Patterned Ground on Mars: Evidence for Recent Climatic Variations

Patterned ground on Mars: Evidence for recent climatic variations

N. Mangold Orsayterre, FRE2566, CNRS and Université Paris-Sud, France

ABSTRACT: New high resolution images (10 m/pixel) of the space probe Mars Global Surveyor display polygonal features very similar to terrestrial patterned ground. Some features are similar to terrestrial ice-wedge type of polygons, with local association of possible pingo-like features. Regular networks of polygons at the ten meters scale have a geometry similar to sorted nets. On Earth, these features are usually related to the presence of ground ice and freeze-thaw cycles. Martian patterned grounds are geologically recent, being less than 10 millions years old, and are located between 50° and 70° latitude in both hemispheres where ground ice is believed to exist. Current Martian temperatures do not allow temperatures above 0°C at these latitudes, but orbital variations of Mars in the last 100,000 or millions years may have provided the possibility of above 0°C summer maximum temperatures.

1 INTRODUCTION released with MOC data acquisition sequences M01 to M18. Topographic profiles from the Mars Observer Mars is believed to have been a cold and dry planet for Laser Altimeter (MOLA) have been used, when avail- the last 3 billion years (Carr, 1996). Liquid water is not able, in order to extract slopes and difference in eleva- stable at the surface because of the low atmospheric tion of the studied landforms. These profiles have a pressure (600 Pa) and the cold surface average annual vertical accuracy of less than 1 metre, but only have a air temperatures of 60°C. Recent data from the Mars horizontal accuracy of 300 m. This makes the study of Global Surveyor (MGS) space probe shows young gul- smaller features impossible. The images of possible lies whose origin is related to liquid water however periglacial features on Mars are compared to terres- (Malin & Edgett, 2000). These flows could be the result trial counterparts in the following sections. of recent obliquity changes which resulted in summer temperatures above 0°C (Costard et al., 2002). The occurrence of recent periods of warmer summer tem- 2.1 Cracks and ice-wedge like polygons peratures renews the idea that polygonal terrains on Mars could be compared to terrestrial patterned ground Figure 1 is a close-up within a 20 km large crater cor- in periglacial environment. Indeed, wide polygonal sys- responding to a natural depression. Polygons 50 to tems have previously been identified in the Martian 300 meters large like those in figure 1 are observed in Northern plains (Pechmann, 1980, Lucchita, 1983, many regions of mid- and high latitudes of Mars. Both Masson, et al. 2001). These polygons, 2–10 km across images on figure 1 display orthogonal networks of with bounding through widths of 200 to 800 m, are cracks. Orthogonal junctions are explained by a suc- probably too large to correspond to periglacial pat- cessive growth of fissures (Lachenbruch, 1966). terned ground (Hiesinger & Head, 2000). Nevertheless, Polygons of this type on Earth are formed by thermal new high resolution images of the Mars Observer cracking and the subsequent filling of cracks by seg- Camera (MOC) of MGS show small-scale polygons regation ice. The features on Mars therefore display a and periglacial features much more similar in size to geometry and shape very similar to the ice-wedge terrestrial patterned ground (Yoshikawa, 2000, Masson type of polygons on Earth, despite the fact that the et al., 2001). In this study, we examine several Martian observation of ice within the cracks is evidently not features that are potential analogues to terrestrial ones. possible. Thermokarst-like depressions and the occurrence of dense networks of cracks inside depressions favour the role of ground water in the formation of 2 OBSERVATIONS cracks as on Earth (Seibert & Kargel, 2001). If similar to terrestrial ice-wedge polygons, these polygons MOC high resolution images (1.5 to 10 m/pixel) are are potential evidence for freeze-thaw cycles and the elongated in an orbital North-South direction with a presence of ice. Thermal cracks could be formed only typical width of several kilometres and length of sev- by temperature variations below ice melting point. eral tens of kilometres. We have completed a system- Indeed, large variations of temperatures between atic analysis of possible periglacial features on the Martian summers and winters favour thermal contrac- Martian surface from more than 10,000 images tion without the necessity of ground thaw (Mellon,

723 Figure 1. Ice-wedge like polygons on Mars, MOC image M00-00602 (MSSS/NASA/JPL), 232°W, 6 5 °N (top). Ice- wedge polygons in Northern Canada, free document from National Resources Canada (NRC), http://www.rncan.gc.ca.

1997). The possibility of thermal contraction at temperatures below 0°C without thaw therefore remains possible in several cases. A classification and locali- sation of all of these polygons should help to discrim- Figure 2. Network of cracks associated to a 300 m large inate between the two end-members and test these pingo-like feature on Mars, MOC image M08-03679 hypotheses. (MSSS/NASA/JPL), 98°W, 6 9 °S (top). 100 m large pingos (Prince Patrick Island, NW Territories, free document from NRC)(bottom). 2.2 Pingos associated to polygons?

On Earth, various types of frost mounds, such as pin- between a pingo hypothesis and other round shaped gos, exist in regions affected by patterned ground. features like impact craters. Impact craters covered by Pingos are formed by the upheaval of frozen ground sediments do show soft rounded rims in comparison from the freezing of injected water (French, 1996). As to fresh craters. In figure 2, the round feature top of a consequence their development requires the presence the image has a positive topography (light is from the of unfrozen water. Features similar to pingos are left), and is therefore different from impact craters. observed in the southern high latitudes of Mars (Fig. 2). The MOLA topographic profile confirms this inter- Without field data, it is difficult to distinguish pretation showing a difference of elevation of about

724 10 meters. Terrestrial pingos have a diameter of up to 300 m (French, 1996). Its size and flat circular shape make the round shaped Martian feature in figure 2 very similar to the flat pingos observed in the Northern Canada. This possible pingo is not isolated in the sur- rounding plains. Several similar features are observed nearby but they are not frequent across the entire Martian surface. The presence of cracks around the postulated pingo, as well as on the terrestrial one (Fig. 2), provides potential evidence for the occurrence of ice-rich ground in this region. Whether these cracks are related to freeze-thaw cycles or not is uncertain. Indeed, the network of cracks is more irregular than that of figure 1, displaying a more random orientation. Nevertheless, the combination of a possible pingo with these cracks, similar to those found in the terrestrial example, could favour the role of a transient period of liquid water in this location. Further data with higher resolution images would show if such pingo-like features are frequent, different from impact craters and commonly associated with polygons.

2.3 Sorted patterned ground

The ice-wedge style of polygons (Fig. 1) is observed accurately at the MOC scale, but smaller polygons exist with more regular patterns and constant spacing (Fig. 3). The maximum size of these regular polygons is 30 m, but polygons smaller than 10 m would be at the limit of the resolution and are therefore impossible to detect. They are observed on more than 200 MOC images in very young and flat regions in the high latitudes of both North and South Martian hemisphere. Their geometry is more consistent with sorted nets than polygons formed by thermal cracking. Indeed, Figure 3. Regular nets of polygons on Mars, MOC image ice-wedge like polygons usually exhibit orthogonal M04-02503 (MSSS/NASA/JPL), 275°W, 6 3 °S (top). One junctions forming polygons of various sizes and irreg- meter large sorted polygons in Jameson Land, Greenland (image J.-P. Peulvast). ular geometry. The networks of polygon observed on Figure 3 are distinguishable from ice-wedge polygons because of the homogeneous spacing of polygons and (plateau or depression) and are sometimes related in the lack of orthogonal junctions. On Earth, the size of continuity to sorted stripes on gentle hillslopes sorted polygons varies from a few centimetres to less (Mangold et al., 2002). These networks of polygons than ten metres. If related to the same processes, are therefore comparable with sorted polygons like Martian polygons of up to 30 m are therefore larger those observed in Greenland. These have segregation than terrestrial ones. Fine grained sediments and/or of stones in metre scale regular polygons (Fig. 3). The high porosities usually found in Martian surface, size difference remains to be explained however. especially in regions close to the poles, could result in Conditions necessary for the formation of sorted differences from their terrestrial counterparts. This polygonal nets are a significant soil moisture content interpretation is limited by the absence of field data, and a sufficient number of freeze-thaw cycles (French, such as rock size and distribution however. 1996). Different physical processes have been invoked Such regular polygons can also be produced by to explain their formation. These include thermally- other processes such as desiccation. Desiccation how- driven convection (Gleason et al., 1986), diapirism ever, would mainly create polygons in flat depressions (Hallet & Prestrud, 1986), differential or secondary frost previously filled by water. Conversely, regular nets of heave (Fowler & Krantz, 1994) or ice segregation (Van polygons are observed independently of location Vliet Lanoë, 1991). The diversity of sorted polygons

725 can probably be explained by one, or a combination of these physical processes depending on local conditions. No single mechanism can explain the initiation and regularity of all patterned ground (Krantz, 1990). Despite the fact that we cannot exclude alternative explanations, the regularity and homogeneity of these polygons contrasts with ice-wedge kind of polygons. As a result, sorted polygons related to cyclic episodes of ground thaw are likely to be appropriate terrestrial analogues for these features.

2.4 Sorted stripes

Terrestrial patterned grounds are observed on flat plains while striated soils or stripes usually exist on gentle slopes of more than 3° (Washburn, 1956). Stripes are often due to the elongation of convection cells or diapirs in the direction of the slope (Krantz, 1990). They can also form by other processes like ice needles, a form of ice segregation (Werner and Hallet, 1993). In all the cases, they are always the consequence of repetitive freeze-thaw cycles. The Martian surface does display sorted stripes 20–30 m in length, the same spacing postulated for sorted polygons (Fig. 4). In the proposed example, stripes are located on a gentle slope of on average 6°, obtained from MOLA altimetry. Like sorted polygons, their width is large in comparison to large stripes from Earth, and no evidence of surface characteristics can be obtained to further the interpretation of these features. Alternative explanations are however difficult to propose. The geometry of these stripes can be explained neither by sedimentary layers, nor by glacier-like flows or other equivalent processes. The similarity of this image with terrestrial sorted stripes is therefore striking enough to propose that similar processes associated with freeze-thaw cycles may have formed them.

3 CLIMATIC IMPLICATIONS

The 4 Martian examples shown in this study are all located at high latitudes polewards of 60°. Most of the more than 200 images of polygons and associated features are located poleward of 50°. All images showing any kind of patterned ground are devoid of fresh impact craters with the exception of some ice-wedge like polygons observed in northern mid-latitude regions by Seibert & Kargel (2001). If present, impact craters are systematically affected by polygons. Figure 4. 20 m spaced sorted stripes on Mars, MOC According to recent models, the absence of impact image M02-02175 (MSSS/JPL/NASA), 15°W, 6 5 °S (top). craters determines the age of these features to be less 2 meters spaced sorted stripes (Ellesmere, NW Territories, than 10Ma (Hartmann, 2000). These features are free document from NRC). therefore formed at high latitudes and in a relatively

726 recent epoch. Due to low temperatures and air pres- ACKNOWLEDGEMENTS sure, the current Martian climate is too cold and dry without stable liquid water at the surface to enable The author thanks two anonymous reviewers for their formation. Only equatorial regions display diurnal detailed comments and F. Costard, F. Forget, maximum above 0°C, but this is only over a maximum V. Jomelli and J.-P. Peulvast for helpful discussions. thickness of a few cm. At these latitudes, the surface has been desiccated for millions of years due to continuous sublimation of ice (e.g. Mellon & Jakosky, REFERENCES 1995). No ice and therefore no liquid water in the surface layers can exist at these latitudes. At the latitudes Costard, F., F. Forget, N. Mangold & J.-P. 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