Conditions there make Dante's Inferno positively appealing. Venus' surface is completely covered by a thick cloud layer made of dirty yellow sulfuric acid. The clouds reflect much of the sun's incident light back into space, which led some to argue that its surface might be cooler than expected, but this neglects the greenhouse effect.
Venus' surface temperature is 880 degrees Fahrenheit, hot enough to melt lead. Its atmosphere is mostly CO2, so thick that its surface pressure is 90 atmospheres, about the same as the pressure on Earth - under 3000 feet of water. No life as we know it can survive there.
Both Earth and Venus were hot 4.5 billion years ago. As they cooled, each accumulated an atmosphere consisting mostly of water vapor and CO2, released by extensive volcanic outgassing. The sun was only 70 percent as bright as it is now and water vapor in the planets' atmospheres condensed and fell as rain, covering them with oceans.
In Earth's case, its then thick CO2 atmosphere caused a strong enough greenhouse effect for it to retain liquid oceans. CO2, washed out of its atmosphere by rainfall, dissolved in the oceans. The mildly acidic water converted some silicate rock in Earth's crust into carbonate rock, effectively storing up the CO2 washed from the atmosphere.
This slow CO2 removal process reduced Earth's greenhouse effect at about the same rate that the sun was slowly heating up (as all stars do as they age). This balance kept Earth's surface temperature fairly constant and its oceans stayed liquid.
Venus wasn't so fortunate. About 30 percent closer to the sun than Earth, it suffered more solar heating, making it a much warmer and steamier place. It didn't rain as much and its infant oceans were shallower, severely compromising its ability to absorb the CO2 belching into its atmosphere from its volcanoes.
Thus, Venus' greenhouse effect remained stronger than Earth's. This extra heat warmed its already warm surface even more, which evaporated more surface water, which released more dissolved CO2 into its atmosphere, which increased its greenhouse effect, etc. So, Venus suffered a "runaway greenhouse effect," which caused its atmospheric CO2 and water vapor concentration - and associated surface temperature - to spiral wildly upward, out of control.
Its water vapor rose higher in its atmosphere than its CO2 and it was broken apart by ultraviolet light. The freed-up hydrogen atoms escaped into space and Venus slowly lost all its water, leaving behind a dry, thick, CO2-rich atmosphere covering a hellishly hot surface.
For the past 4 billion years, Earth has avoided succumbing to such a scenario because of its: (1) active plate tectonics that regulate its atmospheric CO2 concentration, (2) strong magnetic field that shields its atmosphere from the solar wind and (3) its ozone layer that shields its water from the ravages of ultraviolet light.
These protective mechanisms will eventually fail. The sun will continue to heat up, ultimately triggering a runaway greenhouse effect on Earth about a billion years from now. The real question is . . . can humans pull the trigger prematurely by continuing to inject CO2 into the atmosphere at an unprecedented rate?
If we do pass some "tipping point," we will render our planet completely uninhabitable, for any type of life as we know it. One way or another, though, Venus is our next stop.
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* GEORGE L. CASSIDAY teaches in the physics department at the University of Utah.
