37th DPS Meeting, 4-9 September 2005
Session 32 Mars' Surface
Poster, Tuesday, September 6, 2005, 6:00-7:15pm, Music Recital Room

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[32.08] The origin of polygonal impact craters - Evidence from Argyre region, Mars.

T. Ohman (Department of Geosciences, Division of Geology, University of Oulu, Finland), M. Aittola, V.-P. Kostama, M. Hyvarinen, J. Raitala (Department of Physical Sciences, Division of Astronomy, Uiversity of Oulu, Finland)

Polygonal impact craters are a ubiquitous feature on the surfaces of various bodies throughout the Solar System (Ohman et al., 2005). We studied the polygonal craters in the Argyre region, Mars, with two goals in mind: a) to better constrain the origin of polygonality in impact craters, and thus, understand the effects of pre-existing structures of the target material during the formation of impact craters (this work), and b) to see how polygonal craters reflect the complex geotectonic history of Argyre impact basin's surroundings.

Differential erosion is still often regarded as the cause for crater's polygonality, although already Eppler et al. (1983) showed that erosion actually increases the circularity, not the polygonality of impact craters. Our work reveals that both eroded (no rim wall), rimmed, and fresh (preserved ejecta blanket) craters all display the same amount of polygonality, as measured by the number of straight rim segments. Also the directions of the rim segments in all erosional stages are statistically generally the same. This suggests that the fracture systems these polygonal craters reflect have quite ancient origins. These observations are very difficult to understand by means of erosion, but instead are a natural outcome if, as we believe, the polygonal plan view stems already from the formation of the crater.

The currently favoured acoustic fluidisation model of impact crater's modification (collapse) stage requires that the rim material is nearly strengthless during the collapse, and thus can not have a ``memory" of the pre-existing crustal structures (e.g. Melosh and Ivanov, 1999). In the view of polygonal crater data, this is not the case. Therefore, at least a slight adjustment is required to the current cratering models for them to correctly depict nature.

Funding from the Vaisala Foundation is gratefully acknowledged.

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