Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes

Icarus 202 (2009) 462–476

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Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes Joseph S. Levy a , James W. Head a,∗ , David R. Marchant b a b

Brown University, Department of Geological Sciences, 324 Brook Street, Box 1846, Providence, RI 02912, United States Boston University, Department of Earth Sciences, 675 Commonwealth Ave., Boston, MA 02215, United States

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 21 October 2008 Revised 27 January 2009 Accepted 24 February 2009 Available online 3 March 2009

At martian mid-to-high latitudes, the surfaces of potentially ice-rich features, including concentric crater fill, lobate debris aprons, and lineated valley fill, typically display a complex texture known as “brain terrain,” due to its resemblance to the complex patterns on brain surfaces. In order to determine the structure and developmental history of concentric crater fill and overlying latitude-dependent mantle (LDM) material, “brain terrain” and polygonally-patterned LDM surfaces are analyzed using HiRISE images from four craters in Utopia Planitia containing concentric crater fill. “Brain terrain” and mantle surface textures are classified based on morphological characteristics: (1) closed-cell “brain terrain,” (2) open-cell “brain terrain,” (3) high-center mantle polygons, and (4) low-center mantle polygons. A combined glacial and thermal-contraction cracking model is proposed for the formation and modification of the “brain terrain” texture of concentric crater fill. A similar model, related to thermal contraction cracking and differential sublimation of underlying ice, is proposed for the formation and development of polygonally patterned mantle material. Both models require atmospheric deposition of ice, likely during periods of high obliquity, but do not require wet active layer processes. Crater dating of “brain terrain” and mantled surfaces suggests a transition at martian mid-latitudes from peak “glacial” conditions occurring within the past ∼10–100 My to a quiescent period followed by a cold-desert “periglacial” period during the past ∼1–2 My. © 2009 Elsevier Inc. All rights reserved.

Keywords: Mars Mars, climate Mars, surface Ices

1. Introduction Glaciation, and ice-related processes, have shaped martian middle and high latitudes during the late Amazonian (Mustard et al., 2001; Kreslavsky and Head, 2002; Head et al., 2003, 2006a; Kuzmin, 2005; Forget et al., 2006; Fastook et al., 2008; Head and Marchant, 2009). The record of variable Amazonian climate conditions is indicated by a variety of martian landforms (Marchant and Head, 2007), including the latitude-dependent mantle (LDM) (Mustard et al., 2001; Head et al., 2003; Milliken et al., 2003; Schon et al., 2008); concentric crater fill, lobate debris aprons, and lineated valley fill (Squyres, 1979; Lucchitta, 1984; Squyres and Carr, 1986; Head et al., 2006b); polygonally patterned ground (Mangold, 2005; Levy et al., 2009; Mellon et al., 2008); and pedestal craters (Kadish and Barlow, 2006; Kadish et al., 2008). Questions arise as to the nature and timing of Amazonian climate change, and whether conditions that might have led to melting have occurred (Kreslavsky et al., 2008; Soare et al., 2008). Further, unusual textures are observed on the surfaces of many Amazonian

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Corresponding author. Fax: +1 401 863 3978. E-mail addresses: [email protected] (J.S. Levy), [email protected] (J.W. Head), [email protected] (D.R. Marchant). 0019-1035/$ – see front matter doi:10.1016/j.icarus.2009.02.018

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2009 Elsevier Inc. All rights reserved.

examples of concentric crater fill, lineated valley fill, and lobate debris aprons. These textures include pit-and-butte texture (Mangold, 2003), “knobs—brain coral” (Williams et al., 2008), “brain coral terrain” (Dobrea et al., 2007), or, succinctly, “brain terrain” (Levy et al., 2009); the origin, age, and climate conditions represented by these surface textures remain an area of active research. We examine concentric crater fill surface textures in Utopia Planitia in order to assess initial emplacement conditions and processes (Figs. 1 and 2). We document relationships between concentric crater fill surfaces and overlying latitude-dependent mantle (LDM) material (hereafter referred to simply as mantle material) in four Utopia Planitia craters. Finally, we date these surfaces using crater retention ages and propose a model for their formation, and modification history. Although several hypotheses have been suggested for the origin of concentric crater fill, ranging from aeolian modification (Zimbelman et al., 1989) to rock-glacier processes (Mangold and Allemand, 2001), lineated and lobate concentric crater fill surface patterns strongly indicate glacier-like flow (Head et al., 2006a; Levy et al., 2007). “Brain terrain” appears to be a modification of concentric crater fill lobe and lineation patterns. In contrast, the latitude-dependent mantle (LDM) is described as a flat-lying, or draped surface unit, meters to tens of meters thick, which has a variety of characteristic surface textures, including

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Fig. 1. Context map of study area in Utopia Planitia. Individual HiRISE image locations are shown. Base map is MOLA shaded relief topography.

Fig. 2. Image location maps for subsequent figures. (a) Concentric crater fill and mantle material from PSP_002782_2230 over CTX image P05_002782_2232. North to image top and illumination is from the lower left. (b) Concentric crater fill and mantle material from PSP_002175_2210 over CTX image P03_002175_2211. North to image top and illumination is from the lower left.

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Fig. 3. (a) Closed-cell “brain terrain.” Sinuous, elevated “cells” are approximately 20 m wide, and up to ∼100 m long. Image is 200 m wide. North to image top. Portion of PSP_002175_2210. (b) Open-cell “brain terrain.” Sinuous ridges of positive topography outline flat-floored “cells.” Images is 200 m wide. North to upper right. Portion of PSP_002175_2210. (c) High-center mantle polygons. Image is 200 m wide. North to image top. Portion of PSP_002175_2210. (d) Low-center mantle polygons. Image is 100 m wide. North to image top. Portion of PSP_002782_2230.

polygonal patterning, pitting, and scalloping (Mustard et al., 2001; Head et al., 2003; Milliken et al., 2003; Schon et al., 2008). In the Utopia Planitia study region, both “brain terrain” and polygonally patterned mantle are present and in some locations, “brain terrain” underlies the polygonally patterned mantle. Detailed analysis of polygon morphology at the mantle surface provides insight into the processes of mantle emplacement, and in places, of interactions and modification of the underlying “brain terrain.” These morphological and stratigraphic relationships permit us to reconstruct a history of ice-related processes in the martian middleto-high latitudes during the recent Amazonian. These processes range from debris-covered glacial activity (i.e., the formation of alpine and debris-covered glaciers that remain long-lived due to the preservative effects of capping sublimation lags) (Marchant et al., 2002; Kowalewski et al., 2006) to cold desert “periglacial” processes (cold-climate, non-glacial geomorphological processes such as permafrost development, thermal contraction cracking, etc.) (Washburn, 1973). 2. Morphology 2.1. “Brain terrain” At HiRISE resolution (∼30 cm/pixel), the surface of concentriccrater-fill “brain terrain” displays a complex morphology composed of smaller, discrete surface structures that we term “cells.” Two distinct textures are commonly present in “brain terrain” observed in Utopia Planitia concentric crater fill: closed-cell “brain terrain”

and open-cell “brain terrain” (Figs. 3–5). Similar features have been observed elsewhere, notably on lineated valley fill and lobate debris apron surfaces (Williams et al., 2008; Dobrea et al., 2007). Closed-cell “brain terrain” in Utopia Planitia exhibits arcuate, mounded cells with both flat and rounded upper surfaces. Cells are commonly ∼10–20 m wide, ∼10–100 m long, and ∼4–5 m high (based on HiRISE image shadow measurements). Some closed-cell “brain terrain” cells have surface grooves or furrows located near the centerline of the long axis (Figs. 6–8). Closed-cell “brain terrain” cells occur singly, or in linked groups (Figs. 6–8). Closed-cell “brain terrain” cells commonly form lineations that are oriented concentrically to the crater in which the unit is present (Figs. 5, 9 and 10). Spacing between closed-cell “brain terrain” lineations is variable, but is commonly ∼20 m (Figs. 5–8). Closed-cell “brain terrain” is commonly present on undulating topography, at the top of concentric ridges (and sometimes in the concentric valleys between ridges) (Figs. 5, 9 and 10). Some closed-cell “brain terrain” units have a strongly polygonal surface texture (compare closed-cell “brain terrain” mounds in Fig. 6 to Figs. 7 and 8). Polygonal “brain terrain” commonly features a surface furrow oriented axially at the center of the moundshaped cell (Figs. 7 and 8). Open-cell “brain terrain” is composed of arcuate and cuspate cells that are delimited by a convex-up boundary ridge, commonly ∼4–6 m wide and ∼2 m high (based on HiRISE image shadow measurements), surrounding a flat-floored depression (Figs. 3–5). Open-cell “brain terrain” cells are of similar dimensions to closedcell “brain terrain” cells. Open-cell “brain terrain” boundary ridges

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Fig. 4. An elevated region (center) surfaced by closed-cell “brain terrain” (CC-BT) and surrounded by open-cell “brain terrain” (OC-BT). Positive topography can be seen in the stereo view in Fig. 9. The “brain terrain” is ringed by mantling (LDM) surfaces. High-center mantle polygons (HC-MP) are visible immediately surrounding the open-cell “brain terrain.” Low-center mantle polygons (LC-MP) are present to the lower left. Illumination is from the left. North to image top. Portion of PSP_002175_2210.

Fig. 5. Alternating open-cell “brain terrain” (OC-BT) in topographic lows between ridges surfaced by closed-cell “brain terrain” (CC-BT). A schematic topographic profile representing surface undulations across the middle of the image is included in white, with increasing elevation towards image top. Illumination is from the left. North to image top. Portion of PSP_002175_2210.

are commonly parallel along the long axis, but may be tightly rounded or gradually tapered along the short axis (Figs. 3–5). Open-cell “brain terrain” cells occur singly, or in linked groups (Figs. 3–5). Open-cell “brain terrain” cells commonly form lineations that are oriented concentrically to the crater in which the unit is present (Figs. 5, 9 and 10). Open-cell “brain terrain” lineation spacing is variable, but is commonly ∼20 m. Open-cell “brain terrain” is commonly present at the lateral contact between “brain terrain” and the mantle (Figs. 3–5), and in topographically low valleys between “brain terrain” ridges (Figs. 9 and 10). Opencell “brain terrain” is also common in topographic lows between closed-cell “brain terrain”-patterned ridges and hills (Figs. 3–5 and 10). Boulders are present on the surface of both open-cell and closed-cell “brain terrain,” and are typically less than 2 m in di-

ameter (Figs. 3 and 5). Boulders are most commonly found atop closed-cell mounds and on open-cell boundary bands, although some boulders are present in topographic lows. 2.2. Polygonalized mantle In the analyzed craters, mantle material is present in a continuous ring around the crater interior wall, and as patchy occurrences in topographic lows between crater fill ridges. Mantle material present in proximity to “brain terrain” is polygonally patterned and displays two distinct textures (Fig. 3): high-center mantle polygons and low-center mantle polygons (Figs. 11 and 12). High-center mantle polygons are bounded by depressed surface troughs which intersect at both near-orthogonal and near-hexagonal intersections, forming polygonal patterns with topographically high interiors rel-

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Fig. 6. Closed-cell “brain terrain” present in PSP_002175_2210. Axial furrows are present along some “brain terrain” mounds (arrows). North to image top. Illumination is from the left.

Fig. 7. Contact between strongly lineated closed-cell “brain terrain” (left) and polygonal closed-cell “brain terrain” (right). Axial furrows are present in many closed-cell “brain terrain” mounds (Arrows). Portion of PSP_002782_2230. Illumination is from the lower left. North to image top.

ative to their boundaries (Figs. 11 and 12). High-center mantle polygons are commonly ∼10 m in diameter, with slightly convexup interiors. High-center mantle polygon troughs are commonly ∼2–3 m across. The mantle material in which high-center polygons are present can be up to ∼40 m thick, based on MOLA point measurements, but thins and pinches out at lateral contacts with “brain terrain” and steep crater wall surfaces. On average, mantle material in the examined craters must be thick enough to smoothover topographic undulations in underlying “brain terrain”—likely indicating a typical depth of 10–20 m. The mantle unit is generally flat, and is bounded by gently sloping margins, as well as by steeply scarped, scalloped margins (Figs. 11 and 12). Low-center mantle polygons (Figs. 3, 11 and 12) are composed of troughs with raised shoulders that intersect at nearorthogonal and near-hexagonal intersections, forming polygons with depressed centers, relative to their raised rims (Figs. 11

and 12). Low-center mantle polygons are commonly ∼10 m in diameter (but may be as small as ∼5 m in diameter, e.g., Fig. 12), and have smooth, flat, depressed interiors. Low-center mantle polygon troughs are commonly ∼3–4 m wide. Low-center mantle polygons are found at the fringes of the mantle, at both gradual and scalloped margins (Figs. 11 and 12). 2.3. Spatial relationships between brain terrain and mantle polygons Whereas the superposition relationship between mantle material and “brain terrain” is generally clear, detailed mapping reveals locally complex lateral contacts between “brain terrain” and mantle material (Fig. 13). At lateral contacts between mantle and “brain terrain” units, mantle material is commonly draped on, and interfingered between, “brain terrain” “cells,” suggesting that mantle material superposes, and in places, embays “brain terrain” units

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Fig. 8. Closed-cell “brain terrain” with pronounced axial furrows (arrows) at the confluence of two concentric crater fill lobes in PSP_002782_2230. North to image top. Illumination is from the left.

Fig. 10. Close-up view of concentric crater fill ridges and valleys present in Fig. 9 (right side, below middle). Ridges are surfaced with closed-cell and some opencell “brain terrain.” Valleys are surfaced primarily by open-cell “brain terrain.” Red–blue (left/right) anaglyph is a portion of stereo pair PSP_002175_2210 and PSP_001410_2210, and is ∼2 km wide. North to image top. Anaglyph produced by James Dickson.

Fig. 9. Three-dimensional surface structure of concentric crater fill and mantle material. “Brain terrain” visible at full resolution appears as knobby surface roughness and is strongly lineated concentric to the crater, with concentric ridges and valleys observable. “Brain terrain”-covering LDM material is located around the crater interior wall, and in low areas between “brain terrain”-patterned ridges. Crater is ∼10 km in diameter. North to image top. Red–blue (left/right) anaglyph is a portion of stereo pair PSP_002175_2210 and PSP_001410_2210. Anaglyph produced by James Dickson.

(Figs. 4, 14, and 15). Mantle material is commonly found at the foot of crater interior wall slopes, and in topographic lows between “brain terrain”-surfaced concentric ridges within craters (Figs. 9 and 10). Exposures of underlying “brain terrain” microtopography crop out through the mantle, suggesting that the mantle thins and pinches out at lateral contacts with “brain terrain” (Figs. 14 and 15). Topographically high exposures of closed-cell “brain terrain” are commonly ringed by open-cell “brain terrain,” which is in turn ringed by mantled surfaces patterned with low-center, and/or high-center mantle polygons (Figs. 4 and 5). Concentric crater fill ridges surfaced by closed-cell “brain terrain” are commonly flanked by open-cell “brain terrain” in the lows between ridges, particularly in lows that also have exposures of mantle material (Figs. 10,

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Fig. 11. Low-center mantle polygons (LC-MP) present in a scalloped depression in PSP_002782_2230. High-center mantle polygons (HC-MP) surround the low-center mantle polygons, both within, and outside the depression. Illumination is from the lower left. North to image top.

Fig. 12. Low-center mantle polygons (LC-MP) present along a gently sloping contact between high-center mantle polygons (HC-MP) and “brain terrain” (CC-BT) in PSP_002782_2230. Illumination is from the lower left. North to image top.

14 and 15). Open-cell “brain terrain” is rarely observed without near-by and/or overlying mantle material, and low-center mantle polygons are not observed in isolation from extensive regions patterned by high-center mantle polygons. Complete transitions between surfaces dominated by highcenter mantle polygons, low-center mantle polygons, open-cell “brain terrain,” and closed-cell “brain terrain” occur on length scales of ∼100–500 m in the analyzed images (Fig. 15). Lateral contacts between mantle surfaces patterned with high-center and low-center mantle polygons are gradational on gentle slopes and abrupt on steeply scalloped slopes (Figs. 11 and 12). Lateral contacts between closed-cell “brain terrain” and open-cell “brain terrain” are gradational, consisting of closed-cell “brain terrain” cells that are partially depressed, or that transition into open-cell “brain terrain”-like boundary ridges (Figs. 4 and 5). A schematic stratig-

raphy of “brain terrain” and polygonally patterned mantle surface textures is shown in Fig. 16. 3. Surface ages Crater counting on concentric crater fill and mantle terrains is complicated by the obscuration and disruption of craters by brain terrain and mantle polygons (Mangold, 2003; Kostama et al., 2006). In this study, 168 craters were counted on mantle and “brain terrain” surfaces present in four HiRISE images. Only fresh craters showing no evidence of subsequent polygon formation within the crater were counted on mantle surfaces (e.g., Fig. 17a). Given that thermal contraction crack polygons can form during permafrost formation (syngenetic wedges) or after substrate degradation begins (epigenetic and anti-syngenetic wedges) (Mackay,

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Fig. 13. Sketch map of “brain terrain” and mantle in PSP_002782_2230. Closed-Cell BT indicates closed-cell “brain terrain;” Open-Cell BT indicates open-cell “brain terrain;” High-Center MP indicates high-center mantle polygons; and Low-Center MP indicates low-center mantle polygons. Closed-Cell “brain terrain” is more widely distributed than open-cell “brain terrain,” and high-center mantle polygons are more widely distributed than low-center mantle polygons. Mapping was conducted on the HiRISE image only, overlaid on a CTX background image.

1990), counting only fresh, unfractured craters gives us a minimum age for the cessation of mantle emplacement. Counts on mantle surfaces indicate a minimum age of ∼1.5 My (best-fit ages span 1.3 to 1.7 My using the Neukum (Neukum and Ivanov, 2001) and Hartmann (2005) production functions, respectively) (Fig. 18)— a youthful age consistent with earlier estimates of mantle ages elsewhere in the northern hemisphere (Head et al., 2003). For mantle surfaces displaying small thermal contraction crack polygons, a reduction in the abundance of ∼20 m diameter craters

on mantle surfaces is typical, and may result from the removal or obscuration of craters with diameters comparable to polygon diameters (Levy et al., 2009). Crater counts on “brain terrain” are likewise complicated, owing to the obscuration of small craters by “brain terrain” cells, the destruction of small and medium-sized craters by “brain terrain” cell development, and the presence of craters with unusually modified morphologies (e.g., Mangold, 2003) (Fig. 17). Combined counts of fresh and modified craters (including “ring-mold” and/or “oyster

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4. Discussion 4.1. Brain terrain origin

Fig. 14. (a) Closed-cell “brain terrain” (center) surrounded by open-cell “brain terrain” on the flanks of a crater wall. Open-cell “brain terrain” grades into high-center mantle polygons, in a relationship suggesting overprinting of “brain terrain” by mantle material. Illumination is from the bottom of the image. North to right. (b) A mantle surface with high-center mantle polygons (image center) transitioning to open-cell “brain terrain,” and to closed-cell “brain terrain.” Exposure of open-cell “brain terrain” bounding ridges within the high-center mantle polygon surface suggests overprinting of “brain terrain” by mantle material. Illumination is from image left. North to image top. (a) and (b) are portions of PSP_002175_2210.

shell” craters (e.g., Mangold, 2003; Kress and Head, 2008) (Fig. 18), show a roll-off at small crater sizes typical of counts on lobate debris aprons and lineated valley fill. This is unlikely to be dominantly a resolution effect, as counts were made on HiRISE images with spatial resolution of ∼30 cm/pixel. Deformed and extensively modified craters were included in the “brain terrain” surface count in order to capture the minimum age of glacial activity (see below) in concentric crater fill during which massive ice was present shallowly enough to modify impact craters (Kress and Head, 2008). Critically, even deformed and modified craters are present within the brain terrain, rather than underlying it (as would be the case for large craters superposed by “brain terrain”). Counts of craters larger than 125 m in diameter suggest a surface age for “brain terrain” of ∼10–100 My, consistent with, if slightly younger, than counts on lobate debris aprons (Mangold, 2003). The difference between surface ages of ∼1.5 My for mantle, and ∼10–100 My for “brain terrain,” suggests that “brain terrain” and mantle materials are two stratigraphically distinct units that were deposited at markedly different times, even when scaling effects from impacts into ice are considered (Kress and Head, 2009). The aerially small exposures of open-cell “brain terrain” and low-center mantle polygons compared to the more common closed-cell “brain terrain” and high-center mantle polygon surfaces (e.g., Fig. 13) preclude separate age determinations.

What processes result in the formation of “brain terrain” at the surface of concentric crater fill? To answer this question, it is first necessary to consider potential origins for concentric crater fill itself. First-order lineation patterns on concentric crater fill (on which “brain terrain” has developed) have been interpreted to be flow lineations and lobes analogous to those formed by the flow of terrestrial debris-covered glaciers (Head and Marchant, 2006; Head et al., 2006a, 2006b; Shean et al., 2007). The presence of “ringmold” craters on concentric crater fill surfaces further suggests that ice-rich material may be present within meters to tens of meters of the surface beneath a relatively ice-free lag layer (Kress and Head, 2008). Based on MOLA point topography measurements and typical martian crater depth-diameter ratios (Garvin et al., 2002), the analyzed craters are up to 80% filled, accounting for volumes of crater-fill material up to 800 m thick (Fig. 19). Recent radar observations of lobate debris aprons with analogous flow, lineation, and crater structures raises the possibility that much of the concentric crater fill material may be ice (e.g., Holt et al., 2008; Plaut et al., 2008), consistent with predictions from global circulation models (Madeleine et al., 2007). An ice-rich concentric crater fill (e.g., Fig. 20a) is consistent with mapping of 10–25 km diameter craters in the martian northern plains indicating widespread filling of large impact craters with hundreds of meters of ice-rich materials (Kreslavsky and Head, 2006). Thick accumulations of atmospherically deposited, ice-rich, dusty material (Forget et al., 2006; Head et al., 2005, 2006a) (Fig. 20a) deposited in crater interiors could readily flow under current martian conditions (Milliken et al., 2003), sufficient to produce observed strains (e.g., ∼60% crater deformation in Fig. 17). Up to eight broad-scale concentric crater fill ridges can be observed per crater analyzed in this study, suggesting a complex history of deformation of ice-rich materials. Boulder-sized clasts present on the concentric crater fill surface could represent rock-fall entrained at crater wall ice-accumulation zones, and transported to their present location by glacial flow (e.g., Marchant et al., 2002; Head and Marchant, 2006; Marchant and Head, 2007). Gravitational stresses that drive brittle deformation of nearsurface glacier ice, coupled with thermal stresses generated by seasonal heating and cooling (Mellon, 1997) would fracture the ice-rich crater fill, resulting in the generation of oriented fracture networks, analogous to those observed on flowing debris-covered glaciers on Earth (Fig. 20b) (Levy et al., 2006). This combination of stresses—thermal and glacial—can account for the variety of surface patterns and textures observed in “brain terrain.” Internal deformation of flowing ice-rich material generates the broad, first-order concentric lineation patterns observed in “brain terrain.” Lateral flow orients the near-surface stress field (Benn et al., 2003), aligning thermal contraction cracks both normal and orthogonal to the flow direction; alternatively, cracks may initially form in random patterns, but are oriented during subsequent flow of subsurface ice and debris (Marchant et al., 2002; Levy et al., 2006). These oriented fractures will develop into strongly lineated elongate and mound-shaped closed-cell “brain terrain” (see below). Thermal contraction crack fractures will be less strongly oriented on “brain terrain” that has experienced minimal flow, resulting in only moderately oriented, and largely hexagonal fractures (Levy et al., 2006). In addition to the above, concentric crater fill might be thickest near the center of crater floors where transported debris overlying glacier ice is typically at a maximum (e.g., Head et al., 2008; Kowalewski, 2008) and where the temperature conditions are most favorable for ice preservation (Russell et al., 2004). These factors could result in a rise in the elevation of the crater fill at the center

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Fig. 15. Contacts between “brain terrain” and mantle polygon textures. A complete transition between high-center mantle polygons (HC-MP), low-center mantle polygons (LC-MP), open-cell “brain terrain” (OC-BT) and closed-cell “brain terrain” (CC-BT) is present in each panel. All panels are excerpted from PSP_002782_2230, with illumination from the left and north to image top in all panels.

Fig. 16. Schematic stratigraphy of “brain terrain” and mantle textures. The upper line is MOLA point topography. Lower lines delimit estimated subsurface boundaries between “brain terrain” and mantle. Brackets illustrate surface exposures of closed-cell “brain terrain” (CC-BT), open-cell “brain terrain” (OC-BT), high-center mantle polygons and lowcenter mantle polygons. MOLA is derived from a segment of orbit 12303 passing over HiRISE image PSP_002782_2230.

of craters relative to surrounding, lower-elevation, ablated surfaces of crater fill. If an ice-free sublimation lag deposit is present on concentric crater fill surfaces (generated by a combination of aeolian

sediment deposition, sublimation of underlying debris-rich ice, and desert pavement formation), interactions between thermalcontraction surface fractures and overlying lag deposits could result in the observed complex morphology of “brain terrain”

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Fig. 18. Crater counts for fresh craters on mantle material (open circles) and all craters on “brain terrain” surfaces (filled circles) using Hartmann (2005) isochrons. Counts combine data from four concentric crater fill surfaces in Utopia Planitia, and strongly indicate a surface at of ∼1.5 My for mantle polygons. Brain terrain counts show a roll-off at small crater sizes typical of lobate debris apron and lineated valley fill surfaces. Craters in the 50–250 m diameter range suggest an age of ∼10–100 My for brain terrain.

Fig. 17. (a) Fresh crater in LDM with high-center mantle polygons. Portion of PSP_002782_2230. Illumination is from the left. North to image top. (b) “Ringmold”-like crater (Kress and Head, 2008) in closed-cell “brain terrain.” Portion of PSP_002175_2210. Illumination is from the left. North to image top. (c) Elongate crater in closed-cell “brain terrain.” If the crater was deformed by glacier-like flow, linear strain of ∼60% is documented. Portion of PSP_002175_2210. Illumination is from the left. North to image top.

(e.g., Marchant et al., 2002; Schorghofer and Aharonson, 2005; Schorghofer, 2007). Fractures would initially represent sites of depressed surface troughs (Fig. 20c) (Marchant et al., 2002). This is because enhanced contact with the dry atmosphere at polygon fractures would result in greater sublimation of subsurface ice along polygon cracks (Marchant et al., 2002; Kowalewski et al., 2006, 2007), generating widened, deepened troughs analogous to those outlining terrestrial sublimation polygons (Marchant et al., 2002). Thermal fractures would accumulate sediment derived from winnowing of overlying lag deposits (e.g., Marchant et al., 2002) and infiltration of aeolian sediments, forming wedges

Fig. 19. MOLA shot-point profile across a concentric crater fill-filled crater (present in PSP_002175_2210). Inset. The MOLA shot-point profile reprojected to show the actual profile (near-horizontal line) and the modeled depth of the crater, assuming a parabolic profile, and depth-diameter ratios documented by Garvin et al. (2002). Up to ∼80% of the crater has been filled by concentric crater fill material.

analogous to sand-wedge structures common in cold and arid terrestrial environments (Fig. 20d) (Péwé, 1959; Berg and Black, 1966; Marchant et al., 2002; Marchant and Head, 2007). Lateral transport of surface materials into deepening troughs would result in ever-thicker accumulations of sediment relative to “stable” polygon interiors (e.g., Marchant et al., 2002). Inversion of sublimation-polygon-like topography (polygons with convex-up centers) could occur if sublimation continued to remove near-surface ice at polygon centers, but slowed at polygon troughs due to the presence of thickened accumulations of sediment along polygon troughs (Fig. 20e) (e.g., Marchant et al., 2002). Cementation of wedge sediments at polygon margins by ice deposited during periods of reversed vapor flux would enhance the protection of ice located beneath the sediment wedges

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process could account for the formation of closed-cell “brain terrain” contemporaneously with and continuing after, the period of concentric crater fill flow. The development of open-cell “brain terrain” could be accounted for in this topographic inversion model by the continued removal of ice from beneath closed-cell “brain terrain” cells. Continued removal of subsurface ice over time may explain the presence of open-cell “brain terrain” in topographic lows between closed-cell-patterned concentric ridges (e.g., Fig. 5). Additionally, where thin, dessicated remnants of low-albedo mantle material are draped over “brain terrain” cells at the pinched-out margins of the mantle (Fig. 15) we hypothesize that enhanced sublimation of residual ice beneath “brain terrain” cells (Williams et al., 2008) could further promote the collapse of closed-cell “brain terrain” cells, generating open-cell “brain terrain” cells (Fig. 20g). This process for the formation and modification of “brain terrain” textures differs from that outlined by Mangold (2003) in that it invokes both glacial stress orientation, as well as thermal contraction cracking and sand-wedge development, to account for the detailed microrelief of “brain terrain” textures. It differs from the Dobrea et al. (2007) model in that the preservation of broadscale glacial flow features and of thermal contraction crack polygon wedges are preserved in the “brain terrain”—which would be lost if cryoturbation or thermokarst formation (melting) had resulted in widespread reworking of the concentric crater fill surface. Our proposed process is illustrated schematically in Fig. 20. Lastly, some fractures, particularly those located near the margins of the crater, may have formed more recently than the observed “brain terrain” textures. Such fractures (e.g., Fig. 13) are interpreted to represent late-stage mechanical failure of concentric crater fill glacial remnants. 4.2. Mantle origin

Fig. 20. Schematic illustration of “brain terrain” development by fracturing, differential sublimation, and topographic inversion. (a) Ice-rich deposits (white) are emplaced by atmospheric deposition. Sublimation of near-surface ice results in the formation of a sublimation lag deposit (light gray), consisting of dust entrained during deposition, wind-blown deposits, rock-fall, etc. (e.g., Marchant et al., 2002). (b) The combined action of annual thermal contraction cracking and glacio-tectonic fracturing crack the ice-rich surface. Overlying fines winnow in, forming sand-wedges (dark gray). (c) Enhanced sublimation at fractures may result in lowering of troughs relative to polygon interiors. (d) Continued fracture expansion, wedge infilling, and fracture-dominated sublimation further depresses troughs, and thicken wedge sediments. (e) Continuing sublimation results in a lowering of the depth of the ice table, possibly caused by obliquity-driven lowering of the depth of ice stability (e.g., Mellon and Jakosky, 1995). Thickened sediment accumulations at wedges may preferentially preserve underlying ice. Original wedge structures may be preserved in some locations. (f) Where glacier-like flow occurs, polygons/fracture networks may be deformed by flow. (g) Continued removal of subsurface ice, in places enhanced by the presence of thin, dark, dessicated mantle remnants (black), results in the collapse of closed-cell “brain terrain,” and the formation of open-cell “brain terrain.”

(Kowalewski, 2008). Such an inversion could result in the formation of raised cells and chains of raised cells at the loci of relict polygon troughs. Some raised cells may preserve the original, axial surface trough (e.g., the axial furrows observed in some “brain terrain” cells, Figs. 6–8). These cells may remain ice-cored. This

What is mantle material, and how might its deposition and modification relate to “brain terrain”? In the analyzed images, the presence of mantle material at the inner margins of crater walls, and in topographic lows between concentric crater fill ridges, suggests that mantle material could be an atmospherically emplaced, ice-rich deposit, that preferentially accumulates in shadowed areas (Mustard et al., 2001; Hecht, 2002; Head et al., 2003). Surface ages of ∼1.5 My suggest that mantle material in Utopia Planitia concentric crater fill craters is temporally associated with recent hemisphere-wide latitude-dependent mantle (LDM) deposition events (Mustard et al., 2001; Head et al., 2003; Kreslavsky and Head, 2006): deposits which are thought to contain sufficient dusty material to generate a surficial lag deposit during sublimation of near-surface ice (Marchant et al., 2002). 4.3. Polygonalized mantle Seasonal thermal contraction cracking generates contractioncrack polygons in ice-rich sediment on Earth and Mars (Washburn, 1973; Mellon, 1997; Mangold, 2005; Marchant and Head, 2007). As in the formation of “brain terrain,” sublimation may initially be locally enhanced at polygon margins, generating high-center mantle polygons, which may be analogous to terrestrial sublimation polygons (Washburn, 1973; Mellon, 1997; Marchant et al., 2002; Kowalewski et al., 2006; Marchant and Head, 2007). The lack of strongly oriented mantle polygons suggests that young mantle material has not flowed significantly (Zimbelman et al., 1989; Milliken et al., 2003), consistent with thicknesses of <40 m— although a preferred orientation may arise where mantle material overlies sloped crater walls, and where it occurs proximally to well-developed brain terrain. For the latter, contraction cracks may

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Fig. 21. Examples of “brain terrain” and mantle stratigraphy in Eastern Hellas (a and b), and in Deuteronilus Mensae (c and d). (a) Lineated fill in “The Hourglass” (Head et al., 2005) crater on the eastern rim of the Hellas basin. White box indicates location of panel North to left. (b) Portion of PSP_002782_1405. (b) Closed-cell “brain terrain” (CC-BT); sinuous, open-cell “brain terrain” (OC-BT); and high-center mantle polygons (HC-MP) present in a similar arrangement to that observed in Utopia Planitia. (c) A lobate debris apron in Deuteronilus Mensae. White box indicates location of panel (d). North to image top. Portion of P06_003246_2178. (d) Closed-cell “brain terrain” on lobate debris apron ridges; open-cell “brain terrain” in topographic lows between ridges. Dunes partially obscure both surfaces. Portion of PSP_003246_2180.

be aligned with those of nearby or underlying brain terrain. Infilling of polygonal fractures with overlying lag deposit fines could generate subsurface wedges, similar to the initial steps in the formation of “brain terrain.” As with “brain terrain” discussed earlier, inversion of sublimation-polygon-like topography (polygons with convex-up centers) could occur if sublimation continued to remove near-surface ice at polygon centers, but slowed at polygon troughs due to the presence of thickened accumulations of sediment along polygon troughs (e.g., Marchant et al., 2002). This differential sublimation process could also account for the formation of low-center mantle polygons with relatively flat, low-lying interiors, and elevated margins. This shared inversion process accounts for the similarity in morphology between low-center mantle polygons and opencell “brain terrain” cells, although open-cell “brain terrain” cells are generally larger than mantle polygons, and form in gradational contacts with closed-cell “brain terrain,” rather than in mantle material. Across the martian mid-to-high latitudes, low-center polygons are exceptionally uncommon in HiRISE images (Levy et al., 2009), suggesting that unique conditions may exist at the margins of mantle surfaces in concentric crater fill terrains. The concentration of low-center polygons within scalloped depressions in the mantle (that may have a sublimation origin) (Kadish et al., 2008; Zanetti et al., 2008; Lefort et al., 2009), and in regions where the mantle thins and pinches out, suggests that enhanced removal of subsurface ice by locally intense sublimation may be critical for the formation of low-center mantle polygons.

4.4. Climate implications: Local and global What do the proposed mechanisms for “brain terrain” and mantle formation and modification suggest about climate conditions in Utopia Planitia at ∼1.5 My and ∼10–100 My timescales? Debris-covered-glacier-like landforms have been documented extensively in martian mid-latitudes, and have been interpreted to be geomorphic evidence of cold and arid climate conditions, dominated by sublimation of ground ice during periods of low obliquity, and dramatic redistribution and deposition of ice on the surface during periods of high obliquity (Mustard et al., 2001; Kreslavsky and Head, 2002, 2006; Laskar et al., 2002, 2004; Head et al., 2003). Glacier-like deformation of thick accumulations of ice-rich material during high-obliquity (∼35–45◦ ) (Madeleine et al., 2007) redistribution/depositional periods during the past ∼10– 100 My may account for the emplacement of concentric crater fill, while more subtle obliquity excursions over the past ∼1–2 My (up to ∼35◦ ) (Head et al., 2003) may account for the emplacement of thinner, static mantle material. Near-surface modification of both “brain terrain” and mantle is consistent with cold and arid climate conditions and sublimation-driven processes. These lines of evidence suggest an Amazonian hydrological cycle dominated by solid-vapor transitions rather than by widespread melting and flow of liquid water. Recent work (e.g., Soare et al., 2007, 2008) has concluded that widespread melting and thermokarst erosion has been instrumental in the formation of some Utopia Planitia surfaces (e.g., scalloped terrain present in mantle material). What do mantle polygons and

Concentric crater fill: Glacial and periglacial change

“brain terrain” indicate about the presence and duration of saturated active layers in Utopia Planitia during the recent Amazonian? From terrestrial experience, the formation of low-center polygons on Earth is most dramatic in ice-wedge polygons, which require seasonal input and freezing of liquid water (Washburn, 1973; Root, 1975; Marchant and Head, 2007). However, raised shoulders also form on sand-wedge polygons that develop in cold and arid climates in which liquid water is not available in significant quantities (Péwé, 1959; Berg and Black, 1966; Washburn, 1973; Root, 1975; Marchant and Head, 2007). If ephemeral liquid water was present during the development of low-center mantle polygons, driving the formation of pronounced polygon-margin shoulders, then its spatial extent was limited to concentrated occurrences at the tapering margins of the mantle, and to both the floors and steep slopes within scalloped depressions. This range of locations in which low-center mantle polygons are observed is inconsistent with simple ponding and saturation of sediments. Further, exposures of pristine “brain terrain” in locations where the mantle has been completely removed from the surface strongly suggest a cold and dry (e.g., sublimation) mechanism for the removal of mantle material to form windows down to the “brain terrain;” had these pits been water-saturated to form ice-wedge polygons, it is likely that the underlying “brain terrain” would have been reworked and extensively modified. Lastly, the preservation of original “brain terrain” cell axial furrows suggests that subsequent, widespread cryoturbation has not disrupted these fine-scale surface features. Rather, we suggest that the morphology of “brain terrain” and mantle material can be accounted for by atmospheric deposition of ice during periods of high obliquity, and modification of ice-rich units by a suite of cold-desert processes including glacial deformation, thermal contraction cracking, and differential sublimation in the absence of abundant near-surface liquid water. Finally, these analyses provide a framework for understanding relationships between “brain terrain” textures present on lobate debris aprons and lineated valley fill elsewhere on Mars and the latitude-dependent mantle (LDM). Contacts between “brain terrain” and LDM occur extensively at martian midlatitudes, including transects in eastern Hellas, Deuteronilus Mensae and Nilosyrtis Mensae (Fig. 21). The wide distribution of contacts between “brain terrain” and LDM material suggests that the transition between mid-Amazonian glacial periods, and more recent Amazonian, dry “periglacial” conditions are not restricted to local occurrences and may indicate global climate processes. 5. Summary and conclusions “Brain terrain” and polygonally-patterned latitude-dependent mantle (LDM) surfaces were analyzed in Utopia Planitia. Stratigraphic relationships and crater-count ages show that the mantle surface postdates concentric crater fill “brain terrain” by ∼10– 100 My. Two unique surface textures were identified in each unit, respectively, closed-cell “brain terrain” and open-cell “brain terrain,” and high-center mantle polygons and low-center mantle polygons. Lateral contacts between textures are shown to be gradational, suggesting modification of “brain terrain” and mantle material into the current range of surface morphologies. A combined glacial-stress/thermal-contraction and differential sublimation mechanism is proposed for the formation and modification of “brain terrain” on concentric crater fill. A similar model, driven by thermal contraction cracking and differential sublimation of underlying ice, but without glacier-like ice flow, is also proposed for the formation and development of the polygonally patterned mantle. This explanation for examined “brain terrain” and polygonalized mantle material requires two different styles of atmospheric deposition of ice: an older, “glacial” deposition period for the formation of concentric crater fill “brain terrain,” and a more recent “ice

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age” mantling period for the deposition of LDM. Both deposition styles result in the formation of near-surface excess ice (ice volume exceeding available pore space). Glacier-like concentric crater fill with closed-cell “brain terrain” is interpreted to have formed during a mid-latitude peak-glaciation period that ended at ∼10– 100 My. This was followed by quiescent cold desert conditions, preceding the most recent “ice age” (Head et al., 2003) period at ∼1–2 My, responsible for the deposition of mid-high latitude dependent mantle material, the formation of mantle polygons, and the modification of closed-cell “brain terrain” present at mantle margins into open-cell “brain terrain.” Understanding the development of “brain terrain” on Utopia Planitia concentric crater fill provides insight into analysis of “brain terrain” present on lobate debris aprons and lineated valley fill across the martian midlatitudes, suggesting a dynamic history of ice redistribution under cold desert conditions during the late Amazonian. Acknowledgments We gratefully acknowledge support from the NASA Mars Data Analysis Program (NNX07AN95G to J.W.H.), the Mars Fundamental Research Program (NNX06AE32G to D.R.M. and J.W.H.), the Applied Information Systems Research Program (NNG05GA61G to J.W.H.), and the Mars Express HRSC Participating Scientist Program (JPL 1237163 to J.W.H.), and the National Science Foundation, Office of Polar Programs (through grant ANT-0338291 to D.R.M. and J.W.H.). Special thanks to Caleb Fassett for assistance in processing and interpreting crater count information and for image processing; to James Dickson for coordination of image processing and anaglyph production; and to Dr. Matt Balme and one anonymous reviewer for their efforts in improving the manuscript. References Benn, D.I., Kirkbride, M.P., Owen, L.A., Brazier, V., 2003. Glaciated valley landsystems. In: Evans, D.J.A. (Ed.), Glacial Landsystems. Edward Arnold, London, UK, pp. 372– 406. Berg, T.E., Black, R.F., 1966. Preliminary measurements of growth of nonsorted polygons, Victoria Land, Antarctica. In: Tedrow, J.C.F. (Ed.), Antarctic Soils and SoilForming Processes. American Geophysical Union, Washington, DC, pp. 61–108. Dobrea, E.Z.N., Asphaug, E., Grant, J.A., Kessler, M.A., Mellon, M. T., 2007. Patterned ground as an alternative explanation for the formation of brain coral textures in the mid latitudes of Mars: HiRISE observations of lineated valley fill textures. In: Proc. 7th International Conference on Mars. Abstract #3358. Fastook, J.L., Head, J.W., Marchant, D.R., 2008. Dichotomy boundary glaciation models: Implications for timing and glacial processes. Lunar Planet. Sci. 39. Abstract #1109. Forget, F., Haberle, R.M., Montmessin, F., Levrard, B., Head, J.W., 2006. Formation of glaciers on Mars by atmospheric precipitation at high obliquity. Science 311, 368–371. Garvin, J.B., Sakimoto, S.E.H., Frawley, J.J., Schnetzler, C., 2002. Global geometric properties of martian impact craters. Lunar Planet. Sci. 33. Abstract #1255. Hartmann, W.K., 2005. Martian cratering 8: Isochron refinement and the chronology of Mars. Icarus 174, 294–320. Head, J.W., Marchant, D.R., 2006. Modification of the walls of a Noachian crater in northern Arabia Terra (24E, 39N) during northern mid-latitude Amazonian glacial epochs on Mars: Nature and evolution of lobate debris aprons and their relationships to lineated valley fill and glacial systems. Lunar Planet. Sci. 37. Abstract #1126. Head, J.W., Marchant, D.R., 2009. Inventory of ice-related deposits on Mars: Evidence for burial and long-term sequestration of ice in non-polar regions and implications for the water budget and climate evolution. Lunar Planet. Sci. 40. Abstract #1356. Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent ice ages on Mars. Nature 426, 797–802. Head, J.W., and 13 colleagues, 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature 434, 346–351. Head, J.W., Marchant, D.R., Agnew, M.C., Fassett, C.I., Kreslavsky, M.A., 2006a. Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for late Amazonian obliquity-driven climate change. Earth Planet. Sci. Lett. 241, 663–671. Head, J.W., Nahm, A.L., Marchant, D.R., Neukum, G., 2006b. Modification of the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation. Geophys. Res. Lett. 33, doi:10.1029/2005GL024360.

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