Lunar Impact ejecta: The View from Radar and Thermal Infrared Observations

Abstract:

The lunar regolith is derived from impact ejecta, and so understanding ejecta characteristics is a key element of regolith studies. Here we report on the physical properties of impact ejecta from Earth-based and orbital radar (Arecibo / Green Bank telescopes at 12.6- and 70-cm wavelengths, and the Lunar Reconnaissance Orbiter Mini-RF) and LRO Diviner thermal infrared observations. Diviner thermal IR data provide estimates of surface rock abundance, and the radar datasets reveal rocks buried beneath up to several meters of regolith. These four datasets represent a spectrum of observational wavelengths, and their intercomparison provides a means of constraining both ejecta block content and depth extent. We examine all craters on the lunar nearside that show evidence for blocky ejecta, whether buried or on the surface, in order to investigate spatial, crater size-, and time-dependent variations in ejecta block size distribution and spatial extent. Previous work has shown that it is possible to quantify the relationship between surface ejecta rock content and crater age for craters younger than ~1 Gyr; we will now present the results of our effort to extend the age range over which this relationship can be applied by investigating buried ejecta as well. Further, we report on the thermophysical characteristics of distal fine-grained ejecta that have previously been shown to exhibit unique radar characteristics (low radar return and circular polarization ratio). Our results constrain the physical properties of ejecta, and variations in those properties with time and depth, to an extent not possible from examination of visible-wavelength images and multispectral data alone.