BASEMENT CONTROLS ON THE SCALE OF GIANT POLYGONS IN UTOPIA PLANITIA, MARS

We investigate the role of buried topography in controlling the scale of the giant polygons of Utopia Planitia (49ºN, 233ºW), Mars. Our approach is two-fold: observations and measurements of giant polygons and simulation of polygon development using a numerical model of a layer of wet sediments undergoing volume contraction. Using a 100m/pixel THEMIS Infrared Daytime mosaic of Utopia, the spacings of the giant polygons were measured along 7 basin-radial transects in order to obtain a moving average of trough spacing along each transect. The variation in running averages support a layer of sediments within Utopia basin overlying an irregular buried surface. In contrast to inferences of sediment thickness from 1-2 km, the measured average 5-6.5 km trough spacing suggests a layer thickness of 2.5-3.3 km and further supports the primary role of buried topography controlling the scale trough spacing. The change in elevation along MOLA tracks of a selected region of the Highlands, is used to characterize the buried topography expected beneath the basin. The surface topography of this region has a power-law distribution of elevation with best fitting power-law exponent of -1.998. These findings are used to create a numerical model of the Utopia Basin, where fracture seeds of variable height along the base of the model represent variations in observed highland topography that expected to underlie the basin. At low strain, our models produce fractures with ~3-8 km spacing similar in scale to spacing of troughs in the polygonal terrain (5-6.5 km) in the Utopia Basin, supporting the idea that buried topography could be the primary scaling factor for the polygon grabens. The results do not constrain an upper limit for strain; the observed trough widths suggest that further strain was expressed by the widening of the troughs. Material properties for wet, fine sediments, analogous to the terrestrial counterpart, are appropriate for the model to match observed polygon trough spacing in Utopia.