Boulder Distribution Analysis of Impact Craters in the Tharsis Region and Elysium Planitia, Mars
Semester of Graduation
Geology and Geophysics
Crater counting is an accepted method for dating the surface of a planetary body. An older landscape will have a higher areal density of craters due to exposure of impacts occurring over a longer time period. Since each initial impact only forms one primary crater, secondary craters need to be excluded in the crater counting method to appropriately catalog only distinct impact events. However, small primary craters may be difficult to distinguish from secondary craters. Understanding physical features of impact craters, such as the ejecta blanket, would improve the planetary community’s ability to differentiate between the two types of craters. Boulder distribution analysis of individually measured boulders in an ejecta blanket includes the comparison of a boulder’s area to its distance from the crater, the comparison of a boulder’s axial ratio to its distance from the crater, and the cumulative size frequency distribution. Krishna and Kumar (2016) applied boulder distribution analysis to Censorinus Crater on the Moon. Through cumulative size frequency distribution and power-law fit to calculate the largest b-value, they determined the impact direction. On Mars, boulder distribution analysis has yet to be applied to impact craters observed in high resolution images. This study investigated two Martian impact craters using boulder distribution and improved the technique of previous boulder distribution studies by incorporating error propagation. These craters were: 1. a speculated primary crater located in the Tharsis region that was approximately 175 m in diameter (Banks, 2008), and 2. a speculated secondary crater located in Elysium Planitia that was approximately 320 m in diameter (Hart & Gulick, 2010; McEwen et al., 2005). Variations and similarities between the two types of craters on the Martian surface could provide insight about the typical boulder distribution for each crater type and the impact direction. For a secondary crater, the impact direction may indicate its associated primary crater. At these two Martian impact craters, the power-law fit to the cumulative size distribution frequency, boulder population density, and presence of a forbidden zone, defined as a region where ejecta is absent or nearly absent for an oblique impact, did not conclusively indicate the impact direction of either crater. While this study serves as a starting point for understanding the boulder distribution patterns around small Martian primary and secondary craters, it also motivates future work to determine whether the impact direction is indeterminable for small craters and to establish a database of Martian primary and secondary craters for which boulder distribution was applied.
Button, Nicole, "Boulder Distribution Analysis of Impact Craters in the Tharsis Region and Elysium Planitia, Mars" (2017). LSU Master's Theses. 4379.