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Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail-Stop 300-227, Pasadena, California 91109-8099, USA
² National Air and Space Museum, Center for Earth and Planetary Studies, Smithsonian Institution, MRC 315, P.O. Box 37012, Washington, D.C. 20013-7012, USA
³ University of Toronto, Department of Geology, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
⁴ Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i, 1680 East-West Road, Honolulu, Hawaii 96822, USA

Correspondence: *E-mail: Thomas.W.Thompson{at}jpl.nasa.gov.

The southern highlands of the Moon comprise superposed ejecta layers, individually as thick as a few kilometers, from the major basins. Smaller (1–16-km-diameter) impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks. We observe a significant difference in the population of radar-bright craters, 1–16 km and larger in diameter, between regions of the southeastern near-side highlands north and south of ~lat 48°S. There are about one-third more radar-bright craters north of this line than to the south, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian–Imbrian age to the north. The radar-bright crater population is consistent with a megaregolith thickness of ~1.5 km in the north and ~2.5 km in the south, a difference we attribute to South Pole–Aitken basin ejecta.