HR: 0800h
AN: V11A-0368    [Abstracts]
TI: Quantitative Analysis of Circular Symmetry of Venus Coronae and Craters
AU: * Stoddard, P R
EM: prs@geol.niu.edu
AF: Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115-2854, United States
AU: Jurdy, D M
EM: donna@earth.northwestern.edu
AF: Dept. of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208-2150, United States
AB: The origin of craters has long been debated: Exogenic or endogenic? Impact or volcanic? While for the craters of the Earth and Moon the issue has been largely resolved, it has flared anew in recent papers by Hamilton (2005, 2007), Vita-Finzi et al. (2005), and Jurdy and Stoddard (2005, 2007). We weigh in with a quantitative technique to differentiate between these possible mechanisms. Craters by their nature are circular. They are excavated by a roughly hemispherical shock wave, and thus almost regardless of impact angle, will be round rim-and-basin structures (Melosh, 1989). Although underlying structural features, such as faults, and later tectonic deformation can affect crater shape we suggest that the strongest test of an impact origin for coronae is the circularity of these features. Here we introduce an approach for the assessment of a feature's circular symmetry. Using altimetry data we compare, by cross-correlation, multiple profiles across a single feature. Jurdy and Stoddard (2005) provided an example in which Mead crater and two coronae were analyzed. They found that for each corona, profiles cross- correlated at only 25-30% of perfect cross-correlation. Profiles for Mead crater, however, correlated at a much higher level, 80%. Here, we perform an expanded study of features generally classified as craters, and others whose classification as coronae has been questioned by Hamilton (2007). We choose only the largest craters, since altimetry data are too coarse to allow enough data points for analyses of smaller features, and also because they are of similar size to the coronae in our study. For each feature, 36 profiles are extracted from the altimetry data, de-sloped, and averaged together. For each feature, the individual profiles are correlated against the average, and the correlations themselves were averaged to give an assessment of circular symmetry. Results indicate accepted craters have the highest correlation averages (are most circular) and suggest that a few coronae may actually be of impact origin. We propose that this type of correlation analysis can be used in an objective assessment of circularity, and therefore the origin, of the remaining catalogue of similar features.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 6295 Venus
DE: 8440 Calderas
DE: 8450 Planetary volcanism (5480, 6063, 8148)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting