Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..459l&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Polygonally patterned ground has been identified on Mars since the Viking era [1], and has long been interpreted as a signal of the presence of subsurface ice deposits [2-4]. The origin of ice in the shallow martian subsurface, whether by cyclical vapour diffusion or primary deposition, remains an area of active inquiry [5- 9]. Recent modelling suggests that high-latitude terrains on Mars may support buried ice sheets and glaciers, produced by direct atmospheric deposition within the past 5 My [5], overlain by a sublimation lag deposit ranging in thickness from 10s to 100s of cm [8]. These results are consistent with coarse-resolution (100s of km per pixel) neutron-spectrometer results correlating highlatitude patterned ground with subsurface water [4, 10, 11], as well as a suite of geomorphological observations linking young terrains to recently deposited, ice-rich units [5-7]. Polygon classification in terrestrial polar environments is based on morphology, structure, and origin processes. On Earth, thermal contraction crack polygons can be divided into three types: ice-wedge, sand-wedge, and sublimation polygons; each of which forms under a unique set of climate and substrate-composition conditions [12-14]. Although the thermal contraction cracking process under martian conditions is well understood [15], classification systems for polygonally patterned ground on Mars have until now relied primarily on imaging data at resolutions comparable to the scale of the polygons of interest [3]. We build on the identification of sublimation polygons in the NASA Phoenix landing area [16], and preliminary classification of polygons into morphological species (groups distinguishable by characteristic surface morphologies) [17] across the northern hemisphere of Mars. We present an integrated assessment of martian polygon morphological variation as a function of latitude, and suggest links between polygon morphology, origin timing, and global climate conditions. This analysis provides context for interpretation of contraction crack polygons at the NASA Phoenix landing site. Survey Parameters We present an updated survey of 413 HiRISE images of the martian northern hemisphere (30-80°N), at resolutions of ~30 cm/pixel, spanning primary science phase orbits 001331 to 007492 [18]. Of the surveyed images, 276 contain polygonally patterned ground (69%). Polygon diameters were measured using centre-to-centre point distance measurements between adjacent polygons, and were averaged across the four nearest-neighbour points to give a representative diameter. Polygon Classification and Distribution We divide polygons into 8 morphological types (Figure 1). Commonly more than one type is present in a single HiRISE image, suggesting variability in polygon-forming substrate conditions on 100 m to km length scales [19]. Polygon types are strongly grouped by latitude, transitioning from one to the next across the studied latitude range. High Relief (HR). Initially described by [16], HR polygons are well-formed and have strong contrasts between polygon centres and depressed polygon margin troughs. HR are morphologically similar to S1 terrain described by [3], averaging ~6 m in diameter (N = 160). HR polygon distribution is centred at 69.4°N. Northern Plains (NP1-3). Northern plains polygons are the predominant northern hemisphere polygon species (196 occurrences). NP are present in the vicinity of the Phoenix lander [16], and occur in three varieties: NP1, consisting of well-formed, low-troughed polygons present on boulder-topped mounds (mean diameter ~5 m, N = 138); NP2, consisting of less-sharply defined polygons, present on both smooth, and gently hummocked surfaces (mean diameter ~5 m, N = 121); and NP3, which are poorly polygonalized features, commonly consisting of long, sinuous cracks which form coarse networks (mean diameter ~6 m, N = 147). Polygons at the Phoenix lander site appear to be NP2/NP3. NP polygons are all broadly similar to the S group described by [3]. On the basis of aspect-dependent asymmetries (steep pole-facing slopes and shallow equator-facing slopes, determined from HiRISE stereo topography), NP1 were identified as features genetically similar to terrestrial sublimation polygons, which form on a substrate of buried excess ice (ice > pore space) and in the absence of an active layer [14, 16]. NP polygons are distributed latitudinally in the northern hemisphere, with NP1 occurring most commonly to the north (distribution centre at 69.8°N), NP3 occurring to the south (distribution centred at 66.9°N), and NP2 occurring at intermediate latitudes (distribution centred at 67.9°N). Northern Plains Subdued (NPS). NPS polygons average ~16 m in diameter (N = 145) and are bounded by depressed troughs that are less sharply defined than those outlining NP1-3 polygons. NPS commonly have small, meter-scale boulders accumulated in inter-polygon troughs, forming lineated networks. If these boulders have been gravitationally sorted into polygon troughs by oversteepening of polygon interiors [13], which have subsequently been flattened, these observations may suggest significant removal of subsurface ice and deflation of NPS-surfaced terrains. NPS distribution is centered on 53.4°N. Peak-Topped ( P T ) . Peak-topped polygons are characterized by steeply peaked polygon interiors, surrounded by shallow, narrow polygon troughs. PT form in association with Brain-Terrain Covering (BTC) and Scalloped polygons as well as in isolation. PT average ~9 m in diameter, and are present in a range centred on 48.5°N. Scalloped Terrain. Scalloped terrain present in Utopia Planitia has been interpreted to be an ice-rich sublimation residue [21]. Polygons exist at two scales in scalloped terrain: a large scale with pitted troughs (mean diameter ~70 m, N = 125), and a small scale, with welldefined mounds and troughs (mean diameter ~11 m, N = 117). Features similar to small-scale scalloped polygons are also present in south-circum-Argyre images (for example, PSP_002914_1275). Northern hemisphere scalloped terrain is centred on 46.3°N. "Brain Terrain" Covering (BTC). Initially described by [22] as "brain coral terrain," brain terrain is an arcuate and cuspate surface texture commonly associated with lineated valley fill (LVF), lobate debris aprons (LDA), and concentric crater fill (CCF). Polygons present on a mantling unit overlying the brain terrain, brain-terraincovering (BTC) polygons, average ~11 m in diameter (N = 123). BTC distribution is centred at 41.7°N, with a comparable distribution to brain terrain. Discussion Gradational contacts between polygon species—locally in the Northern Plains group, and more broadly across the entire northern hemisphere— suggest that localized changes in surface conditions can have pronounced effects on polygon morphology. Crater-dating of polygonally patterned surfaces analyzed in this survey reveals a complex history of deposition and modification. Published crater counts of mantled terrains containing NP and HR polygons range from <3 MY to <0.3 MY, depending on latitude and exposure [6], providing a recent benchmark for polygon development. A past point for polygon-related activity is provided by brain terrain, which is dated in a variety of locations to within the last ~100 MY [e.g., 23]. Intermediate between these extremes, crater counts on BTC units suggest an age of ~1 MY, consistent with older ages of the latitudedependent mantle [6], but younger than the most poleward polygons. Continued crater counting by polygon morphological group will reveal how subtle changes in surface evolution, shallow-subsurface ice distribution, and obliquity-driven thermal history may be recorded in northern hemisphere polygon morphology. References [1] Lucchitta, B.K. (1981) Icarus, 45, 264-303. [2] Malin, M.C. and K.S. Edgett (2001) JGR, 106, 23,429- 23,540. [3] Mangold, N. (2005) Icarus, 174, 336-359. [4] Mangold, N., et al. (2004) JGR, 109, doi:10.1029/ 2004JE002235. [5] Head, J.W., et al. (2003) Nature, 426, 797-802. [6] Kostama, V.-P., et al. (2006) GRL, 33, doi:10.1029/2006GL025946. [7] Mustard, J.F., e
Head James
Levy Jeremy
Marchant David
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