Accurate and Consistent Microwave Observations of Venus and Their Implications

Abstract

We present observations of Venus at six frequencies: 1.385, 1.465, 4.86, 8.44, 14.94, and 22.46 GHz. These were obtained with the Very Large Array and calibrated in a consistent fashion. The brightness temperature of Venus at the six frequencies is derived and compared to a model which includes the emission from the atmosphere, surface, and subsurface of Venus. A single set of model inputs can fit the higher frequency data, where the emission comes mostly from the atmosphere. At the lower frequencies, where emission comes mostly from the surface and subsurface, the model has difficulty reproducing the measurements, and the lowest two frequencies are not reproduced well at all. Improvements in this model over past models include new formalisms for the microwave opacity of SO 2 and H 2SO 4 and incorporation of measured topography and surface characteristics from Magellan. The inputs to the model which best fit the higher frequency data include a relatively warm temperature profile (more consistent with the Mariner V occultation temperature profile than that from the Pioneer–Venus sounders), a disk-averaged abundance of gaseous H 2SO 4 of 1–2.5 ppm at and just below the lower cloud layer, and an average SO 2 abundance of ⋦50 ppm for all altitudes below the cloud layers. This H 2SO 4 abundance is consistent with the values inferred from Magellan occultations, taking into account the differences between equatorial and polar locations. The SO 2 abundance in the lower atmosphere is in agreement with earlier results of P. G. Steffes, M. J. Klein, and J. M. Jenkins (1990, Icarus 84, 83–92), but much lower than the 180 ppm inferred from the Pioneer sounder probe or the 130 ppm inferred from infrared observations. Models with as much as 130 ppm of SO 2 fit the measurements only if the atmosphere is nearly completely devoid of H 2SO 4 and the temperature profile which is much warmer than that measured by the Pioneer–Venus probes is used.