Degradation of mid-latitude craters on Mars
Introduction
The history of water on Mars is a fundamental question central to our understanding of the planet. From current and previous spacecraft missions, it is clear that the evolution of the martian surface has been diverse with respect to volatile distribution and abundance as well as in regard to the myriad roles volatiles play in geologic processes. Both liquid water and water ice have likely had a significant impact in shaping martian landscapes, with specific landforms and deposits interpreted to result from fluvial, lacustrine, periglacial, and glacial processes (e.g., Baker et al., 1991, Kargel and Strom, 1992, Scott et al., 1995, Carr, 1996, Cabrol and Grin, 1999, Cabrol and Grin, 2001, Baker, 2001, Malin and Edgett, 2001). Water and/or ice also influence mass-wasting, volcanism, and the degradation of surficial materials (e.g., Lucchitta, 1987, Squyres et al., 1987, Crown et al., 1992, Mangold, 2003, Milliken et al., 2003, Tanaka et al., 2005). Geomorphic indicators of ground ice have been identified and described (e.g., Carr and Schaber, 1977, Squyres and Carr, 1986), theoretical constraints for the distribution of subsurface ice developed (e.g., Squyres et al., 1992, Clifford, 1993, Mellon and Jakosky, 1995, Barlow and Perez, 2003), and recent spacecraft observations have revealed the variability of ice content in the regolith (Boynton et al., 2002, Feldman et al., 2007). The martian mid-latitudes are thought to be zones where the effects of ice on the surface geology are prominent and representative of geologically recent periods of volatile-driven activity (e.g., Malin and Edgett, 2000, Mustard et al., 2001, Christensen, 2003, Head et al., 2003, Head et al., 2005).
The degradation of craters in ice-rich environments is vital to our understanding of the geologic evolution of the terrain surrounding the crater, as well as for assessing whether the source of the ice is atmospheric or from the subsurface. Features such as arcuate ridges, gullies, and small flow lobes can be used to understand the distribution, abundance, and styles of emplacement of the ice. In this investigation, we explore the distribution and nature of geomorphic indicators of ground ice found in association with impact craters >20 km in diameter in mid-latitude regions in the northern and southern hemispheres of Mars in order to assess their associations, describe and interpret observed suites of features, and examine dependencies on such factors as slope, orientation, latitude, and geologic setting.
Section snippets
Background
Geomorphic indicators of ground ice have long been acknowledged, having been first observed in Viking Orbiter images, and continue to be cited as evidence for volatile-rich zones within the martian mid-latitudes. Among the most prominent of these features are lobate debris aprons, lineated valley fill, and concentric crater fill, which are found in the fretted terrain of the northern hemisphere and in areas surrounding the Argyre and Hellas impact basins in the southern hemisphere (Carr and
Previous results
A thorough survey of craters in the region surrounding Newton Basin by Berman et al. (2005) identified a suite of geomorphic features on their interior rims and suggested a sequence of crater degradation for mid-latitude regions. Two hundred and twenty-five craters between 2–30 km in diameter with sufficient image coverage (e.g., both northern and southern walls imaged) were examined. One hundred eighty-eight of these had gullies on some portion of their walls; 118 of these contained arcuate
Survey
Our general approach for this study was to explore the distribution and nature of geomorphic indicators of ground ice found in association with impact craters, and to assess their associations and dependencies on such factors as slope, orientation, and latitude. Our specific approach included the following steps: (1) identification of mid-latitude study areas; (2) identification of all craters greater than 20 km in diameter in these study regions; (3) assembly of a GIS database that allowed us
Crater morphologies
We have found a series of landforms on the walls and floors of craters indicative of the presence of water or ice, including lobate flows; narrow channels within and around the crater; trough-like crater-wall valleys (sometimes filled with mantling deposits); gullies (with accompanying fans and alcoves); filled and unfilled alcoves; arcuate ridges; and pitted and lineated debris aprons.
All of the above features were found in both the eastern Hellas and Arabia Terra study regions. However,
Morphology
Morphologic observations of individual features and the suites of features found in mid-latitude craters are suggestive of significant glacial and fluvial activity. Features indicative of glacial activity (or ice-related mass wasting) include lobate flows, arcuate ridges, crater-wall valley deposits, and debris aprons. Lobate flows have been previously compared to terrestrial debris-covered glaciers (Milliken et al., 2003, Arfstrom and Hartmann, 2005) and rock glaciers (Mahaney et al., 2007).
Conclusions
The specific results from this study can be summarized as follows:
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In our two martian mid-latitude study regions, lobate flows, channels, and other fluvial and glacial features are common in craters with diameters >20 km.
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Pole-facing orientations for lobate flows, gullies and arcuate ridges are prevalent in latitudes between 30° and ∼45°.
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Equator-facing orientations are more prevalent than pole-facing orientations between ∼45° and 60°.
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In our southern mid-latitude study region, narrow channels
Acknowledgments
The authors thank Nathan Bridges and Nathalie Cabrol for their insightful reviews, as well as Matt Balme for his helpful suggestions. All MOC images courtesy of NASA/JPL/Malin Space Science Systems. MOLA data courtesy of NASA/GSFC. THEMIS data courtesy of NASA/JPL/ASU. This work was supported by NASA Grants Nos. NNG05GQ43G and NNG05GQ78G from the Mars Data Analysis Program. This is PSI Contribution 445.
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