Geomorphic Evidence for Martian Ground Ice and Climate Change

Introduction: Recent results from gamma-ray and neutron spectrometers on Mars Odyssey indicate the presence of a hydrogen-rich layer tens of centimeters thick in the uppermost meter in high latitudes (>60 ̊) on Mars [1]. This hydrogen-rich layer correlates to regions of ice stability [2]. Thus, the subsurface hydrogen is thought to be water ice constituting 35+/15% by weight near the north and south polar regions [1]. We refine the location of subsurface ice deposits at a < km scale by combining existing spectroscopy data with surface features indicative of subsurface ice. A positive correlation between spectroscopy data and geomorphic ice indicators has been previously suggested for high latitudes [3]. Here we expand the comparative study to northern mid latitudes (30 ̊N65 ̊N). One of the most recognizable terrestrial geomorphic indicators of subsurface ice is an organized network of polygonal fractures. Polygonal terrain is typically found in Arctic and Antarctica permafrost environments and is demarcated by troughs, beneath which grow vertical wedges comprised of ice, sand or combination. Polygonal terrain has also been identified on the Martian surface in high-resolution Viking Orbiter images [4,5] and Viking Lander 2 images [6]. Recent narrow-angle images from the Mars Orbital Camera (MOC) on Mars Global Surveyor reveal the planet’s surface features in unprecedented, meter-scale detail. Our study uses these MOC images to locate and map the distribution of small-scale polygons (~10-250 m). It is hypothesized that these polygons form from similar thermal contraction processes as those on Earth [7]. Polygonal terrain on Mars is one of several geomorphic ground ice indicators identified at mid and low latitudes even though water ice is not currently stable equatorward of 60 ̊ for the present obliquity of 25 ̊ [8,9]. Climatic variations resulting from dramatic changes in orbital parameters can explain the current spatial distribution of these features [10]. The combination of geomorphic evidence and modeling provides strong indication that ice may have accumulated at mid and low latitudes during periods of high obliquity, 35 ̊< 45 ̊ [2,11]. Methods: Using high-resolution, narrow-angle MOC images (1.55-12.39 m/pixel), we mapped polygonal terrain in a latitude band from 30 ̊N to 65 ̊N [12]. We used data from August 1997 to February 2003, excluding the September 1999 to February 2000 set. A total of 5,782 images were analyzed. Nearly all images span an area of 3 km in width and 5 to 20 km in length. Because epithermal neutron flux rates are inversely related to water ice abundance in the uppermost meter, we can map the distribution of ice in the near subsurface. Using corrected epithermal neutron flux rate data in 2 x 2 degree cells, we mapped the relative distribution of hydrogen between 30 ̊N to 65 ̊N and used flux rate data in 0.5 x 0.5 degree cells for 30 ̊N to 65 ̊N and 300 ̊W to 240 ̊W, the Casius Quadrangle. We then compared the surface ice abundance with the surface features in the MOC images. MOC images showing the presence of polygonal terrain in the Casius Quadrangle were also analyzed to determine polygonal geometry, dominant spatial trends, and indications subsurface water ice. Observations: Of the total MOC images analyzed, 4.2 % revealed the presence of polygonal terrain, identified in this study as having the following characteristics: diameters of polygons ranging in size from 25 m to 250 m, nearly straight bounding troughs or rims, and angular joins. Frequently, the longest edges of the polygons run parallel and strike north-south. Polygonal terrain exists at low elevations (<0 km); few polygons were identified south of 35 ̊N or in the cratered highlands. A particularly high concentration of polygonal ground is present between 288 ̊W-258 ̊W and 40 ̊N-50 ̊N. In this region, known as Utopia Planitia, 52% of the total 191 images analyzed showed the presence of polygonal terrain. The distribution is similar to that found by Seibert and Kargel (2001) us-