Thermal Contraction Crack Polygons on Mars: Classification, Distribution, and Con- Text for Phoenix from North and South Polar Hirise Observations

Introduction: The detection of subsurface ice at the po-lygonally-patterned Phoenix landing site [1] is the culmination of over thirty years of Mars permafrost and polygons research [2-6]. The exploration of martian polygonally pat-terned ground addresses several critical questions related to recent martian habitability and climate processes: 1) what is the origin of ice in the shallow martian subsurface (e.g., cyclical vapor diffusion or primary deposition of now-buried ice sheets) [7-11]; 2) to what extent is liquid water involved in martian polygon development [5]; does thermal contraction cracking account for the complete range of polygonally patterned ground observed on Mars [5]; do the ages of different polygonally patterned units vary systematically [5, 8]; and how does polygonally patterned ground fit into broader-scale ice-related activity during the late Amazonian [7, 9, 12]? Polygon classification in terrestrial polar environments is based on morphology, structure, and origin processes. On Earth, thermal contraction crack polygons can be divided into three types: ice-wedge, sand-wedge, and sublimation polygons ; each of which forms under a unique set of climate and substrate-composition conditions [13, 14]. Although the thermal contraction cracking process under martian conditions is well understood [15], classification systems for poly-gonally patterned ground on Mars have until now relied primarily on imaging data at resolutions comparable to the scale of the polygons of interest [5]. Here, we classify small polygons (<25 m diameter) visible in HiRISE images into seven morphological groups distinguishable by characteristic surface morphologies. This morphological classification can provide the basis for further analysis of polygon evolution processes [16]. Survey Parameters: We present a survey of 823 full-resolution HiRISE images of martian polar regions (30-80°N), spanning primary science phase orbits 001331 to 006981 [17]. Of the surveyed images, 483 contain polygo-nally patterned ground (59%), with polygon frequency higher at higher latitudes. Polygon Classification and Distribution: We divide polygons into 7 morphological varieties (Fig. 1). Commonly more than one variety is present in a single HiRISE image, suggesting variability in polygon-forming substrate conditions on 100 m to km length scales [18]. Polygon groups are latitude-dependent, suggesting a climatic control on polygon morphology. High Relief (HR). High relief polygons have strong to-pographic contrasts between polygon interiors and topog-raphically-depressed, straight-sided polygon troughs. HR are morphologically similar to S1 terrain described by [5], with diameters averaging 6.1 m in the northern hemisphere, and 5.6 in the southern hemisphere. In places, well-developed HR polygons fringe central clusters of HR polygons …