Microwave Remote Sensing of the Temperature and Distribution of Sulfur Compounds in the Lower Atmosphere of Venus

Abstract

A multi-wavelength radio frequency observation of Venus was performed on April 5, 1996, with the Very Large Array to investigate potential variations in the vertical and horizontal distribution of temperature and the sulfur compounds sulfur dioxide (SO 2) and sulfuric acid vapor (H 2SO 4(g)) in the atmosphere of the planet. Brightness temperature maps were produced which feature significantly darkened polar regions compared to the brighter low-latitude regions at both observed frequencies. This is the first time such polar features have been seen unambiguously in radio wavelength observations of Venus. The limb-darkening displayed in the maps helps to constrain the vertical profile of H 2SO 4(g), temperature, and to some degree SO 2. The maps were interpreted by applying a retrieval algorithm to produce vertical profiles of temperature and abundance of H 2SO 4(g) given an assumed sub-cloud abundance of SO 2. The results indicate a substantially higher abundance of H 2SO 4(g) at high latitudes (above 45°) than in the low-latitude regions. The retrieved temperature profiles are up to 25 K warmer than the profile obtained by the Pioneer Venus sounder probe at altitudes below 40 km (depending on location and assumed SO 2 abundance). For 150 ppm of SO 2, it is more consistent with the temperature profile obtained by Mariner 5, extrapolated to the surface via a dry adiabat. The profiles obtained for H 2SO 4(g) at high latitudes are consistent with those derived from the Magellan radio occultation experiments, peaking at around 8 ppm at an altitude of 46 km and decaying rapidly away from that altitude. At low latitudes, no significant H 2SO 4(g) is observed, regardless of the assumed SO 2 content. This is well below that measured by Mariner 10 (Lipa and Tyler 1979, Icarus 39, 192–208), which peaked at ∼14 ppm near 47 km. Our results favor ≤100 ppm of SO 2 at low latitudes and ≤50 ppm in polar regions. The low-latitude value is statistically consistent with the results of Bézard et al. (1983, Geophs. Res. Lett. 20, 1587–1590), who found that a sub-cloud SO 2 abundance of 130±40 ppm best matched their observations in the near-IR. The retrieved temperature profile and higher abundance of H 2SO 4(g) in polar regions are consistent with a strong equatorial-to-polar, cloud-level flow due to a Hadley cell in the atmosphere of Venus.