Results—planets and Comets

FIGURE 7 (Left) Diurnal path of the Sun about Mercury and (right) representative brightness temperature curves near 4-cm wavelengths for hot, warm, and cold regions on Mercury. 

Venus has the densest atmosphere of all the terrestrial planets. The principal atmospheric constituents are carbon dioxide and nitrogen (N2) their mixing ratios are approximately 96.5% and 3.5%, respectively, below, 100-km altitude. Trace constituents below 100-km altitude are in the range of 0.1%. The Venus atmosphere is covered with thick clouds composed primarily of sulfuric acid and contaminants, making the surface invisible from above. The total pressure at the bottom of the cloud layer ( 47 km) is approximately 1.3 atm. Water is highly depleted throughout the atmosphere. The mean physical structure of the atmosphere (pressure and temperature profile) is reasonably well known from the data returned by a number of space probes. The surface pressure and temperature (on a mean surface) are approximately 94 atm and 737 K, respectively. 

Carbon monoxide is an important constituent of the upper atmosphere of Venus. It is formed primarily by the dissociation of carbon dioxide by solar ultraviolet radiation and is removed by chemical and transport processes in the atmosphere. The ground and first excited rotational 

FIGURE 9 Schematic representation of the spectrum of Venus from 1 mm to 6 cm (right) and temperature versus aititude profiie (ieft). The figure iiiustrates how the atmosphere is probed in altitude by changing the wavelength of the observations. 

It has been possible to derive the vertical temperature and mixing ratio profiles of CO in the upper atmosphere of Venus from such observations. As shown in Fig. 9, the spectra reveal considerable variability of CO abundance, which varies with solar phase angle. The variability is believed to be the result of large-scale circulation in the upper atmosphere of the planet. Detailed studies of this variability have been undertaken with arrays of telescopes which operate at millimeter wavelengths. With such arrays the planet can be imaged in the CO lines, so the CO concentration and temperature profile at all solar phase an- 

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Instrumentation for Planetary Radio Astronomy Results—Planets and Comets Prospects for the Future... 

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