Investigation of the distal margins of the LVF/LDA environments in the northern midlatitudes of Mars.

Details Investigation of the distal margins of the LVF/LDA environments in the northern midlatitudes of Mars. Investigation of the distal margins of the LVF/LDA environments in the northern midlatitudes of Mars.

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..638m&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

Computer Science

Scientific paper

Introduction The northern dichotomy boundary on Mars consists of an abrupt escarpment between the cratered uplands and the northern plains. The boundary is characterized by fretted valleys which divide the uplands into a series of plateaus and mesas which become progressively smaller to the north and eventually merge with the northern plains [1]. Lineated Valley Fill (LVF) and Lobate Debris Aprons (LDA) deposits are a prominent feature of the dichotomy boundary [2] and have been attributed by some [e.g., 3] to be the remains of debris-covered glaciers that were active during previous climatic regimes when snow was deposited along the dichotomy boundary [4-5]. Recent in depth analysis of LVF/LDA has been made possible by new comprehensive high-resolution data from Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter. and has revealed evidence for multiple LVF/LDA emplacement events [6-8], as well as evidence for plateau glaciation [4] and evidence that glacial conditions extended much further from the margins of the dichotomy boundary than previously thought [4]. However, remaining unexamined, however, are the regions adjacent to the current positions of the LVF/LDA deposits where one can explore the relationship between the distal landforms present there and the main bodies of LVF/LDA. We have therefore investigated an area of the northern plains directly north of a region of the dichotomy boundary where detailed studies have reported largescale integrated LVF systems >10,000 km3 in extent, that are analogous to terrestrial debris covered glacial systems [3,6-8]. Through the utilization of the most recent data sets (CTX and HiRISE) we have categorized the distinct landform assemblages associated with this area and documented the degraded nature of lobate debris aprons and related features present there. Degraded Lobate Debris Aprons All of the LDAs in the study region have troughs cut into their surface which are aligned in the same direction as the flanks of the mesa from which the LDA originates. In the case of the most northerly LDAs, the contact between the upper most portions of the LDAs and the mesa flanks forms a prominent scarp. Scalloped morphology can be seen along portions of the scarps, forming pits (Fig. 1). Such features are similar to pits formed at the edge of pedestal craters, which have been attributed to sublimation [9]. Elongated troughs, which extend laterally in a direction away from the flanks of the mesas, are also observable on some of the LDAs in the region (Fig. 1). These features are found further downslope along the surface of the LDAs than the first type of trough. Closer inspection of these troughs reveals that the majority of them consist of a series of circular pits. In some of the areas, the troughs form broad triangular indentations that are wider in the downslope direction (Fig. 1). The 90º difference in orientation between the two troughs may reflect structural differences between the material close to the mesa flanks and the material that is further downslope. Elongated alcoves several hundred meters across are commonplace along the exposed flanks of the mesas and likely represent the erosional scars produced by mass wasting processes which have supplied the debris that currently covers the surface of the LDAs. Destabilization of the debris that constitutes the upper portion of the aprons that are in direct contact with the mesa flanks (as a result of deflation) may expose ice and permit enhanced sublimation, forming the scalloped pits. Conversely the chains of pits orientated in the downslope direction may be related to the flow patterns present in the LDA when they were active. Regions of divergence formed by the outward expansion of the LDA during its formation may have generated areas which had a thinner debris content than the adjacent surfaces. Such areas would tend to experience enhanced sublimation in the current climate, causing the formation of the pits. This would explain why some of the troughs become wider as a function of distance away from the mesa flanks. Layered Deposits A landform of particular interest within the areas adjacent to the large scale expanses of LVF/LDA are small scale (~ 500 m) deposits that appear to consist of relatively evenly spaced layers (Fig. 2). Carr (2001) [10], utilizing the then newly acquired MOC data first observed these small scale remnants of layered deposits and noted they were superimposed on the pre-existing terrain and were found across regions throughout the fretted terrain. The deposits are typically found adjacent to the flanks of isolated small scale ridges and craters. They are angled in the upslope direction with a dip that matches the broad trend of the slopes, and causes the layers (which are ~ 10s of meters thick) to be orientated in an upward direction. High resolution views of the surface of the deposits reveal that they appear relatively softened and subdued (Fig. 2). Boulders ~1 meter in diameter occur in large numbers across the surface of the deposits, but appear to be just as numerous as on the surface of the adjacent slopes and surrounding plains (Fig. 2). Broad rectangular polygonal patterns ~20 m across consisting of narrow troughs (~ 1 m wide) are present on the surface of all the layered deposits. However, this pattern does not extend into the surrounding terrain; this supports the idea that the layered terrain might be more ice-rich. The polygons maintain a constant pattern over the entire surface of the deposits and do not appear to be affected by the occurrence of the layers. We tentatively interpret these features as remnants of a once much more widely-distributed layered ice sheet that extended into the northern lowlands [4]. Detailed mapping of their characteristics and locations is underway to assess the full former extent of such a deposit. References [1] Sharp, R., 1973. JGR, 78, 4073-4083. [2] Squyres S.. 1979, Icarus, 34, 600. [3] Head, J.W. et al., 2006. Earth and Planet Sci Lets, 241, 663-671. [4] Head, J. and Marchant, D., 2008, EPSC, this volume. [5] Madeleine, J-B. et al, 2007, LPSC 38, 1778. [6] Dickson, J. et al, 2008. Geology, 36, 411-414, doi: 10.1130/G24382A.1. [7] Levy. J. et al, 2007. JGR, 112, E08004, doi: 10.1029/2006JE002852. [8] Morgan, G. et al, Icarus, in revision. [9] Kadish, S. et al, GRL, in review.. [10] Carr, M., 2001, JGR, 106, 23,571-23,593.

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