Clouds and Atmosphere of Venus
The atmospheric pressure at the surface of Venus is about 90 times greater than that of the air in your room. Analysis of the lower atmosphere suggests that it is about 96 percent carbon dioxide and about 3.5 percent nitrogen, with the remainder water vapor and some others. Because of the high surface temperature (about 730 K), the carbon dioxide was apparently not depleted, as it was on the earth, by reacting with the primitive rocks to form carbonates and limestones and by absorption by water. Above 150 kilometers atomic oxygen is the most abundant species. And finally a huge cloud of hydrogen surrounds the planet far above the atmosphere.
More than almost any other aspect of Venus, the mysterious clouds that perpetually obscure the surface have been the subject of extensive speculation. In 1973 it was suggested that the clouds are composed of sulfuric acid droplets. The clouds begin around 46 kilometers above the planet's surface and seem to be confined to a fairly distinct layer, rising up to about 70 kilometers. Thin-haze regions lie above and below the cloud layer; the lower one has a surprisingly abrupt cutoff some 32 kilometers above the surface.
From the Pioneer Venus results we think that the clouds are indeed composed of sulfuric acid droplets and other particles, possibly free sulfur, so thick that during the probes' descent they appeared to be passing through a blizzard. Early analysis of the data also suggests that possibly several sulfur compounds also exist in the atmosphere. From the bottom of the haze down to the su rface the atmosphere appears to be surprisingly clear.
If one assumes that Venus formed with about the same relative amount of water as the earth did, then the challenging question is what happened to it since it is not on the surface in pools nor in the atmosphere. Most likely it stayed in the vapor form because of the high temperature. Incoming ultraviolet photons from the sun could dissociate the molecule, with free hydrogen then able to escape over the planet's lifetime. Some water is also consumed in making the sulfuric acid droplets in the clouds, and a tiny amount of water vapor is still present in the atmosphere.
Why did this same escape of water not occu r on the earth since its early atmosphere was probably similar in composition to that of Venus? It is probable that the advent of life and photosynthesis on the earth began to replace carbon dioxide with oxygen and prevent a substantial greenhouse effect; thus the water on the earth stayed primarily in pools on the surface. Moreover Venus receives about twice as much radiant energy from the sun as does the earth.
The atmospheric circulation is the same in both hemispheres. A vigorous, equatorial, east-west jet stream is quite evident in the upper atmosphere, moving around the planet in only 4 days, opposite to the direction of the planet's slow spin. The wind velocity decreases at lower altitudes until at the surface it slows to a gentle breeze. The lower atmosphere apparently circulates because of differences in solar heating between the equatorial and polar regions. Clouds rise near the equator, spiral toward the poles, and descend in what appears to be an almost continuous flow. But why such high winds reverse direction in the upper atmosphere we do not know.
