viable explanation for the observed evidence. Other features such as sedimentary deposits, mounds, and shorelines, corroborates past lake formation and eventual desiccation to form crater floor polygons. Furthermore, the variation of crater floor polygons sizes with location can be indicative of different hydrologic environments.
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El Maarry, M.R., Markiewicz, W.J., Mellon, M.T., Goetz, W., Dohm, J.M., andPack, A., 2010: Crater floor polygons: Desiccation patterns of ancient lakes on Mars? Journal of Geophysical Research (Planets), 115, E10006.
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