Possible Endogenic Features in the South Polar Region of Mars From Mars Orbiter Camera Images

The narrow angle (NA) Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) mission has provided detailed imagery of the south polar layered deposits (SPLD) and the south polar residual cap revealing previously unknown features. There are a multitude of these features which we have identified and which we believe have a possible endogenic origin. The observed endogenic features often have particular associations and negative, circular, but non crater-like forms and are morphologically different in different areas of the south polar region. Ideas on the origin of these features are generally discussed, including possible terrestrial analogues. The possible endogenic features on the south polar residual cap are all located in or near the termed "Swiss cheese" terrain [Ingersoll et al., 2000; Thomas et al., 2000]. Near 100 - 115 W, 86 S many, semi-circular, conical pits with diameters all less than approximately 600 m are found. All of these residual cap negative features are at least partly if not completely covered with the Swiss cheese terrain. Another section of these pits is near 340 - 355 W, 86 S. The region of the SPLD from at least 190 - 230 W, 85 - 87 S, just west of Chasma Australe, is the primary location for a different clustering of negative, circular pits. This area is topographically unique because of the extensive grooves of an undetermined origin cut into it. These pits are all approximately 10 - 100 m in diameter and have been ruled out as small primary impact craters or secondary craters based on the lack of crater-like associations such as raised rims or ejecta blankets and the high number of features observed over this entire area. A significant association that has been noted in some images is that the pits are grouped in circular patches of slightly lower topography over a particular region, as found in MOC NA m11-02900. Other pit-like features are noted in different areas on the SPLD, particularly from at least 120-190 W, 85- 87 S. These features are irregular in shape and are also found commonly within this region. An example of a one of these pit-like groupings can be seen in MOC NA m15-01126. There are terrestrial features that have the same size range and are similar in appearance and distribution to the south polar pits. These termed "glacial potholes" are thought to occur in association with glacial surging on Earth but a detailed process of formation is not well constrained [Post and LaChapelle, 1971; Sturm, 1987]. The analogue to the observed Martian features is only by appearance and size distribution thus far. It is also considered that these features may be associated with the Swiss cheese terrain found on the residual cap. It is possible that these features are remnants of ancient Swiss cheese. The origin of these endogenic features can not yet be concluded and it is possible that different mechanisms are responsible for the features on the residual cap and on different regions of the SPLD. Ingersoll, A.P., et al. 2000. Spring and Summer Changes at the south pole as seen by the Mars Orbiter Camera. Mars Polar Science II (2000), 4076. Post, A. and E. LaChapelle. Glaicer Ice. University of Washington Press, 2000. Sturm, M. 1987. Observations on the Distribution and Characteristics of Potholes on Surging Glaciers. Journ. Geophys. Res. 92, 9015-9022. Thomas, P.C., et al. 2000. North-South geological differences between the residual polar caps on Mars. Nature 404, 161-163.