Testing the Regional Ice-sheet Collapse Model: Evidence of Glacial Modifica- Tion of Plains Units in Deuteronilus

Introduction: Lobate debris aprons (LDA) (Fig. 1) surround steep escarpments such as the walls of isolated plateaus and massifs in the northern and southern mid-latitudes (~30 to 50) on Mars and are Middle to Late Amazonian in age [1]. Much evidence supports the formation of these features by flow of ice and debris in a manner similar to glacial systems on Earth [1-9]. Additional evidence has suggested that this glaciation may have been more extensive in the past [10-11] and recent modeling [12] supports the hypothesis that LDA are the preserved, debris-covered remnants of the retreat and down-wasting of a much more regional ice sheet. To test the regional ice-sheet collapse model, we investigated the morphology and topography of plains units distal to LDA in Deuteronilus Mensae (39.6N to 50.2N and 13.6E to 35.4E) [see also, 13] using image data from the Mars Reconnaissance Orbiter (MRO) Context (CTX) camera at ~6 m/pixel resolution and topo-graphic data from High Resolution Stereo Camera (HRSC) digital terrain models (DTMs). We find evidence of a unique plains unit (" Upper plains unit ") at the distal areas of LDA that exhibits characteristics indicative of a former glacial extent. Regional Ice-Sheet Model Predictions: Model results [12] show that LDA could not have historically attained temperatures near or above the ice melting point and retained their current shape. As a result, geomorphic evidence of former glacial maxima is most likely to resemble cold-based glacial landsystems and not wet-based landsystems such as those associated with many temperate glaciers on Earth. Glacial retreat and down-wasting will therefore be largely driven by sublimation of ice. On Earth, cold-based glaciation can produce a number of geomorphic features, including drop mo-raines, lateral moraines, sublimation tills, and lateral melt-water channels [14]. Other unique landforms can be produced from the retreat and deposition of glacial sediments over down-wasting, buried glacial ice. Such land-forms include controlled and hummocky moraines, with the details of moraine topography being largely related to the spatial pattern, distribution, and concentration of englacial and supraglacial debris concentrations over stagnant glacial ice at the glacier's terminus [15]. Plains Units Morphology and Stratigraphy: An Upper plains unit (Upl) covers much of the intermediate areas between plateaus and surrounding LDA north of ~43N in Deuteronilus Mensae (Fig. 1). The unit is relatively smooth at Viking and CTX image resolution but is pockmarked with numerous muted impact craters and depressions formed by surficial collapse. …