Results and Interpretation of the S-Band Occultation Experiments on Mars and Venus

Abstract

The S-band occultation experiments have provided important observational data on the structure of the atmospheres of Mars and Venus. The results for Mars show a very cold region in the middle atmosphere where CO₂, the main constituent of the Martian atmosphere, will saturate and may condense. Also, at the surface boundary of Mars there appears to be a temperature discontinuity between the air and ground in the local afternoon and a temperature inversion in the atmosphere in the nightside of the planet. On Venus, the Mariner 5 results show that the diurnal variation of temperature is small not only in the lower atmosphere but also in the stratosphere, implying strong zonal mixing at those levels. Also, the amplitude data from the Mariner 5 experiment has provided evidence for the existence of cloud layer or layers in the lower atmosphere where the ambient temperatures range between 350 and 450K. Such high temperatures preclude water as the constituent of these clouds. As for the upper atmospheres of both Mars and Venus, the occultation experiments indicate that the ionospheres of these planets contain up to a factor of two more electrons than can be explained in terms of the presently accepted values of the EUV flux. At the same time the exospheric temperature of Mars appears to be as low as 350–450K, about 100K lower than the closest predicted value.