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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.
Image 1. Typical crater floor polygons. [A] CTX (a 6 meter/pixel camera onboard the Mars Reconnaissance Orbiter, P16_007372_2474).of a 14 km‐sized impact crater (location: 67.2°N, 47.8°E). [B] Close-up from the same image. Two distinct size groups can be seen: A large 70-350 m sized polygons with an average polygon diameter of 120 m and mainly orthogonal trough intersection, and a smaller group, not always present, ranging in size from 5 to 20 m. [C] High resolution HiRISE (a telescopic camera with an impressive 25 cm/pixel resolution onboard the same spacecraft as the CTX, PSP_007372_2247) sub-image for the same crater of a 100 m‐wide polygon with a 6-8 m-wide, frost‐filled troughs surrounding it. Secondary troughs within the larger features form polygons with an average diameter of 10 m. These embedded features are probably periglacial thermal contraction polygons.
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.
Buczkowski, D.L., and Mcgill, G.E., 2002: Topography within circular grabens: Implications for polygon origin, Utopia Planitia, Mars. Geophysical Research Letters, 29, 1155, doi:1110.1029/2001GL014100.
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.
Levy, J., Head, J., and Marchant, D., 2009: Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations. Journal of Geophysical Research (Planets)
Mangold, N., 2005: High latitude patterned grounds on Mars: Classification, distribution and climatic control. Icarus, 174, 336-359.
Marchant, D.R., and Head, J.W., 2007: Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars. Icarus, 192, 187-222.
Mcgill, G.E., and Hills, L.S., 1992: Origin of giant Martian polygons. Journal of Geophysical Research (Planets), 97, 2633-2647.
Osinski, G.R., Lee, P., Parnell, J., Spray, J.G., andBaron, M., 2005: A case study of impact-induced hydrothermal activity: The Haughton impact structure, Devon Island, Canadian High Arctic. Meteoritics and Planetary Science, 40, 1859-1878.
Yoshikawa, K., 2003: Origin of the polygons and the thickness of Vastitas Borealis Formation in Western Utopia Planitia on Mars. Geophysical Research Letters, 30, 1603, doi:1610.1029/2003GL017165.