Gullies, polygons and mantles in Martian permafrost environments: Cold desert landforms and sedimentary processes during recent Martian geological history

A range of cold desert landforms are found on the Martian surface that have been interpreted to indicate prevailing frozen and hyper-arid conditions for at least the past several million years. These cold desert conditions are punctuated by brief periods of localized surficial liquid water flow. Sediment transport pathways operate under these conditions of extreme cold and aridity and the processes involved generate permafrost landforms that are recognizable from spacecraft at local, regional and global scales. Thermal-contraction-crack polygons are associated with hemisphere-spanning mantle units that contain excess ice in the immediate subsurface. Sublimation is the dominant phase transition rather than melting under present Martian conditions. Evidence is presented for melting of near-surface snow, frost and/or ground ice in protected gully alcove microclimates during the most recent several million years. Mars is a permafrost planet. The Martian surface supports a wide range of fluvial, volcanic and aeolian landforms analogous to features found on Earth (Chapman 2007; Carr & Head 2010). During most of the geological history of Mars (Laskar et al. 2002, 2004), the entire Martian surface and shallow subsurface have experienced mean annual temperatures well below 273 K (0 8C), commonly dipping below 220 K (Mellon & Jakosky 1993). Accordingly, the entire face of Mars meets the standard definition of a permafrost terrain (Gold & Lachenbruch 1973; Washburn 1973; French 2007). These permafrost conditions likely extend to a depth of several kilometres (Clifford & Parker 2001). Indeed, Mars may be considered a cryotic planet insofar as, at present, mean annual surface temperatures are below the melting temperature of several water-ice compounds and solutions (Yershov 1998). Is the permafrost terrain of Mars similar to that of Earth? Although permafrost conditions persist over c. 20% of the Earth’s land surface, much of Earth’s permafrost is found in the continental and maritime regions of the North American and Eurasian Arctic (French 2007). In these warmer climate zones, permafrost commonly experiences summertime melting as the 0 8C (273 K) isotherm penetrates the frozen ground surface (Washburn 1973; Williams & Smith 1989; Yershov 1998; French 2007). This seasonally thawed portion of terrestrial permafrost is referred to as the ‘active layer’ and, when water-saturated (‘wet’), it is the horizon in which many of the classic permafrost landforms arise (Williams & Smith 1989; Vliet-Lanoe 1991; Yershov 1998). In contrast, Mars currently lacks a wet active layer, and has probably not experienced climate conditions permitting the widespread development of a wet active layer over at least the last 5– 10 Ma (Kreslavsky et al. 2008). Interestingly, though, many of the most dramatic Martian permafrost landforms (Fig. 1) including gullies, thermal-contraction-crack polygons and the latitude-dependent mantle (LDM), all formed more recently than c. 5 Ma (Mustard et al. 2001; Head et al. 2003; Milliken et al. 2003; Kuzmin et al. 2004; Riess et al. 2004; Levy et al. 2009a; Schon et al. 2009a) Accordingly, it is essential to consider Martian permafrost from a cold desert climate perspective in which wet active layers are rare or absent (Anderson et al. 1972; Gibson 1980; Marchant & Head 2007). From: Martini, I. P., French, H. M. & Perez Alberti, A. (eds) Ice-Marginal and Periglacial Processes and Sediments. Geological Society, London, Special Publications, 354, 167–182. DOI: 10.1144/SP354.10 0305-8719/11/$15.00 # The Geological Society of London 2011. Fig. 1. Plot of permafrost landforms diagnostic of a range of morphogenetic climate regions on Earth and Mars (adapted from Baker 2001 and Marchant & Head 2007). Oval represents mean annual climate conditions typical of the Antarctic Dry Valleys. SUZ indicates the Antarctic Stable Upland Zone (Marchant & Head 2007). TD indicates Taylor Dome and LGM indicates the Last Glacial Maximum in interior Antarctica. Modern conditions at a range of latitudes on Mars and representative thermal-contraction-crack polygon populations typical of those latitudes are plotted, as are conditions modelled for ancient Mars at higher atmospheric pressures. For Martian polygons, field of view is c. 300 m in all cases (nomenclature from Levy et al. 2009d ). Flat-top small polygons are excerpted from PSP_001959_2485; peak-top polygons from HiRISE image PSP_001737_2250 and mixed-centre polygons from PSP_002175_2210. The field of view in the illustration of sublimation polygons in Beacon Valley, Antarctica is c. 200 m wide. Oblique aerial view of sand-wedge polygons in lower Beacon Valley, Antarctica, has a field of view c. 50 m wide. Composite-wedge polygons are illustrated in Wright Valley, Antarctica, cross-cut by a gully channel with a field of view c. 75 m wide. Aerial view of ice-wedge polygons in Taylor Valley, Antarctica has a field of view c. 75 m wide. J. S. LEVY ET AL. 168