Post contributed by Dr. Chloe B. Beddingfield, The SETI Institute and NASA Ames Research Center
Some impact craters are classified as polygonal impact craters (PICs), which have at least one straight rim segment, as shown in Image 1. The morphologies of PICs are shaped by pre-existing, sub-vertical structures in the target material, such as normal and strike-slip faults, joint sets, and lithologic boundaries. Because the straight rim segments of PICs only form where pre-existing structures are present, PIC morphologies can be used to analyze fractures that are buried by regolith or too small to be seen in available spacecraft images. On the icy Uranian moon Miranda, PICs are widespread across its southern hemisphere, which was imaged by the Imaging Science System (ISS) onboard the Voyager 2 spacecraft. Some of these PICs reveal previously undetected fractures that suggest Miranda has experienced multiple periods of tectonic activity.
Image 1: Examples of two PICs identified on the Uranian moon Miranda. Black arrows indicate the straight rims of these PICs. The Voyager 2 ISS image mosaic shown here includes the following images, from top to bottom: c2684620 (light blue box), c2684629, c2684617 (dark blue box).
Image 2 shows PICs identified on Pluto’s icy moon Charon. The surface of Charon exhibits an abundance of impact craters overprinting Oz Terra in the north, as well as Vulcan Planitia in the south. Rilles, which are linear features attributed to extensional faulting and fracturing, are present in this area as well. The PICs in this image exhibit straight rim segments that parallel these rills, indicating that these rilles predate the PICs in this area.
Image 2: Examples of PICs on Pluto’s icy moon Charon. Arrows show examples of nearby rilles (black) and PIC straight rim segments (white), which have similar orientations. The images used are image lor_0299171413 (large image), and portions of images lor_0299180418 (top), lor_0299175682 (top right), and lor_0299180421 (right) from the LORRI instrument onboard the New Horizons spacecraft.
Image 3 shows PICs that have been identified and analyzed on Saturn’s icy moon Dione. The presence of PICs in this area indicates that hidden fractures are also present in the heavily cratered regions outside Dione’s heavily tectonized Wispy Terrain. Therefore, analysis of PICs can provide important information for deciphering the tectonic histories of planetary bodies throughout the Solar System.
Image 3: Examples of PICs on Saturn’s icy moon Dione. White arrows highlight the straight rim segments of different PICs. The images here are Cassini ISS images N1578081030_1 (left) and N1507741460 (right).
Further Reading
Beddingfield, C.B. and Cartwright, R.J., 2020. Hidden tectonism on Miranda’s Elsinore Corona revealed by polygonal impact craters. Icarus, 343, p.113687. https://doi.org/10.1016/j.icarus.2020.113687
Öhman, T., Aittola, M., Korteniemi, J., Kostama, V.P., Raitala, J., 2010. Polygonal impact craters in the solar system: Observations and implications. Geological Society of America Special Papers, 465, 51-65. https://doi.org/10.1130/2010.2465(04)
Beddingfield, C.B., Beyer, R., Cartwright, R.J., Singer, K., Robbins, S., Stern, S.A., Bray, V., Moore, J.M., Ennico, K., Olkin, C.B. and Spencer, J.R., 2020. Polygonal Impact Craters on Charon. LPI, (2326), p.1241. https://www.hou.usra.edu/meetings/lpsc2020/pdf/1241.pdf
Beddingfield, C.B., D.M. Burr, L.T. Tran, 2016, Polygonal impact craters on Dione: Evidence for tectonic structures outside the Wispy Terrain. Icarus, 274, 163-194. https://doi.org/10.1016/j.icarus.2016.03.020
Öhman T, Aittola M, Kostama VP, Hyvärinen M, Raitala J. Polygonal impact craters in the Argyre region, Mars: Evidence for influence of target structure on the final crater morphology. Meteoritics & Planetary Science. 2006 Aug;41(8):1163-73. https://doi.org/10.1111/j.1945-5100.2006.tb00513.x
Cartwright, R.J., Beddingfield, C.B., Nordheim, T., Elder, C., Grundy, W., Bramson, A., Sori, M., Pappalardo, R., Neveu, M., Burr, D. and Ermakov, A., 2020. The science case for spacecraft exploration of the Uranian satellites. https://arxiv.org/pdf/2007.07284.pdf
