SOUTH-POLAR POLYGONAL PATTERNS – PHENOTYPES AND LOCAL GEOMORPHOLOGICCONTEXT. S. van Gasselt1, D. Reiss1, G. Neukum2, 1German Aerospace Center, Institute of PlanetaryResearch, Rutherfordstrasse 2, D-12489 Berlin, Germany, 2Freie Universitaet Berlin, Germany(Stephan.vanGasselt@dlr.de).

Introduction: The general shape anddistribution of small scaled polygonal patterns onMars have been attributed to ice-wedge polygonssimilar to their terrestrial analogs of high latitudeperiglacial environments, sand-wedges, desiccationcracks of dried water- or mudlakes, or to contractioncracks of cooled lava. Whatever the exact process isbased on, the composition of the (sub-)surfacemedium, its mechanical properties, as well as thedynamics of the fracturing process still remainunknown to us. The global distribution of polygonalpatterns (e.g., [1]) is in general accordance with thedistribution of water and ice in the shallow subsurface of Mars, according to modelled dataprovided by Mars Odyssey’s HEND experiment [2]and theoretical modelling of earlier years [3], stillthere is a lack of evidence for polygonal patternscaused by thermal contraction cracking and theexistence of ice wedges. As an approach for furtherunderstanding of the phenomena we haveinvestigated the south polar region of Mars as aprimary target for ice-wedge formation andcatalogued the small-scale properties of polygonalnetworks and their geologic and geomorphologiccontext.

Geologic Settings: The south-polar regionbetween 80°S and 90°S consists primarily of theAmazonian aged polar layered deposits (Apl) [4]and a patchy distribution of old highland terrainadjacent to the Apl materials. Towards the polecenter the residual ice cap (Api) is present andshifted towards longitudes between 0°W and 90°W.At longitudes between 265°W and 90°W parts of theupper and lower Hesperian-aged Dorsa ArgenteaFormation are exposed (Hdu, Hdl). The areal extentof this unit corresponds with the topography in termsof elevation values of 1200 m (fig. 1). At 10°W andbetween 70°W to 95°W remnants of the undividedHesperian and Noachian material (HNu) occur andshow degradation by possible removal of ground ice,mass wasting and eolian processes occur [4].

Observations: From a number of ~6000 MOC-NA images between 80°S-90°S we have found over700 images showing features of small-scalepolygonal patterns. The resolution of the cataloguedimages ranges from 1.37 m/pixel to 14.45 m/pixelwith 40% of all images ranging from 2 m/pixel to 3m/pixel. Image acquisition ranges from Ls=7-356.The cluster effect of the distribution (fig. 1) is notbound to local settings mostly, but is constrained bythe coverage density of image data of the MarsOrbiter Camera. This results in large clusters at IncaCity, the Chasma Australe system and the

distribution at the 87°S cirum polar circle. Thegeographic distribution of polygonal terrain isdescribed in [5]. On the basis of these observations itis possible (with some restrictions concerningtransitional morphologies) to distinguish at least fivedominant types of polygonal patterns. About 75% ofthe mapped polygon types fit into this scheme, butstill, in order to understand the mechanical andclimatic conditions, this classification needs to beenhanced.The polar trough type polygons (fig. 2a) arerestricted to the polar layered deposits, troughs andre-entrants (Apl) of the south polar region. About10% of the catalogued imagery show polygonalpatterns on a single layer or across multiple layers.The polygons are characterized by orthogonal aswell as hexagonal trough intersections with apolygon diameter of 80 m to 150 m but diameters of20 m to more than 300 m are occasionally observed.The polygon troughs have a constant width ofseveral meters across each investigation area, theyappear fresh to moderately degraded. Only fewpolar layer type polygons present higher degreepolygonal patterns with diameters of 5 m to 6 mwithin the main polygons. Lateral limitations in theirextension are due to topographical steps or changesin surface material which cause changes in materialstrengths. The complex type polygons (fig. 2b) are aset of primary polygonal troughs and high orderpolygons, which vary in trough widths and

Fig. 1: Topographic shaded relief and color coded map of thesouth polar region with observations of classified polygonalpatterns.

Microsymposium 38, MS095, 2003(A4 format)

SOUTH POLAR POLYGONAL PATTERNS: S. van Gasselt et al.

intersecting angles. The major troughs are radial toan initial crack center, the troughs are interconnectedby higher order troughs orthogonal to the maintroughs. Higher order troughs form intersectionswith lower angles. Their troughs are filled withbright frost as observations have been made within alimited seasonal period of Ls=250-290, when frostmaterial has not been removed completely from thetroughs. The polygonal features can be observed ontop of the dark deposits of the Nplh, Hd(u,l), andHnu units at 80°W and 340°W, only. The smoothplains type polygons (fig. 2c) are the commonesttypes of polygons and are distributed on smoothplains of average albedo values across the polarlayered deposits (Apl) and on the residual ice caparea (Api). The surface topography on which theyappear is flat or undulated. Commonly, they appearon small low albedo patches near the lee-ward sidesof knobs or dunes. The polygonal troughs are filledwith dark material and their polygons range from 20to 60 m in average, but observations with diametersof up to 100 m can also be observed. Their troughintersections are orthogonal. The polygonal networkconsists either of a mixture of short straight troughsegments and highly arcuate troughs or rectangularshort trough polygons. The surroundings of patcheswith polygonal patterns are covered by deposits ofeolian material and frost or they terminate at

topographical steps. The regularity in terms ofpolygonal shape implies a homogeneous surfacematerial in which they form [6]. Their occurencesare restricted to imagery of spring to late summerseasons. The fretted trough type polygons (fig. 2d)are characterized by small straight trough segmentsfilled with either high or very low albedo material.The polygons extend across large areas in the Aplunit independent of the topographical situation andare situated along the 87°S circum-polar region.Their seasonal context is late southern spring to latesouthern summer, when the dissappearing frostuncovers the surface polygons. The short troughsshow a dendritic pattern, in which short secondary(high order) troughs originate at the main trough andpropagate in orthogonal direction. The size of thepolygons reach up to 100 m, the side arms havelengths of a few meters only. The polygonal patternis very distorted and interrupted, but it seems, thatthey originate in one point growing into a radialdirection. The small side troughs are already formedin this early stage of development as it can beobserved in several images. The dissected dunerelated type polygons (fig. 2e) are a set of smalland short orthogonal troughs which have a highlyuncomplete appearance. The polygons havediameters of a few tens of metres. The low albedopolygonal troughs occur between small dark patchesof dune fields on a bright frost covered surface.Their appearance is bound to the 87°S-Apl units andthe northernmost Nplh units. They appearthroughout the seasons but the well developedpolygons are restricted to about Ls=200 (earlywinter). At this time we can expect that the circum-polar areas are covered by large amounts of frost andthat in contrast to other polygonal patterns describedabove, the troughs appear relatively fresh.

Conclusions: The observability of polygonalpatterns and trough fill on Mars are directly tied toMartian seasons [e.g., 7]. The Amazonian polarlayers and adjacent units present a variety ofpolygonal patterns which occur after removal offrost cover or at winter time. The characteristics interms of shape and sizes can be attributed to theappearance of the surface material, to variations intopography and to geologic units, according to [8],but for a specification of surface material propertiesand crack processes data from mid-latitude areasneed to be added and compared with the results forthe south-polar area.

References: [1] Kuzmin, R. and E. Zabalueva (2003), LPSC,34, #1912 , [2] Mitrofanov, I. et al. (2002), Science, 297, 78+, [3]Squyres and Carr (1986), Science, 231, 249++; [4] Tanaka andScott (1987) Geologic Map of the polar regions of Mars, USGS I-1802-C, [5] van Gasselt et al. (2003), 3rd Mars Polar Sci. Conf.,#8088 ; [6] Yershov, E. D., General Geocryology, 580 pp.,Cambridge Univ. Press, New York, 1998; [7] Kossacki andMarkiewicz (2002), Icarus, 160, 73++; [8] Lachenbruch (1962)Spec. GSA Papers, 70.

Fig. 2: South Polar polygonal networks. Polar trough typepolygons (a), complex polygon type (b), smooth plainspolygon type (c), fretted polygons type (d), dissected dunerelated polygons type (e). The scalebar represents 1km.