Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..156k&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction: Pedestal craters (Pd) are a subclass of impact craters unique to Mars [1] characterized by a crater perched near the center of a pedestal (mesa or plateau) that is surrounded by a quasi-circular, outward-facing scarp. The marginal scarp is usually several crater diameters from the crater rim (Figs. 2,4,5), and tens to over 100 meters above the surrounding plains (Fig. 2). Pd have been interpreted to form by armoring of the proximal substrate during the impact event. Hypotheses for the armoring mechanism include an ejecta covering [e.g., 3], increased ejecta mobilization caused by volatile substrates [4], distal glassy/melt-rich veneers [5], and/or an atmospheric blast/thermal effect [6]. Subsequently, a marginal scarp forms by preferential erosion of the substrate surrounding the armored region, most commonly thought to involve eolian removal of fine-grained, non-armored material [e.g., 3]. An understanding of the distribution of Pd, which form predominantly poleward of ~40°N and S latitude [7-9] (Fig. 1), and the role of redistribution of ice and dust during periods of climate change [e.g., 10-11], suggests that the substrate might have been volatile-rich [8-9, 12-14]. As such, some researchers [e.g., 8-9] have proposed a model for Pd formation that involves impact during periods of higher obliquity, when mid- to high-latitude substrates were characterized by thick deposits of snow and ice [e.g., 15]. Subsequent sublimation of the volatile units, except below the armored regions, yielded the perched Pd. Thus, this model predicts that thick deposits of snow/ice should underlie Pd. This is in contrast to the eolian model [3], which calls primarily for deflation of sand and dust. Here, we show the results of our study [8,16] that has documented and characterized 2461 Pd on Mars equatorward of ~65° N and S latitude (Fig. 1) in order to test these hypotheses for the origin of pedestal craters. In particular, we report on the detection of 50 Pd in Utopia Planitia and 21 Pd in Malea Planum that have pits in their marginal scarps [17]. We interpret these as sublimation pits (Fig. 3), providing evidence for snow/ice deposits preserved below the protective cover of the Pd. Marginal Pits in Pedestal Craters: Pedestal craters with marginal pits are a newly identified crater morphology in which one or more pits exist along the marginal scarp of a Pd (Figs. 2,4,5). The ejecta deposit surface (top of the pedestal) is perched ~100 m above the surrounding terrain (Fig. 2), about twice as high as a typical Pd crater. At the Pd plateau edge, the marginal scarp slopes down to the surrounding terrain, except where it is interrupted by a pit. The pits have a typical depth of ~20 m, often contain isolated mesas (Fig. 2), and are elongated, generally spanning <3 km in length (measured tangential to the pedestal margin) and <1 km in width (measured normal to the pedestal margin). In some cases, pits appear to coalesce to form larger pits (Fig. 5), and can yield a marginal, moatlike depression along a significant part of the pedestal circumference. Altimetry data from MOLA indicate that pits form in the side of the pedestal scarp; they do not extend below the elevation of the surrounding substrate (profiles in Fig. 2). Pd containing scarp pits identified thus far occur poleward of 48°N in Utopia Planitia and 58°S in Malea Planum (orange dots in Fig. 1). Pits are similar in morphology to dissected terrain [11,18] and pits on the floors of some ancient outflow channels [19], both thought to represent sublimation of an ice-rich substrate. They are also similar to formerly ice-rich and now beheaded pits in the proximal part of debris-covered glaciers on Earth [20] and Mars [21] (see also [22]). Both of the regions in which we observe Pd with marginal pits also exhibit scallop-shaped depressions, indicative of sublimation of interstitial ice [e.g. 23-25]. Climate models show that these specific regions are both predicted to have high seasonal water-ice accumulations during periods of high obliquity [26,27]. Discussion: The morphologic similarity between the marginal pits associated with Pd and ice sublimation pits leads us to favor an origin of preferential sublimation of ice/snow from the Pd scarp. In this interpretation, an impact crater forms in a thick (~10s to ~100s m) regional highlatitude deposit of ice and snow, mixed with dust. The area around the crater (the future pedestal surface) is armored by proximal ejecta and distal sintering effects of impact melt and atmospheric blast/thermal effects accompanying crater formation [5-6]. Following crater formation, obliquitydriven climate change leads to removal of the intervening snow and ice, leaving the Pd perched. Over time, the volatile-rich scarp margins, where the armoring tapers off, undergo continued sublimation to produce the pits, while the heavily armored Pd surface inhibits/prevents sublimation of underlying volatiles (Fig. 3). Ice-rich layered substrates are thus interpreted to be preserved under Pd. On the basis of our analysis, Pd represent the remnants of a past extensive, layered, climate-related deposit, similar to, but thicker than the latitude-dependent mantle emplaced in a recent ice age [11,18]. Due to the large number and widespread distribution of Pd (Fig. 1) [8,9,16], we believe that this climate-related deposit persisted for a considerable part of the recent past, implying that obliquity was relatively higher than at present during a significant portion of the Amazonian period of the history of Mars. References: [1] Barlow, N. et al. (2000) JGR, 105, 26733. [2] McCauley, J. (1973) JGR, 78, 4123. [3] Arvidson, R. (1976) Icarus, 27, 503. [4] Osinski, G. (2006) MAPS, 41, 1571. [5] Schultz, P. and Mustard, J. (2004) JGR, 109, E01001. [6] Wrobel, K. et al. (2006) MAPS, 41, 1539. [7] Mouginis-Mark, P. (1979) JGR, 84, 8011. [8] Kadish, S. and Barlow, N. (2006) LPSC 37, #1254. [9] Kadish, S. et al. (2008) LPSC 39, #1766. [10] Jakosky, B. et al. (1995) JGR, 100, 1579. [11] Head, J. et al. (2003) Nature, 426, 797. [12] Barlow, N. (2005) RVAMIC, #3041. [13] Head, J. and Roth, R. (1976) LSI, 50-52. [14] Schultz, P. and Lutz, A. (1988) Icarus, 73, 91. [15] Levrard, B. et al. (2004) Nature, 431, 1072. [16] Kadish, S. et al. (2008) JGR, in progress. [17] Kadish, S. et al. (2008) GRL, in progress. [18] Mustard, J. et al. (2001) Nature, 412, 411. [19] Levy, J. and Head, J. (2005) Terra Nova, 17, 503. [20] Marchant, D. and Head, J. (2007) Icarus, 192, 187. [21] Head, J. and Marchant, D. (2008) Workshop on Martian Gullies, #8009. [22] Moore, J. et al. (1996) Icarus, 122, 63. [23] Lefort, A. et al. (2006) 4th Mars Polar Science Conf., #8061. [24] Zanetti, M. et al. (2008) LPSC 39, 1682. [25] Morgenstern, A. et al. (2007) JGR, 112, E06010. [26] Forget, F. et al. (2006) Science, 311, 368-371. [27] Madeleine, B. et al. (2007) LPSC 38, #1778.
Barlow Nadine G.
Head James W.
Kadish Seth J.
Marchant David R.
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