Characterization of Boulders Ejected from Small Impact Craters

When an asteroid or comet impacts the surface of a solid body, some of the surface material is often ejected from the crater in the form of blocks. We are characterizing the size and location of such blocks around craters on the Moon and Mars. The lunar craters were observed in Lunar Orbiter III images from P-12 and S-18. The Mars crater was observed in Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) Release No. MOC2-712. The craters range in size from 300 m to 3 km diameter. We measured the diameters of boulders observed around the craters, and also measured the distance between the boulder and the crater center. We then calculate the ejection velocity of each boulder based on how far the block was from the crater. The data indicate that larger boulders are more frequently found close to the crater rim rather than far away. The size of the ejecta drops off as a power law with distance from the crater. Our results are consistent with studies by Vickery (1986, 1987), which indirectly found the distribution of ejecta sizes from large craters by analyzing the size and distribution of their secondary craters. Our work characterizes the other end of the ejecta spectrum --- low velocity boulders ejected from small craters. We have also constructed R-plots of the boulder diameters for each crater. We found that the R-plot for the boulders has a dependence remarkably similar to an R-plot of the diameters of secondary craters. This similarity supports the already accepted idea that the impactors that produce secondaries are blocks ejected from larger craters. It is also consistent with the interpretation that the upturn of the cratering curve at small diameters on the terrestrial planets is due to secondary impacts rather than a primary population as some have proposed.