Recent microwave observations of Venus give brightness temperatures near 600°K. The spectrum precludes an origin as synchrotron or cyclotron radiation from a Cytherean Van Allen belt; the emission must be thermal and probably arises from just beneath the surface ofVenus. The radiation temperature of an airless planet with Venus' rotation is much less than the period of revolution or if there is appreciable interhemispheric circulation; it is -350°K if the two periods are comparable. It is evident that a surface temperature of 600°K demands a very efficient greenhouse effect. From the radiation balance, the effective absorptivity of the atmosphere, integrated over all wavelengths, is ? 0.995 for nonsynchronous rotation; the equivalent atmosphere is opaque between 1.5 and 40 11. Water is the only molecule which is likely to be abundant on Venus and which absorbs in the region longward of 20 æ. If there is 1 km-atm of carbon dioxide above the effective reflecting layer for the ?8000 bands, the total carbon dioxide abundance in a convective Cytherean atmosphere is 18 km-atm; the total surface pressure is ? 4 atm. Extrapolation to long paths of CO, and H,O emissivities at elevated temperatures shows that ?10 g cm-' of water vapor is required for a nonsynchronously rotating Venus and ?1 g cm-' for a synchronously rotating Venus in order that the required greenhouse effect be achieved. As a check, the method was applied to Earth; the correct terrestrial water-vapor abundance was predicted. The Venus model atmosphere has an ice-crystal cloud layer about 36 km above the surface. For nonsynchronous rotation, the predicted cloud layer is at the thermocouple temperature but at saturation has five times more water vapor above it than observed by Strong; for synchronous rotation, the cloud layer is 14 K° cooler than the thermocouple temperature but has 2 X 10-² cm-' of water vapor above it, as observed by Strong. More precise agreement with the observed thermocouple temperature and water-vapor abundance cannot be expected, because of CO, band emission from warmer regions above the cloud layer, and because of the possibility that ultraviolet photo dissociation reduces the water-vapor abundance below saturation values. If the mean cloud albedo is >0.75, the overcast is <0.90. At gaps in the clouds, there are windows near 8.7 æ and also at many wavelengths in the near infrared and visible regions. The absence of surface liquid water inhibits the establishment of the Urey equilibrium, and, incidentally, greatly reduces tidal friction. If the Earth had been placed in the orbit of Venus soon after its formation, roughly the same atmospheric carbon dioxide content as on Venus would have resulted, but the surface temperature and water vapor abundance would have been much greater; thus, Earth must have been formed with> 10? times more water than was Venus. Because of the high temperature and the absence of liquid water, it is very unlikely that contemporary indigenous organisms exist on the surface of Venus.
