VENUS SURFACE RADIOTHERMAL EMISSION AS OBSERVED BY MAGELLAN

The Magellan radiometer experiment has observed the 12.6 cm-wavelength radio emissivity of more than 91% of the Venus surface during the first 8 months of its operation. The global mean value of emissivity seen using horizontal linear polarization is 0.845, a value that corresponds to a dielectric permittivity of between 4.0 and 4.5, depending on the surface roughness. These values are consistent with the dry basaltic minerals thought to compose the bulk of the Venus surface. The 2-degrees beam width of the Magellan high-gain antenna yields surface resolutions that vary from 15 by 23 km at periapsis (10-degrees-N latitude) to about 85 km at the north pole; measurements of emissivity carry an absolute error of about 0.02 and can distinguish local variations as small as 0.005. The observations have confirmed earlier findings that a few regions on Venus, primarily located at high elevations, possess unexpectedly low values of radiothermal emissivity, occasionally reaching as low as 0.3. Some members of each of five classes of features are found to display anomalously low values of emissivity: highlands, volcanoes, novae, ridges, and impact craters. Two possible explanations for these low values have been advanced; (1) emission from a highly reflective single interface between the atmosphere and a surface material having a bulk dielectric permittivity of order 80; or (2) emission from the surface of a low-loss medium having a more usual permittivity (of order 5) but which contains many voids permitting efficient internal multiple scattering. Distinguishing between these hypotheses is difficult with the data presently in hand.