Regolith Layer Thickness Mapping of the Moon by Radar and Optical Data

Abstract

Mapping of regolith layer thickness for the lunar nearside is carried out with Arecibo radar data at 70 cm wavelength (T. W. Thompson 1987, Earth Moon Planets 37, 59–70) and distributions of iron and titanium content derived from Earth-based optical data (Yu. Shkuratov et al. 1999, Icarus 137, 222–234). For the mapping a new simple model of radio wave multiple scattering in the regolith layer was used. A comparison of this map with independent estimates of regolith layer thickness for landing sites and other areas of the lunar surface showed a good correlation. It was found that regional variations of thickness are almost the same for maria and highlands, though the average thicknesses are different (5 and 12 m, respectively). A relatively thin regolith layer (4 m) covers a portion of Mare Serenitatis, Mare Tranquillitatis, and Mare Humorum, while the thickest regolith layer occurs in Mare Nectaris (9 m). A thin regolith layer is a characteristic of the cryptomare Schiller-Schickard. Very small regolith thickness corresponds to the crater floors. In this case, the model cannot yield reliable estimates of thickness; it predicts only a characteristic distance between rock inclusions buried in regolith. A thick highland regolith occurs at the southeast portion of the lunar disk and the highland to the north of Mare Imbrium and Sinus Iridium. For the lunar nearside, the regolith thickness generally correlates with the surface age: the greater the age, the thicker the regolith. The results are consistent with a higher rate of regolith growth for times earlier than 3.5 byr ago, when meteorite flux was much higher.