Gasbags of Venus

After four-month journeys through space, Russia's Venus 4 and the U.S. Mariner 5 spacecraft will both reach Venus this week. No matter what the space probes find, most scientists have already written off the possibility that Venusian life exists; the planet's apparent surface temperature is approximately 800DEG F., above the melting point of lead.

Just the same, say Astronomer and Exobiologist* Carl Sagan and Biophysicist Harold Morowitz, it is conceivable that earth's nearest planetary neighbor could be home to living organisms. In balloonlike form, Venusian life could float in the dense atmosphere, never approaching the searing surface.

In the Clouds. Of the planetary environments investigated so far by telescope and space probe, the scientists write in Nature, conditions in the atmosphere of Venus resemble those on earth more than anywhere else. In the lower Venusian clouds, they say, there is carbon dioxide, water and sunshine--prerequisites for photosynthesis. The temperatures are chilly, but above freezing. If small amounts of minerals were stirred up to the clouds from Venus' surface, the scientists believe that an indigenous biology--based entirely on biochemical principles observed on earth--could exist.

Because the organisms would encounter severe cold if they drifted farther up in the clouds, or extreme heat if they descended too far toward the surface, Morowitz and Sagan speculate that they must be regulated to hover at an essentially fixed altitude. Thus, the organisms could well take the form of a gasbag or float bladder containing hydrogen gas--which the organism itself could produce by decomposing water.

Greenhouse Effect. Depending on the thickness of the membrane, they calculate, the organisms could range from the size of a pingpong ball to more complex and thicker-skinned gas spheres many times larger. Despite their internal hydrogen, Sagan jokes scientifically, there would be little danger of miniature Hindenburg disasters; there is little or no free oxygen in the Venusian atmosphere to support an explosion of hydrogen.

To critics who point out that it would be difficult for life to arise spontaneously in the atmosphere, Morowitz and Sagan have a ready answer: it did not. Instead, they postulate, ancient Venus had a much thinner atmosphere; its surface, now superheated by the greenhouse effect of a thick carbon-dioxide-filled atmosphere, was once cool enough to spawn life. As more gas was spewed into the atmosphere by volcanic action, however, the surface temperatures gradually became unbearable and could have driven the more buoyant organisms into the clouds, where they evolved and may well exist today.

*Exobiology: the science of extraterrestrial life.

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