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Impact Ejecta Fragment Size-frequency Distributions

Presentation #119.03 in the session Moon & Earth (Poster + Lightning Talk)

Published onOct 23, 2023
Impact Ejecta Fragment Size-frequency Distributions

We have collected empirical data on ejecta fragment sizes formed through dynamic fragmentation of a target surface during the passage of a shock wave after an impact. On a planetary surface, the shock wave both creates fragments and ejects them at a given velocity. Fragments ejected during the main excavation flow create the majority of the secondary craters seen around craters and can redistribute material across the body and beyond. Secondary crater sizes and distances from the primary record the ejecta fragment sizes and ejection velocities. We mapped secondary crater fields around primary craters on Mercury, the Moon, Mars, and icy satellites.

In addition to our previous work addressing the size-velocity distribution of ejecta fragments (Singer et al. 2020, JGRE 125, e2019JE006313), this new work looks at their intrinsic size-frequency distributions. Although we cannot map every secondary crater, and thus cannot obtain estimated sizes for every fragment produced, we believe we are capturing more of the larger secondaries than the smaller ones. Thus, the slope at the large sizes is a closer approximation for the slope of the fragments and likely represents a minimum slope. For the Moon we find differential power-law exponents/slopes of -6 to -7 for the large secondary craters around primary craters ranging in diameter from 1.8-to-170 km. The fragment differential size-frequency slope would be slightly shallower when scaled from the secondary craters, depending on the assumed target material properties, or approximately -5 to -6.5 (or equivalently, cumulative slopes of -4 to -5.5). These are similar to the slopes found for secondary craters on Europa and Ganymede (Singer et al. 2013, Icarus 226, 865).

We will also present on the size-frequency distributions for secondary craters on Mercury and Mars, and any trends or changes with distance from the primary crater, primary crater size, or target material type. The data from this study can be compared to results from laboratory experiments or numerical models, and used as inputs to models of redistribution of material, dynamic fragmentation (e.g., Grady-Kipp), and ejection of material off a body.

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