CIRCULAR GRABEN AND GIANT POLYGONS IN UTOPIA PLANITIA, MARS

Throughout the giant polygonal terrain of Utopia Planitia are circular graben, inferred to overlie buried crater rims. Thirty-seven circular graben were previously identified southwest of the Utopia Basin in Viking and MOC images [Buczkowski and Cooke, 2004]. Thirty-four of the 37 have two nested rings, while the remaining three have a single ring. The distance of doubled circular graben from the center of the Utopia Basin scales directly with the spacing between the two rings, implying a relation between ring spacing and cover thickness. Numerical models of horizontal shrinkage during volumetric compaction of material over crater rims indicate that double ring formation is likely when cover material is a critical thickness and that these fractures move away from each other with increasing cover thickness. However, where cover material is thin, only a single circular graben forms, accounting for the observation that the 3 single-ringed graben are most distal to the center of the basin.

Differences between the giant polygons north and south of the Utopia basin have been previously examined [Buczkowski and Seelos, 2007]. The southern polygons are larger with bounding troughs that are steeper and deeper. Meanwhile, the northern polygons are smaller, more regular in shape and the bounding troughs are subdued and appear filled. Using THEMIS data, at least 7 circular graben have been identified to the north of the Utopia basin. We examine the geomorphology of the northern circular graben to determine any differences compared to those to the south and evaluate how these differences compare to the differences between the northern and southern giant polygons. We measure the spacing between rings, determine if there is a relationship between ring spacing and distance to the center of the Utopia basin, and assess how this relationship compares to the ring spacing of the southern circular graben. We evaluate what these differences mean for the formation environment of these features.