Microwave Observations and Modeling of a Lunar Eclipse

Abstract

Microwave observations of the Moon during the eclipse of November 28-29, 1993, are reported. Observations were made at 1.327 mm wavelength with a 12-m-aperture radio telescope. Points observed were at the lunar equator and 42° north and south lunar latitude, all near the sub-Earth meridian of lunar longitude. Brightness temperature ( T B) was observed to decrease a maximum of 25% below the pre-eclipse value of T B. Comparisons are made between our observations and previous data sets collected under similar circumstances. A thermal model has been constructed with which we calculate eclipse cooling curves that agree with our observations. From this model inferences are made about the porosity of the lunar regolith necessary to produce the observed changes in brightness temperature throughout the eclipse. The effect of rocks resting on top of the regolith on theoretical eclipse cooling curves is examined. The data are well fit with a regolith-only model that assumes a density of 1.4 g cm -3 (porosity of 59%) for the top 1 cm of regolith. Comparison of lunar high latitude with equatorial observations suggests that regolith density increases with depth within this top 1 cm. Inclusion of rocks in the model increases the amplitude of the eclipse cooling curve for a fixed regolith density, so that a regolith + rocks model fitting the data must have a lower regolith density than a regolith-only model. The regolith model described here to explain observed eclipse cooling curves is shown to be consistent with observed diurnal cooling curves reported by Low and Davidson (1965, Astrophys. J. 142, 1278-1282).