 ROCK BEHAVIOR ON HIGH-LATITUDE PATTERNED GROUND TERRAINS OF MARS.  T. C. Orloff , M. A. Kreslavsky, E. Asphaug. torloff@ucsc.edu,  Department of Earth and Planetary Sciences, University of California Santa Cruz 1156 High St. Santa Cruz, California 95064, USA.   Introduction: Here we investigate the preferred rock arrangements for typical high-latitude Martian patterned ground. Patterned ground covers nearly all high-latitude terrains between 60-75° in both hemispheres. Interconnected polygons make up the patterned ground and form due to environmental fluctuations in the ice-cemented surface.  Many types of patterned ground exist [1] and there are different types of polygons [2].  These polygons can integrate cobbles and boulders (further referred to as rocks) strewn on the surface into the pattern [3].  There are several ways that rocks may organize into the patterning.  Common arrangements include concentration in low polygon interiors, high polygon interiors, low polygon margins (troughs), and along high polygon margins.     Figure 1: Typical high-relief (HR) polygons in the northern hemisphere from image PSP_001474_2520.  Illumination is from the right.  Note rocks in troughs casting shadows.  Background: Individual small-scale polygons on Mars were invisible in the earlier studies [e.g., 1] due to low resolution.  Levy et al. [2] used more detailed images from HiRISE and defined two prominent types of small-scale polygons found at high-latitude.  High relief (HR) polygons (Fig. 1) are linearly dissected plates with diameters 5-6m.  They have relatively deep troughs of variable width surrounding high centers.  HR polygons are found between 67-72° in the northern hemisphere and 70-80° in the south.   Another class is the Flat Top Small (FTS, Fig. 2). These have diameters ~5m in the northern hemisphere and 10-11m in the southern.  These polygons also have crisp troughs surrounding high centers.  FTS polygons are found between 65-73° in the northern hemisphere and 47-79° in the southern.  Together these two polygon types make up the majority of patterned ground terrain at high latitude, according to [2]. Data Set:  This study looks at images containing polygons found at high latitudes.  Images in this study are taken by HiRISE and range in resolution between 25cm/pixel and 100cm/pixel.  Images at 100cm/pixel cannot reliably observe individual rocks on the surface.  The rocks in each image are examined to determine preferences for locations on polygon such as high or low places or interiors or margins.  Presented here are the results of the initial search.     Figure 2:  Typical FTS polygons in the northern hemisphere from image PSP_001607_2510.  Illumination is from the right.  Rocks participate in a patterning at larger scales than individual polygons clustering in highs (upper portion of the image).  Some rocks still prefer low troughs surrounding the smaller polygons (center of the image).  Results:  Figure 3 is a chart of the results of the initial image analysis.  Rocks are found to concentrate in high polygon interiors, low polygon interiors, low polygon margins, and high polygon margins.  The margins of high latitude polygons are overwhelmingly the preferred locations of rocks on small-scale polygons.  Figure 1 and Figure 2 show examples of polygons organizing rocks into the low places in the polygon margins surrounding them. Rocks on HR polygons do not concentrate into low polygon interiors or high polygon margins. At Phoenix landing site much smaller, decimeter-scale rocks have also been found to concentrate preferably in the lowered margins of polygons [4].  In many patterned ground terrains in the northern hemisphere and especially with FTS polygon terrains, rocks organize both at the individual polygon scale as well as clustering on topographic highs.  When such clustering occurs it tends to cover polygons.  In a few instances, rocks are not present on patterned ground terrains at high latitudes. Mantling of the surface may cover rocks previously at the surface.  But generally rocks are present.  Impact craters are one obvious source of rocks to the surface.  Patterned ground polygons may also exhume buried rocks to the surface. "Rocks" may also be formed by the patterning itself, as cohesive chunks of ice-cemented soil.  Figure 4 shows a number of large rocks that appear to have shapes and sizes similar to the polygons themselves.  Cracking of an ice-cemented surface may add rocks to the surface in this way.  As cracks propagate into the soil they may intersect and separate into discrete blocks, which after further erosion become fully exhumed.                  Figure 3:  Table of Results of Rock Concentration Analysis of HR and FTS Polygons.  Rocks are commonly unorganized in patterned ground terrains, but when rocks are organized, they do so in polygon troughs.     Conclusions: Each patterned ground type observed is capable of organizing rocks.  Their preferred method of rock concentration is to strand rocks in low polygon margins although other arrangements are possible.  This affect may be disrupted by larger scale patterning also capable of organizing rocks.  When multiple scales of patterning are present the rocks may respond to each.      Figure 4:  Polygonal shaped rocks being exhumed from FTS polygons in the southern hemisphere.  These "rocks" may be exhumed blocks of ice-cemented material that originated as part of the patterning process.  This image is from PSP_003918_1170.  Illumination is from the left.  Further Research: We continue looking at images of high latitude patterned ground in order to expand the catalog of [2] and also make observations on the details of rock concentration. This study may be extended to study other types of polygonal terrains and lower latitudes. References:  [1] Mangold N. (2005) Icarus 174, pg.336-359. [2] Levy J. et al. (2009) JGR, 114, E01007.  [3] Orloff T.C. et al (2009) LPS XL, Abstract #2205. [4] Heet et al. (2009) JGR 114, E00E04.   
