th Lunar and Planetary Science Conference ( 2009 )

Introduction: During the Viking era, Mars' recent climatic history was held to be cold and dry with little evidence for long-lived liquid water near the surface; signs of a past wetter, warmer climate were confined to ancient Noachian or Hesperian-aged terrains. However, data from recent Mars missions have revealed contemporary near-surface water-ice to be abundant at high latitudes [1, 2], and a population of mid-latitude flu-vial-like gullies [3] that appear to have formed by transient melting of ice or snow [4]. Thus today's view of Mars' recent surface evolution is one of global perma-frost conditions [5], with the latitudinal distribution of near-surface ice depending on changing climatic conditions ; the timescales of which are governed by obliq-uity cycles with periods of tens to hundreds of thousands of years [6]. However, in recent geomorphological studies of the equatorial Elysium Planitia region using very high resolution images (HiRISE; 25cm/pixel) we have identified landforms that almost certainly formed by the action of repeated thaw of ice-rich ground. These observations appear to be at odds with the equilibrium, obliquity-cycle based model of near-surface ice distribution on Mars Study area: The Elysium Planitia region of Mars is geologically young (late Amazonian; < 100-200 Ma [7]) and hosts a variety of landforms that are morphologically similar to those of periglacial and permafrost environments on Earth [8-11]. The region was exposed to massive flooding from deep underground sources during the late Amazonian, as demonstrated by the distinctive fluvial morphologies seen in the outflow channel Athabasca Vallis [12, 13]. These floods would have provided both the source of ice and particulate material required for a periglacial or permafrost landscape and there was probably a long-lived, but slowly freezing, lake or sea in the downstream Elysium basin [14, 15]. However, the provenance of the materials and landforms of this region is disputed: many still regard the Athabasca Vallis and Elysium basin as being flood lava provinces, with effusive volcanic materials reoc-cupying earlier flood landscapes [16-20]. Results: We present context mapping results of this area and show HiRISE images of periglacial land-forms in the region that include sorted stone circles, pingos within thermokarst-like basins, retrogressive thaw slumps and sinuous channels incised into polygo