Periglacial landforms at the Phoenix landing siteand the northern plains of MarsMichael T. Mellon,1Raymond E. Arvidson,2Jeffrey J. Marlow,2,3Roger J. Phillips,2,4and Erik Asphaug5Received 13 November 2007; revised 3 May 2008; accepted 30 May 2008; published 5 November 2008.[1] We examine potentially periglacial landforms in Mars Orbiter Camera (MOC)and High Resolution Imaging Science Experiment (HiRISE) images at the Phoenixlanding site and compare them with numerical models of permafrost processes tobetter understand the origin, nature, and history of the permafrost and the surface of thenorthern plains of Mars. Small-scale (3–6 m) polygonal-patterned ground is ubiquitousthroughout the Phoenix landing site and northern plains. Larger-scale (20–25 m)polygonal patterns and regularly spaced (20–35 m) rubble piles (localized collections ofrocks and boulders) are also common. Rubble piles were previously identified as‘‘basketball terrain’’ in MOC images. The small polygon networks exhibit well-developedand relatively undegraded morphology, and they overlay all other landforms. Comparisonof the small polygons with a numerical model shows that their size is consistent with athermal contraction origin on current-day Mars and are likely active. In addition, theobserved polygon size is consistent with a subsurface rheology of ice-cemented soil ondepth scales of about 10 m. The size and morphology of the larger polygonal patterns andrubble piles indicate a past episode of polygon formation and rock sorting in thermalcontraction polygons, while the ice table was about twice as deep as it is presently.The pervasive nature of small and large polygons, and the extensive sorting of surfacerocks, indicates that widespread overturning of the surface layer to depths of many metershas occurred in the recent geologic past. This periglacial reworking has had a significantinfluence on the landscape at the Phoenix landing site and over the Martian northernplains.Citation: Mellon, M. T., R. E. Arvidson, J. J. Marlow, R. J. Phillips, and E. Asphaug (2008), Periglacial landforms at the Phoenixlanding site and the northern plains of Mars, J. Geophys. Res., 113, E00A23, doi:10.1029/2007JE003039.1. Introduction[2] Permafrost on Mars today is global in extent. Belowabout a centimeter of depth at the equator and below thevery surface at higher latitudes the soil temperatures remainbelow the freezing point of water year-round. At higherlatitudes soil temperatures are cold enough that even inthe dry Martian climate subsurface ground ice has beenpredicted to be stable [e.g., Leighton and Murray, 1966] andindeed the presence of ground ice has been inferred from thedetection of abundant subsurface hydrogen [e.g., Boynton etal., 2002]. It is therefore not surprising that ground icewould have an influence on the geomorphic character of thelandscape. One of the primary goals of the Mars ScoutMission Phoenix is to land in a high-latitude region withabundant ground ice to examine the environ ment, thelandscape, a nd the subsurface ice itself. To this end alocation has been chosen in the northern plains of Mars(near 68!N ! 233!E) where ground ice is expected to beabundant a few centimeters below the surface [Arvidson etal., 2008]. The desire to land in a region with shallowground ice is necessarily coincident with the desire to landin a region exhibiting abundant periglacial landforms. Anabsence of these landforms could suggest an absence ofground ice and would be of concern to the primary scientificobjectives of the Phoenix mission.[3] Perhaps the most widespread landforms in terrestrialpermafrost are thermal-con traction fractures and associatedpolygonal- patterned ground. These patterns form whencohesive permanently ice-cemented soil undergoes seasonalthermal contraction resulting in tensile stresses that exceedthe strength of the froz en ground, thus developing ahoneycomb network of fractures to relieve this stress [e.g.,Leffingwell, 1915; Lachenbruch, 1962]. Individual fracturesmay only open millimeters in one season, but over manyJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, E00A23, doi:10.1029/2007JE003039, 2008ClickHereforFullArticle1Laboratory for Atmospheric and Space Physics, University ofColorado, Boulder, Colorado, USA.2Department of Earth and Planetary Sciences, Washington University,St. Louis, Missouri, USA.3Now at the Department of Earth Science and Engineering, ImperialCollege London, London, UK.4Now at Southwest Research Institute, Boulder, Colorado, USA.5Department of Earth and Planetary Science, University of California,Santa Cruz, California, USA.Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JE003039$09.00E00A23 1 of 15years of repeated cracking at the same previously fracturedzones of weakness, surface material may enter the crack anddevelop a subsurface wedge. In climates where seasonalsurface thaw and runoff are available, such as in Arctictundra, a subsurface ice wedge may form [Black, 1976]. Inarid regions analogous to Mars, such as the Antarctic DryValleys, loose surface sand and dust enter the crack forminga subsurface sand wedge [Pe´we´, 1959, 1974]. The growthof the wedge and subsequent seasonal thermal expansionresult in the development of surface topography in the formof a perimeter trough over the crack and wedge wheresurface material is being consumed, and uplift within part orall of the polygon interior.[4] Another common feature in terrestrial permafrost isthe self organization of rocks, i.e., cobbles and boulders.Most often this sorting occurs through freeze-thaw cycles inthe active layer above the permafrost [e.g., Washburn, 1956,1980; Hallet and Prestrud, 1986], where frost heave drivesan upward and outward convection. Sorting may also occurin permafrost undergoing thermal contraction, where nofreeze-thaw cycle is presen t and rocks slu mp into thepolygon trough driven by gravity [Washburn, 1956; Pe´we´,1959] or combined with differential sublimation [Marchantet al., 2002]. Sletten et al. [2003] suggested such a convec-tion like cycle in some Antarctic sand-wedge polygons,where long-term wedge growth drives fine material inwardand results in upward deformation of the polygon centerfollowed by surface creep toward the troughs. Indeed, theyfound sand-wedge growth in the oldest polygons resulted incomplete replacement of the polygon interior subsurfacewith sand-wedge material. In these cases rocks larger thanthe typical thermal-contraction-crack