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N.M. Seibert, J.S. Kargel (U.S. Geological Survey)
Following from [1], we report results of an extensive and continuing survey and classification of small-scale polygons in MOC images of Mars having resolutions better than 13 meters per pixel. We have examined relationships between polygon occurrence and morphology and between the occurrence of polygons and other ground-ice indicators. Our database includes MOC images that either do or do not contain polygonal terrain, elevation from MOLA, geologic unit, and morphologic type of polygons. Included are (a) all qualifying images from orbital subphases AB, FHA, SP, and M12 through M18; (b) all qualifying MOC images between 30 and 90 degrees north and south from M7 through M11 orbit phases; (c) all qualifying images from all orbits in central Utopia Planitia; and (d) a global sampling of images from orbit phases M0 through M6.
Small polygons are strongly concentrated at middle and high latitudes, somewhat similar to the occurrences of (a) lobate debris aprons, lineated valley fill, concentric crater fill, and terrain-softening features observed by [2], (b) small-scale erosional pits observed by [3], and (c) a variety of patterned ground features reported in a survey by Kargel et al. (2001, submitted). Polygonal terrain is widespread at high southern latitudes, where a morphologic type dominates that in places has a sharp axial ridge within a trough. This type (probably ice-wedge polygons) may have been active recently.
Though polygons are present in northern mid/high latitudes, small polygons are less common there than at equivalent latitudes in the south, except they are pervasive across much of Utopia Planitia. Small polygons observed in Utopia Planitia are mainly the eroded thermokarstic expression of other more widespread polygon forms. The "Utopia Type" is rarely found outside Utopia, but wherever it occurs it is associated with scalloped depressions interpreted as possible thermokarst. Unique polygons occur widely in south polar dry ice deposits.
References. [1] Seibert, N, and Kargel, J., Geophys. Res. Lett., 28, 899-902, 2001. [2] Squyres, S.W. and Carr, M.H., Science, 231, 249-252, 1986. [3] Mustard, J.F., Cooper, C.D., and Rifkin, M.K., Nature, 412, 2001.
This work was supported by a grant to Kargel from the Mars Data Analysis Program.
The author(s) of this abstract have provided an email address for comments about the abstract: nseibert@earthlink.net