The Surface Temperature of Venus.

Recent microwave observations of Venus give brightness temperatures near 600 K. The spectrum precludes an origin as radiation from a cytherean Van Allen belt; the emission must be thermal, and must arise from just beneath the surface of Venus. The radiation temperature of an airless planet with Venus' color-corrected albedo and solar distance is 250 K if the period of 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, and if there is no interhemispheric circulation. It is clear 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 wavelengths, is a =0.995 for rapid rotation; an equivalent atmosphere is one opaque between 1.5 and 40 j . The only molecule which is likely to be abundant on Venus and which absorbs in the region longward of 20 ~ is water. The carbon dioxide abundance in a convective cytherean atmosphere is 18 km-atm; the total surface pressure is 4 atm. Extrapolation to long paths of CO2 and H2O emissivities at elevated temperatures shows that 10 g cm-2 of water vapor is required for a rapidly rotating Venus, and 1 g cm-2 for a slowly rotating Venus, in order that the required greenhouse effect be achieved. As a check, the method w~s applied to the 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 rapid rotation, the predicted cloud layer is at the bolometric temperature, but has five times more water vapor above it than observed by Strong; for slow rotation, the cloud layer is 14 K0 cooler than the bolometric temperature, but has 2 X 10-~ g cm-2 above it, as observed by Strong. If the mean cloud albedo is >0#75, the overcast is <0.90. At gaps in the clouds, there are windows near 8.7 ~, and also at many wavelengths in the near infrared and visible. 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 in primitive times, 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, the earth must have been formed with > 10~ times more water than was Venus. This work was supported in part by the California Institute of Technology Jet Propulsion Laboratory, National Aeronautics and Space Administration, and in part by the National Science Foundation.