High-latitude cold-based glacial deposits on Mars: Multiple superposed drop moraines in a crater interior at 70°N latitude

available online at http://meteoritics.org 1659 © The Meteoritical Society, 2006. Printed in USA. High-latitude cold-based glacial deposits on Mars: Multiple superposed drop moraines in a crater interior at 70°N latitude James B. GARVIN1, James W. HEAD2*, David R. MARCHANT3, and Mikhail A. KRESLAVSKY2, 4 1National Aeronautics and Space Administration Headquarters, Washington, D.C. 20546, USA 2Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA 3Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, USA 4Kharkov Astronomical Institute, 35 Sumska, Kharkov, 61022 Ukraine *Corresponding author. E-mail: [email protected] (Received 03 November 2005; revision accepted 15 March 2006) Abstract–An impact crater 26.8 km in diameter, located in the northern lowlands (70.32°N, 266.45°E) at the base of the flanking slopes of the shield volcano Alba Patera, is characterized by highly unusual deposits on its southeastern floor and interior walls and on its southeastern rim. These include multiple generations of distinctive arcuate ridges about 115–240 m in width and lobate deposits extending down the crater wall and across the crater floor, forming a broad, claw-like, ridged deposit around the central peak. Unusual deposits on the eastern and southeastern crater rim include frost, dunes, and a single distal arcuate ridge. Based on their morphology and geometric relationships, and terrestrial analogs from the Mars-like Antarctic Dry Valleys, the floor ridges are interpreted to represent drop moraines, remnants of the previous accumulation of snow and ice, and formation of cold-based glaciers on the crater rim. The configuration and superposition of the ridges indicate that the accumulated snow and ice formed glaciers that flowed down into the crater and across the crater floor, stabilized, covering an area of about 150 km2, and produced multiple individual drop moraines due to fluctuation in the position of the stable glacier front. Superposition of a thin mantle and textures attributed to a recent ice-age period (~0.5–2 Myr ago) suggest that the glacial deposits date to at least 4–10 Myr before the present. At least five phases of advance and retreat are indicated by the stratigraphic relationships, and these may be related to obliquity excursions. These deposits are in contrast to other ice-related modification and degradation processes typical of craters in the northern lowlands, and may be related to the distinctive position of this crater in the past atmospheric circulation pattern, leading to sufficient preferential local accumulation of snow and ice to cause glacial flow.An impact crater 26.8 km in diameter, located in the northern lowlands (70.32°N, 266.45°E) at the base of the flanking slopes of the shield volcano Alba Patera, is characterized by highly unusual deposits on its southeastern floor and interior walls and on its southeastern rim. These include multiple generations of distinctive arcuate ridges about 115–240 m in width and lobate deposits extending down the crater wall and across the crater floor, forming a broad, claw-like, ridged deposit around the central peak. Unusual deposits on the eastern and southeastern crater rim include frost, dunes, and a single distal arcuate ridge. Based on their morphology and geometric relationships, and terrestrial analogs from the Mars-like Antarctic Dry Valleys, the floor ridges are interpreted to represent drop moraines, remnants of the previous accumulation of snow and ice, and formation of cold-based glaciers on the crater rim. The configuration and superposition of the ridges indicate that the accumulated snow and ice formed glaciers that flowed down into the crater and across the crater floor, stabilized, covering an area of about 150 km2, and produced multiple individual drop moraines due to fluctuation in the position of the stable glacier front. Superposition of a thin mantle and textures attributed to a recent ice-age period (~0.5–2 Myr ago) suggest that the glacial deposits date to at least 4–10 Myr before the present. At least five phases of advance and retreat are indicated by the stratigraphic relationships, and these may be related to obliquity excursions. These deposits are in contrast to other ice-related modification and degradation processes typical of craters in the northern lowlands, and may be related to the distinctive position of this crater in the past atmospheric circulation pattern, leading to sufficient preferential local accumulation of snow and ice to cause glacial flow. INTRODUCTION AND BACKGROUND It has long been known that Mars contains polar caps and that they are largely composed of water ice and dust. Less well understood are the presence and nature of glacial flow in polar regions (e.g., Thomas et al. 1992), the possibility of glacial processes operating outside the polar regions (e.g., Lucchitta 1981), and the mode of formation of circumpolar craters (e.g., Garvin and Frawley 1998; Garvin et al. 2000a, 2000b, 2002) that contain significant high-albedo mounds and deposits (e.g., Garvin et al. 2000b; Russell and Head 2005). Recent studies have shown that deposits similar to those of debris-covered glaciers occur in mid-latitude impact craters (e.g., Marchant and Head 2003; Perron et al. 2003; Kargel 2004), and that circumpolar craters (65–80° latitude) currently contain remnant ice deposits (Garvin et al. 2000b) some of which may be glacial in origin (e.g., Russell and Head 2005). Analysis of new data from spacecraft exploring Mars (Head and Marchant 2003; Shean et al. 2005) and a better understanding of glacial processes in terrestrial, hyperarid, cold polar deserts analogous to the Mars environment (Marchant and Head 2004) have led to the documentation of tropical mountain glaciers and their distinctive deposits (e.g., Head et al. 2003; Shean et al. 2005; Parsons and Head 2005; Milkovich et al. 2006), as well as mid-latitude deposits of apparent glacial origin (e.g., Head et al. 2006a, 2006b). Most circumpolar craters that show evidence of icy fill (65–80° latitude) have distinctive concentrations of ice 1660 J. B. Garvin et al. around the central peak (e.g., Korelev), lobate deposits attached to polar layered terrain, or isolated mounds of material usually along the base of the pole-facing crater wall (e.g., Russell et al. 2004; Russell and Head 2005). Here we report on a crater in the same 65–80° latitude range, but with a distinctly different crater interior deposit. We describe the crater occurrence and characteristics, its distinctive deposits that we interpret to be remnant drop moraines, and the conditions and sequence of events implied in the origin and evolution of the deposits. We conclude that this unusual deposit represents the remnants of a cold-based glacier that formed as a result of snow and ice accumulation on the southeastern rim of the crater due to localized environmental conditions; the resulting cold-based glacier flowed down the crater wall, climbed the central peak structure and was passively diverted by it, and then underwent several phases of advance and retreat. These deposits appear to be very young in age. DESCRIPTION OF THE CRATER AND DEPOSITS The unusual deposits occur in association with a crater 26.8 km in diameter located in the northern lowlands (70.32°N, 266.45°E) at the base of the flanking slopes of the huge shield volcano Alba Patera, which forms the northern flank of the Tharsis Rise (Fig. 1). The crater formed at an elevation of about −4304 m and is about 1.6 km deep (Table 1). The impact occurred on the late-Hesperian-aged to early-Amazonian-aged Vastitas Borealis Formation (Tanaka and Scott 1987; Tanaka et al. 2004; Boyce et al. 2005) (Figs. 1b and 1c). The crater is characterized by a somewhat degraded lobate ejecta deposit (Fig. 1c), a distinctive central peak rising about 430 m from the crater floor (Figs. 2 and 3), and a relatively flat floor, with the northwestern portion of the floor being the deepest (Figs. 2 and 3). Topographic maps and profiles (Figs. 2 and 3) show that the southeast portion of the floor is shallower and lies about 250 m above the northwest floor. Altimetric profiles (Fig. 2) show that at the resolution of the Martian Orbiter Laser Altimeter (MOLA) gridded data set, the upper wall slopes are in the 8–14° range, and the lower wall slopes are in the 3–8° range. The profiles also suggest that crater walls are steeper on the southeast than the northwest (Fig. 3) (A-A’). Thermal Emission Imaging System (THEMIS) data reveal the presence of a complex set of linear and arcuate ridges, ranging from ~115 to 240 m in width (the average of 35 measurements is 175 m), extending from near the southeastern crater wall base out onto the crater floor (Figs. 4 and 5). Due to the small width of the ridges, MOLA data do not provide precise measurements of ridge heights, but multiple individual orbit profiles that cross the ridges (for example, those shown in Fig. 3) suggest that they range up to 20–30 m in height and about 100 m in width, and generally appear symmetrical. Along the crater floor and basal wall, the ridges are oriented generally radially to the crater, extending as linear ridges hundreds of meters to several kilometers long (Figs. 4 and 5). On the crater floor, some of the ridges become arcuate and lobate in form and display complex intersecting and superposition patterns (Fig. 5). In the vicinity of the central peaks, the ridges display multiple tight lobate patterns, forming a bifurcating, claw-like structure around the base and southeastern flank of the central peak (Fig. 5). Some ridges, particularly the marginal ones, are very continuous and extend as much as ~15 km from the lower crater wall out onto the crater floor. Overall, the set of ridges form a distinctive sweeping pattern extending from near the southeastern crater wall base out onto the floor and central peak, where they form the claw-like structure on and around the central peak (Fig. 5). The ridges are more continuous toward the margins of the deposits and in the area surrounding the central peak, and evidence for the multiple phases and overlapping relationships are seen more readily on the floor and area surrounding the central peaks (Fig. 5). Perspective views (Fig. 4) emphasize that the set of ridges forms a contiguous deposit extending down the wall and out onto the floor, rising up onto the central peak summit and then bifurcating and forming two complex and multicomponent marginal lobes. The southern lobe extends ~2 km further than the northern one (Fig. 5), actually rising up onto the base of the western crater wall (Fig. 2). All concentric ridge structures are convex outward, away from the southeastern wall. Overlapping ridges (see arrows in Fig. 5b) indicate superposition relationships that imply at least five phases of successive lobe formation. Although Mars Orbiter Camera (MOC) images of the ridges on the floor are not currently available, the ridges themselves appear generally laterally continuous at local scales and symmetrical in cross-section in THEMIS data. Associated features on the crater floor include a dark patch (Fig. 5a), interpreted to be eolian dune deposits superposed on the lobate deposit on the southeastern slope of the central peak. No evidence of additional structures, such as fractures, gullies, or channels was observed. Superposition relationships show that the deposit largely overlies radial wall textures and deposits at the base of the walls (Figs. 4 and 5). The THEMIS image centered on the crater interior (Fig. 2) does not provide coverage of the upper crater wall, but insight into the geology there is provided by a MOC image (Figs. 6 and 7) that partially overlaps the THEMIS image (Fig. 7a). Here the morphology can be subdivided into four zones (Figs. 7b and 7c). The lowermost zone 1 overlaps with several of the ridges in the THEMIS image and extends for several kilometers across the crater wall and about 150 m up the crater wall (Fig. 7d), where the ridges tend to die out and be replaced by zone 2, which is characterized by a more hummocky terrain that represents the middle parts of the crater interior wall. The boundary between these two zones, and indeed some of the deposits within zone 1, are lobate (see High-latitude cold-based glacial deposits on Mars 1661 the outward facing scarps in zone 1 indicated by tick marks in Fig. 7c). Zone 3 is near the rim crest and is characterized by a very distinctive texture of relatively dark domes 10–20 m in diameter, very similar to the bumpy basketball-textured terrain seen on the latitude dependent mantle (Kreslavsky and Head 2000, 2002; Mustard et al. 2001; Head et al. 2003). Although this terrain is very well developed in this zone, this same texture is seen throughout all four zones in this image. Zone 4 is on the crater rim and consists of a series of dune-like ridges that are oriented parallel to slightly tangential to the crater rim crest. At the innermost ridge (about at the location of the rim crest), a white, frost-like patch is seen and several other patches of bright, frost-like deposits occur in the interridge areas. This image was taken at Ls of 108°, at the beginning of northern summer. An additional MOC image covers a portion of the southeastern rim of the crater (Figs. 6 and 8) and shows that the dune-like features extend for an additional several kilometers out onto the rim. Five zones can be mapped in this image (Figs. 8b and 8c). The boundary between zone 1 and 2 Fig. 1. A location map for the crater. a) A global view centered on 34°N, 265°E, showing the Tharsis Rise (lower left), Alba Patera (center left), and the northern lowlands (upper, purple). The arrow indicates the crater location at 70.32°N, 266.45°E. b) A MOLA gradient context image showing the detailed location and morphology of the crater at the base of the Alba Patera rise (smooth area with several graben in the bottom of the image) and the rougher northern lowlands. c) A MOLA gradient context image showing the detailed morphology of the crater. North is to the top in this and all other images. 1662 J. B. Garvin et al. represents the approximate crater rim crest. Zone 2 contains the dune-like ridges, which generally have a spacing of ~160–200 m, but are spaced up to 300–400 m and appear to represent reworked material on the crater rim. They occur on the steepest portion of the crater rim just exterior to the rim crest (see Figs. 6 and 7d). They could represent the result of viscous flow of ice-rich material perhaps related to the evolution of the ice-rich deposit (e.g., gelifluction or solifluction lobes). Alternatively, they could be eolian dunes related to circulation patterns influenced by the presence of the crater. The distribution of these dune-like features and the interpretation of them as formed by eolian reworking is strengthened by the bright, streak-like deposit seen extending about 60 km downrange from the crater rim in an ESE direction in the MOC narrow-angle (NA) image (Fig. 8a). Southeast of the dune-like features, the surface is broadly smoother (zone 3) and the topography of the underlying ejecta deposit and precrater substrate can be seen (zone 4). At the lower margin of the MOC coverage of this area, a single broadly arcuate ridge about 100 m in width is observed (defining the boundary between zones 3 and 5, marked “A” in Fig. 8c), which is similar in morphology and scale to the ridges seen within the crater (Figs. 5 and 8e). Also observed is a single, fresh-appearing impact crater about 400 m in diameter with a distinctive lobate ejecta deposit (Fig. 8d; marked “B” in Fig. 8c). The western rim and crater interior differ considerably from the eastern rim and interior (Figs. 6 and 9) with no evidence for the large dune-like features, ridges, or bright frost patches seen on the southeastern rim, and no radial ridges or lobe-like deposits on the crater inner wall (compare Figs. 8 and 9). Most of the surface of the rim and wall is characterized by a thin, hummocky mantling material that appears to uniformly cover the area except for topographic prominences on the crater rim and wall. The superposition relationships of the ridges and the internal stratigraphy of the deposit (Figs. 4, 5, and 10) show that the ridges form several sets of broad, continuous lobes of different sizes that are superposed on one another, often with no disruption of the underlying ridges, but in many cases apparently obscuring them due to deposition. Cross-cutting and overlapping relationships were determined and the sequence of lobe emplacement defined by these overlapping relationships is shown in Fig. 10. The first stage appears to be a broad lobe that extends from the wall out onto the crater floor to the base of the central peaks. The second stage extends further out onto the crater floor and bifurcates more around the base of the central peak. The third stage is the most areally extensive, extends down the wall and across the floor, and splits in a claw-like pattern around the central peak, with the southern part of the claw extending up onto the lower part of the far crater wall. The total distance from the crater rim crest to the distal part of this phase is about 20 km. The fourth stage crosses the floor and bifurcates at the central peak, climbing nearly to the peak summit, but does not extend as far across the crater floor as the third phase. The fifth and apparently last phase is limited in both width and extent, extends down the wall and out to the base of the central peak in a swath about 3.5 km wide.