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We present images of Mercury's thermal emission at wavelengths of 0.3, 1.3, 2.0, 3.6, 6.2, 18.0, and 20.5 cm. In addition, images of the linearly polarized component of this emission were obtained from 2.0 to 20.5 cm. Observations were performed with the Hat Creek millimeter interferometer at 0.3 cm and with the Very Large Array at centimeter wavelengths. The images are compared with a thermophysical model that is based on Mariner 10 observations and lunar analogy. A re-analysis of the Mariner 10 Infrared Radiometer data shows that Mercury's regolith, like 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 at least 4 meters. The polarization images reveal an increase in the effective dielectric constant with wavelength and rms surface slopes that range from 15 degrees at 2.0 cm to 10 degrees at 6.2 cm. These trends are caused at least in part by wavelength-dependent scattering at the surface, although the rapid density gradient near the surface may also be a contributing factor. The observed day-night brightness contrast at each wavelength requires a microwave opacity that is a factor of at least 2-3 lower than the opacity of the lunar regolith, which has been determined by laboratory measurements of returned lunar samples and remote microwave observations. This difference is likely due to a lower ilmenite (FeTiO$_3$) abundance in Mercury's regolith, which is expected from visual albedo differences between Mercury and the Moon. Residual images obtained by subtracting the best-fit level-surface models from the data reveal thermal depressions at the poles and along the sunlit side of the morning terminator, which are due to shadowing by surface topography. The strengths and morphologies of the thermal depressions depend on the observing wavelength and on the aspect of the planet.
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