Using lunar boulders to distinguish primary from distant secondary impact craters

Abstract

A high-resolution study of 18 lunar craters, including both primary and distant secondary craters, shows that the secondary craters produce larger ejecta fragments at a given crater size than do the primary craters. The maximum boulder diameter (B) increases with crater size (D) according to the power law B = KD2/3; for primary craters, when B and D are in meters, K is 0.29, whereas for secondary craters, we find that K is 0.46 (60% larger). Next we show that impact fracture theory predicts that secondary craters, because of their lower impact velocity, will produce larger ejecta fragments than primary craters. This result provides an opportunity for distinguishing between primary and secondary craters in high resolution planetary images. The ability to identify distant secondary craters will help constrain primary production rates of small craters and improve surface age determination of small areas based on small crater counts. Copyright 2007 by the American Geophysical Union.