OBSERVATIONS OF MARS, URANUS, NEPTUNE, IO, EUROPA, GANYMEDE, AND CALLISTO AT A WAVELENGTH OF 2.66 MM

Brightness temperature observations at a wavelength of 2.66 mm of Uranus, Neptune, and the Galilean Satellites calibrated against Mars are presented. A detailed procedure is reported for using an accurate thermal model of Mars as a calibrator which yields brightness temperatures for Uranus and Neptune of 122 ± 5 K and 114 ± 7 K in excellent agreement with published "absolutely calibrated" measurements of these bodies. The results for the satellites are: Io, 101 ± 9 K, Europa, 102 ± 10 K, Ganymede, 78 ± 6 and 83 ± 5 K, and Callisto, 105 ± 6 K. The measurements for Io, Europa, and Callisto are consistent with thermal models of the lunar type but Ganymede is strongly anomalous with its 2.7-mm brightness temperature remaining very low and equal to those at centimeter wavelengths. It appears that for Ganymede alone, the millimeter surface emissivity is very low as it is at centimeter wavelengths for all the satellites. Apparently, the near surface of Ganymede is different from that of the other satellites in some unknown way to cause this phenomenon, possibly being more scattering at short wavelengths. © 1991.