Venus has been visited by a number of spacecraft. Those most important in advancing our understanding of the atmosphere include Mariner 10 (1974), Pioneer Venus (1978- 1992), and the series of Venera probes by the former USSR (1982-1986). The Pioneer Venus orbiter conducted more than a decade of monitoring of the upper atmosphere of Venus. The spacecraft data are supplemented by telescopic and satellite (in Earth orbit) observations. The astronomical data for Venus are summarized in table 8.1. Venus is a close but slightly smaller sibling of Earth. The radius is 6051 km, as compared to Earth's 6371 km. The mass is 4.87 x 1024 kg, a little less than Earth's 5.98 x 1024 kg. The gravity of Venus is 8.87 ms-2, as compared to Earth's 9.82 ms-2. The dynamical and orbital parameters of Venus are very different from those of Earth. The rotation of Venus is retrograde, with a period of 225 days. The planet has little obliquity, and its orbit is close to being circular. Thus there is little seasonal variation in insolation over a Cytherian year. Perhaps the greatest surprise about Venus is its dense, dry, and hot atmosphere of 92 bar (see figure 8.1). This is all the more surprising because the planet is completely covered by thick clouds. Figure 8.2 shows the altitude profiles of three modes of cloud particles in the middle atmosphere. The high albedo of Venus implies that the planet receives less energy from the sun than Earth despite its closer proximity to the sun. However, the surface of Venus is hot, with a temperature of 733 K, attributed to the greenhouse effect. The bulk of our knowledge of the atmosphere of Venus is derived from observations in the middle atmosphere (60-100 km altitude). At cloud-top levels (65 km), groundbased ultraviolet (UV) observations revealed a 4-5 day period, east-to-west circulation that is 60 times faster than the solid surface. The mechanism for generating this superrotation is not well understood. The net result of this rotation is that it gives the upper atmosphere an effective diurnal cycle of 4-5 days.
Airglow remote sensing of the seasonal variation of the Martian upper atmosphere: MAVEN limb observations and model comparison