Abstract
We have developed a quantitative model for predicting characteristics
of ejecta deposits that result from basin-sized cratering events.
This model is based on impact crater scaling equations (Housen, Schmitt,
and Holsapple 1983; Holsapple 1993) and the concept of ballistic
sedimentation (Oberbeck 1975), and takes into account the size distribution
of the individual fragments ejected from the primary crater. Using
the model, we can estimate, for an area centered at the chosen location
of interest, the average distribution of thicknesses of basin ejecta
deposits within the area and the fraction of primary ejecta. contained
within the deposits. Model estimates of ejecta deposit thicknesses
are calibrated using those of the Orientale Basin (Moore, Hodges,
and Scott 1974) and of the Ries Basin (Horz, Ostertag, and Rainey
1983). Observed densities of secondary craters surrounding the Imbrium
and Orientale Basins are much lower than the modeled densities. Similarly,
crater counts for part of the northern half of the Copernicus secondary
cratering field are much lower than the model predicts, and variation
in crater densities with distance from Copernicus is less than expected.
These results suggest that mutual obliteration erases essentially
all secondary craters associated with the debris surge that arises
from the impacting primary fragments during ballistic sedimentation;
if so, a process other than ballistic sedimentation is needed to
produce observable secondary craters. Regardless, our ejecta deposit
model can be useful for suggesting provenances of sampled lunar materials,
providing information complementary to photogeological and remote
sensing interpretations, and as a tool for planning rover traverses.
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