Comparative studies of lunar, Martian, and Mercurian craters and plains

The amount of smooth plains material in craters surrounding the Caloris basin on Mercury depends on the extent these craters have been eroded by Caloris ejecta. Therefore smooth plains surrounding Caloris must have been emplaced at least in part by saturated secondary cratering. Mercurian uplands outside the continuous belt of smooth plains deposits have a crater population which is deficient in craters smaller than 50 km relative to extrapolations of craters larger than 50 km and relative to size frequency distributions of craters ≤50 km on typical lunar upland regions. However, the typical lunar upland regions have been masked by the addition of numerous large basin secondary craters. Only rare areas southwest of Mare Serenitatis appear to be similar to Mercurian terrain at great distances from smooth plains because the areas are relatively free of basin secondaries. Martian uplands also exhibit a crater population which is deficient in craters less than 30–50 km, which was previously interpreted to have been caused by obliteration of some craters less than 30 km by surface processes. The observed crater deficiencies on the moon, Mars, and Mercury below 30–50 km are mostly a reflection of the primary crater production population; it is characterized by a power function having two different exponents (−2.0 for craters less than 50 km and −3.2 to 3.5 for craters ≥50 km). The hypothesis that the observed deficiency of small lunar craters southwest of Mare Serenitatis resulted from an episode of premare volcanism which obliterated some small craters was evaluated and rejected because highland volcanic rocks are rare in returned samples, and if significant premare upland basaltic volcanism is hypothesized, it would be circumstantial that it occurred only in those areas least affected by basin ejecta. Premare upland volcanic activity would be required also to explain the Mercurian crater counts. However, the Mercurian mapping results indicate that intercrater plains representative of hypothetical volcanic rocks were formed before craters now observed on their surfaces. The most persuasive reason for rejecting the theory of lunar premare volcanism is that reexamination of all available crater count data for lunar uplands shows no evidence for a primary production population characterized by a single power function. Therefore crater counts showing deficiencies relative to a single power function which previously was acceptable evidence for crater obliteration need no longer be considered as evidence for crater obliteration.