The planet Venus is a textbook case of greenhouse warming run amok. Sunlight filters down through the planet's dense, cloud-choked atmosphere, where the mix of carbon dioxide (CO₂), water, and sulfur compounds in its atmosphere blocks infrared energy from returning to space. As a result, the planet's average surface temperature is a searing 735° Kelvin (865°F) — easily hot enough to melt lead. But things could be worse: Venus's greenhouse has a leaky "window" between 2.1 and 2.7 microns, through which a good deal of infrared energy manages to escape.

Could Venus become even more hellish? Yes, say Mark A. Bullock and David H. Grinspoon (Southwest Research Institute), who yesterday described their analysis to planetary scientists in a meeting in New Orleans. Were the planet's interior to unleash a flood of volcanism, as likely occurred some 600 million years ago, the erupting lava would release even more greenhouse gases into the atmosphere. The two scientists calculate that adding CO₂, carbon monoxide (CO), or sulfur dioxide (SO₂) would have a negligible effect. However, increasing water vapor's abundance to roughly 0.5 percent, about 20 times its current value, would trigger rapid heating.

Fortunately, a natural "thermostat" limits Venus's maximum temperature to about 920°K (1200°F). "It's not possible to get so hot that the surface melts," Bullock says. As the lower atmosphere heats up, he explains, its peak infrared emission shifts to shorter wavelengths. Once it coincides with the window centered at 2.4 microns, the energy escaping to space will equal that coming in.

One interesting consequence of these higher temperatures, Grinspoon notes, is that the massive cloud deck should evaporate. So throughout geologic history the atmosphere may have sporadically been clear enough to reveal Venus's lava-covered surface. Eventually, however, the airmass cools and the clouds return, as the excess SO₂ reacts with surface rocks and the water vapor is gradually destroyed by sunlight.