Radar studies of the Moon

Abstract

Accurate range measurements have been reported only by the group 'working at the Nava l Re· search Laboratory. Their most recent value for the mean center-to-center distance betwee n th e Earth and the Moon is 384,400.2 ± 1.1 km and is based upon a value for the Earth's radius of 6.378,170 m which seemed most cons istent with the observed diurnal variation in range.The absolut e cross section of the Moon has been determined over a wide range of wavelengths to a precision in most cases of ± 3 dB. Unfortunately, this uncertainty is too large to permit any definite conclusions to be drawn concerning the wavelength dependence in the cross section. The observations suggest th at the cross section remains cons tant at about 7 percent of the proj ec ted area of the Moon's disk at wavelengths in the range 1 cm to 1 m, and perhaps rises to 10 percent or higher at wavelengths in the range 1 to 10 m.Short-pulse observations ca n be used to exp lore th e angular dependence in the sca tterin g of radi o waves b y the lunar surface. Usefu l meas urem e nts have bee n tnade at wave le ngth s of 1130, 68,23, 10, and 3.6 em. The angu lar dependence has also been investigated at 8.6 mm , th ough here th e angu lar resolution afforded by a narrow pencil-beam a nte nna was e mployed. At all s ix 'wavele ngth s, it appears that part of th e ec ho arises from a hi ghli ght located at th e ce nt e r of the Moon 's visible disk_ A seco nd com ponent comes almost eq ually from th e re maining parts of th e surface. Th e divi sion of power in th e two compone nts changes mark ed ly as th e wavele ngth is red uced. At 68-cm wavele n gth, 80 percent of th e power is return ed fr om the highli ght , but at 8.6 mm only 15 percent ca n be associated with this co mp one nt. The angu lar power spec trum obse rved for th e power fro m th e highli ght also c ha nges with wavelength, indi cating that th e rm s slope of th e surface in c reases as th e wavelength is red uced. Th ese observations have been interpreted as indicat ing that there is a wid e range of structure sizes on the Moon.The deduced valu es of dielec tric co nstant ran ge from k = 2.79 down to k = 2.13. In view of th e greater experime nt al diffi c ulti es togeth e r with the doubtful validity of th e assumptions at 8.6-mm wavelength, this apparent wavelength dependence should be accepted with caution. If real, it may be ca used by the finit e co ndu ctivity of the mat erial (i.e., s "" a in (6)) or by inh omogeneity in th e surface layers -the den sit y pe rhap s in c reas ing sli ghtly with de pth. These val'ues ce rtainly indi ca te that th e surface is broke n or porous in texture -the material occupying perhaps about 30 percent of the available volume. In short, th e echo inte nsity and angular s pec trum is co mparable to that observed from aircraft over terrestrial des erts.

An upper limit for ice in Shackleton crater as revealed by LRO Mini‐RF orbital radar

“…To place an upper constraint on the amount of water‐ice present in the near surface, we used a semi‐empirical model of a silicate‐ice mixture to create synthetic SAR images containing a set percentage of ice, present as decimeter‐sized or larger blocks [ Lichtenberg , 2000], and compared them to the topographically corrected Mini‐RF swaths. The model utilizes a combination of a theoretical model of the coherent backscatter opposition effect [ Mishchenko , 1992] and ground‐based radar data of the Moon [ Evans et al , 1968]; additional details are given in the Text S1 and Figure S1 in the .…”