Degradation of mid-latitude craters on Mars

Abstract

Recent geomorphic, remote sensing, and atmospheric modeling studies have shown evidence for abundant ground ice deposits in the martian mid-latitudes. Numerous potential water/ice-rich flow features have been identified in craters in these regions, including arcuate ridges, gullies, and small flow lobes. Previous studies (such as in Newton Basin) have shown that arcuate ridges and gullies are mainly found in small craters ($\sim 2\text{–}30\text{km}$ in diameter). These features are located on both pole-facing and equator-facing crater walls, and their orientations have been found to be dependent on latitude. We have conducted surveys of craters $>20\text{km}$ in diameter in two mid-latitude regions, one in the northern hemisphere in Arabia Terra, and one in the southern hemisphere east of Hellas basin. In these regions, prominent lobes, potentially ice-rich, are commonly found on the walls of craters with diameters between ∼20–100 km. Additional water/ice-rich features such as channels, valleys, alcoves, and debris aprons have also been found in association with crater walls. In the eastern Hellas study region, channels were found to be located primarily on pole-facing walls, whereas valleys and alcoves were found primarily on equator-facing walls. In the Arabia Terra study region, these preferences are less distinct. In both study regions, lobate flows, gullies, and arcuate ridges were found to have pole-facing orientation preferences at latitudes below 45° and equator-facing orientation preferences above 45°, similar to preferences previously found for gullies and arcuate ridges in smaller craters. Interrelations between the features suggest they all formed from the mobilization of accumulated ice-rich materials. The dependencies of orientations on latitude suggest a relationship to differences in total solar insolation along the crater walls. Differences in slope of the crater wall, differences in total solar insolation with respect to wall orientation, and variations in topography along the crater rim can explain the variability in morphology of the features studied. The formation and evolution of these landforms may best be explained by multiple cycles of deposition of ice-rich material during periods of high obliquity and subsequent modification and transport of these materials down crater walls.

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.