MICROWAVE IMAGING OF MERCURYS THERMAL EMISSION AT WAVELENGTHS FROM 0.3-CM TO 20.5-CM

We present images of Mercury's thermal emission that were obtained wit h the BIMA millimeter interferometer at a wavelength of 0.3 cm and wit h the VLA (Very Large Array) at wavelengths from 1.3 to 20.5 cm. These images are analyzed with detailed thermophysical and radiative transf er models that are based in part on a lunar analogy. We constrain the thermophysical model with Mariner 10 infrared measurements of Mercury' s night-side surface temperature and show that Mercury's regolith, lik e that of the Moon, consists of a thermally insulating surface layer, with a thickness of a few centimeters, atop a highly compacted region that extends to a depth of several meters. The radiative transfer mode l is constrained in part by linear polarization images that we obtaine d with the VLA at wavelengths from 2.0 to 20.5 cm. These images reveal wavelength-dependent scattering at the surface boundary and rms surfa ce slopes that range from 15 degrees at lambda 2.0 cm to 10 degrees at lambda 6.2 cm. We develop a method for constraining the microwave opa city at each wavelength by modeling diurnal brightness variations over the resolved disk and find that Mercury's regolith is at least two to three times more transparent than the lunar maria and at least 40% mo re transparent than the lunar highlands. This difference is likely due to lower Fe and Ti abundances in Mercury's regolith, which is consist ent with Mercury's high visual albedo and suggests that most of Mercur y's surface is an extreme example of the lunar highlands. (C) 1994 Aca demic Press, Inc.