Origin and Nature of a Debris-tongue in Hellas Montes, Mars

tion of a tongue-shaped landform located in the Hellas Montes area and summarize morphologically derived evidence that this landform (a) originates at a formerly unknown volcanic edifice located in the north, (b) is probably older than previously assumed, (c) has been resurfaced by subsequent mass-wasting processes taking place ontop of the flow, and (c) shows a substantial degradation morphology. The hypotheses contrast to other explanations provided by earlier workers [1-3]. We agree that this landform is connected to landslide mechanisms, as suggested by [2]. Background: Terrestrial debris avalanches are instantaneous and rapidly moving chaotic mass movements of debris and water in varying concentrations allowing transitional morphologies ranging from debris flows to mudflows and other slurries [4]. They are frequently observed at volcanic edifices where their sudden formation is triggered by volcanic eruptions and flank instabilities (e.g., Mount St. Helens, Mt. Shasta [5,6]). In 2003, Baratoux and co-workers published a paper [2] about a 50 km long and 7 km wide tongue-shaped wet debris avalanche located east of the Hellas Planitia impact basin that has been a matter of controversal discussion and was previously interpreted as a possible rock glacier analogue [2-3] (Fig. 1). Based upon the concavity of the longitudinal profile, length estimates as well as width to height ratios, Baratoux et al. [2] came to the conclusion that this landform is best described as a wet debris avalanche when compared with terrestrial data from debris flows and other debris/water mixtures. A possible connection to volcanic processes, as common in the circumHellas area, was established in that context as well and a small knob-like feature (rm3 in Fig. 2) was proposed as possible origin. Observations: For this analysis, which has taken place in the context of characterization of slope morphologies in the Hellas Montes [7], all available image data (Viking, THEMIS-VIS/IR, HRSC, MOC-NA) and topographic data from MOLA have been combined and mapped. The analyses showed that the textural inventory of the surfaces of both, debris tongue and adjacent debris apron lobes (Fig. 2), are different in terms of lineations, alignments of ridges and furrows (i.e., compression pattern) and distribution of degradational pits. The elongated shape clearly indicates a formation mechanism that is different from an earlier proposed ice/debris mixture in terms of volatile contents and grain-size distribution, resulting in flow of material with a reduced yield strength. Nevertheless, as clearly pointed out by [4], various types of debris/water mixtures and other slurries are difficult to distinguish morphologically and flows can have different properties at different locations. Natural limitations in genetic classifications based upon morphometry only are well known ORIGIN AND NATURE OF A DEBRIS-TONGUE IN HELLAS MONTES, MARS S. van Gasselt1 ([email protected]), E. Hauber2, G. Neukum1, and the HRSC Co-Investigator Team, 1Freie Universitaet Berlin, Berlin, Germany, 2Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany.