Concentric Crater Fill in Utopia Planitia: Timing and Transitions Between Glacial and Periglacial Processes.

Details Concentric Crater Fill in Utopia Planitia: Timing and Transitions Between Glacial and Periglacial Processes. Concentric Crater Fill in Utopia Planitia: Timing and Transitions Between Glacial and Periglacial Processes.

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..458l&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

Computer Science

Scientific paper

Concentric crater fill (CCF), lobate debris aprons (LDA), and lineated valley fill (LVF) have long been used as indicators of ground ice on Mars [1-3]. Formation models for these features range from aeolian modification [4], to rock-glacier processes [5], to debris-covered glacier processes [6-7], but are now largely constrained by the detection of material within lobate debris aprons that is 100s of meters thick, and which has dielectric properties consistent with water ice [8-9]. At ~30 cm/pixel HiRISE resolution, LVF, LDA, and CCF show complex surface textures, termed "brain coral terrain" [9], or, succinctly, "brain terrain" (BT) [10]. Polygonally patterned ground commonly is present in proximity to brain terrain, overlying it as "brain terrain-covering" polygons (BTC) [10]. Here we document spatial patterns of BT and BTC morphology present in four CCF-filled, ~10 km diameter craters in Utopia Planitia. We then evaluate formation processes for BT and BTC units. Brain Terrain (BT) Morphology At HiRISE resolution (~30 cm/pixel), concentric crater fill brain terrain displays a complex surface texture. Two distinct sub-textures are commonly present in brain terrain [9]: filled brain terrain (FBT) and hollow brain terrain (HBT) (Figure 1). Filled brain terrain (FBT) is composed of arcuate and cuspate mounds, commonly ~10-20 m wide and 10 - <100 m long. Some FBT mounds have surface grooves located near the centreline of the long axis. FBT mounds occur singly, or in linked groups. FBT mounds are commonly oriented in lineations which are concentric to the crater in which the unit is present. FBT mound lineation spacing is variable, but commonly has a wavelength of ~20 m. FBT is commonly present on undulating topography, at the top of concentric ridges (and sometimes in the concentric valleys between ridges). Hollow brain terrain (HBT) is composed of arcuate and cuspate features that are delimited by a convex-up boundary band, commonly ~4-6 m wide, surrounding a depression. HBT are of similar dimensions to FBT, but are seldom longer than ~100 m. HBT boundary bands are commonly parallel along the long axis, but may be tightly rounded or gradually tapered along the short axis. HBT features occur singly, or in linked groups. HBT features are commonly oriented in lineations which are concentric to the crater in which the unit is present. HBT lineation spacing is variable, but commonly has a wavelength of ~20 m. HBT is commonly present at the contact between BT and BTC units, particularly in topographic lows between FBT-covered ridges and surrounding isolated FBT-covered hills embayed by BTC material. Brain Terrain Covering (BTC) Polygon Morphology Polygonally patterned ground present in proximity to BT, and commonly overlying it, constitutes the brain terrain covering (BTC) unit.. BTC polygons have two distinct morphologies: high centred (HC-BTC) and low centred (LC-BTC) (Figure 1). High centred BTC polygons (HCBTC) are composed of depressed surface troughs which intersect at both near-orthogonal and near-hexagonal intersections, forming polygons that are topographically high relative to their boundaries. HC-BTC polygons are commonly ~10 m in diameter, and have little topographic relief, although most have slightly convex-up interiors, based on shadow observations. HC-BTC troughs are commonly ~2-3 m across. HC-BTC polygons are present in a unit which has a lower albedo than brain terrain, and which can be up to ~40 m thick, based on MOLA point measurements. The BTC unit is generally flat, and is bounded by gently sloping margins, as well as by steeplyscarped, scalloped margins. Low centred BTC polygons (LC-BTC) are composed of troughs with raised shoulders that intersect at near-orthogonal and near-hexagonal intersections, forming polygons with depressed centres, relative to the raised rims. LC-BTC polygons are commonly ~10 m in diameter, and have smooth, flat, depressed interiors. LC-BTC polygon shouldered troughs are commonly ~3-4 m wide. LC-BTC are found at the fringes of the BTC unit, at both low-angle and scalloped margins. Spatial Relationships Between Units FBT surfaces are much more common than HBT surfaces, and HC-BTC surfaces are much more common than LCBTC surfaces. Contacts between FBT and HBT are gradational, consisting of FBT mounds which are partially hollow, or which transition into HBT-like boundary bands. Contacts between HC-BTC and LC-BTC are gradational on gentle slopes, and abrupt on steeply scalloped slopes. BTC surfaces are commonly found at the foot of crater wall interior slopes, and in topographic lows between BTsurfaced concentric ridges. BTC material is commonly draped on, and inter-fingered between, FBT mounds and HBT boundary walls at contacts between the two units, suggesting that BTC units superpose, and in places, embay BT units. FBT-covered hill surfaces are commonly ringed by HBT, which is in turn ringed by LC-BTC, and/or HCBTC. FBT-covered concentric ridges are commonly flanked by HBT in the lows between ridges, particularly in lows which also have exposures of LC- or HC-BTC polygons. Discussion Crater counts on BT material indicate an age of ~100 MY, consistent with counts on LDA [11]; crater counts on BTC units indicate an age of ~1 MY. This age difference suggests that BT and BTC are stratigraphically distinct units that were deposited at markedly different times. The small exposures of HBT and LC-BTC make distinguishing ages for these textures from ages of the more common FBT and HC-BTC surfaces impossible. However, the gradational contacts between each sub-texture, on both steep and gentle slopes, suggests that modification of two distinct units, rather than exposure of four radically different layers, accounts for the differences between sub-textures. On the basis of these observations, we propose the following formation sequence for BT and BTC units. BTC units are an atmospherically-emplaced, ice-rich deposit, temporally associated with recent latitude-dependent mantling events [12-14] containing sufficient dusty material to generate a surficial lag deposit during sublimation of near-surface ice [e.g., 15]. Thermal contraction cracking generates polygonal fractures, which initially enhance sublimation at polygon margins, generated HC-BTC polygons [15]. The lack of strongly lineated BTC polygons suggests that BTC deposits have not significantly flowed on ~1 MY timescales [e.g., 4, 16]. Infilling of polygonal fractures by overlying lag deposit fines generates subsurface wedges, which concentrate nonicy material in polygon troughs (insulating underlying icerich material), and which gradually results in the generation of raised shoulders along polygon troughs due to thermal expansion—a process analogous to sand-wedge formation in terrestrial polygons [17]. As sublimation continues, concentration of ice-free material at trough boundaries results in relatively greater sublimation at polygon interiors, generating lowered polygon interiors, and relatively raised polygon margins (LC-BTC polygons)—a process analogous to the formation of "fortress polygons" [18], but resulting from solid-vapour transitions, rather than solid-liquid transitions. Ice may still be preserved beneath thickened lags at LC-BTC polygon boundaries. A comparable process can be invoked to account for the formation of CCF brain terrain (BT). Based on MOLA point topography measurements and typical martian crater depthdiameter ratios [19], the analyzed CCF-filled craters are up to 80% filled, accounting for volumes up to 800 m thick, which may be ice-rich [8-9]. Thick accumulations of icerich material could readily flow on ~100 MY timescales under current martian conditions, sufficient to produce observed strains (e.g., ~60%, observed in one deformed crater). Internal glacio-tectonic stresses, coupled with surface thermal contraction stresses would fracture the developing BT surface, resulting in the generation of oriented, lineated fracture networks, analogous to those observed on flowing debris-covered glaciers on Earth [20]. Inversion of polygon topography at polygon margins due to differenti

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