Primary, secondary and tertiary microcrater populations on lunar rocks: Effects of hypervelocity impact microejecta on primary populations.

Since return of the first Apollo lunar samples, the definition of the primary cosmic dust impact microcrater distribution on lunar regolith materials has been a prime research ob'ective. Well documented model crater populations have been measured on 60015 tl] and more recently on 12054 [2] representing an extensive and carefully executed observational achievement. We now ask how valid these models are; in particular how are such observed models related to the true primary crater distribution. A newly completed computer program presents calculations of secondary and tertiary microcrater numbers expected to be formed by each of these two model microcrater distributions. Basis for the work is experimentally measured secondary hyperveZocity impact craters formed from laboratory impacts on lunar rock. Previous laboratory measurements have been made of submicron impact spallation ejecta [31 and secondary hypervelocity microcraters 141 produced near impacts of 4-6 lans- iron microspheres on lunar crystalline rock (62235). It has been suggested that accreta and hypervelocity microdebris could be more significant in the development of lunar microfeatures