Space probes unravel Venus's 'greenhouse' mystery

Scientists studying Venus believe they have solved the puzzle of what keeps that planet so hot -- a sizzling 482 degrees C. (900 degrees F.) at the surface.

It is, they say, the so-called "greenhouse effect." Outgoing heat is captured by the thick Venusian atmosphere, 96 percent of which consists of the heat-trapping gas carbon dioxide (CO[2]).

While this has been suspected for a long time, scientists had been unable to balance the planet's heat budget in terms of the greenhouse theory alone. It looked as though Venus radiated some 15 percent more energy back into space than it received from the sun. That would suggest the planet had an internal heat source.

Now, according to a recent announcement from the NASA-Ames Research Center at Moffett Field, Calif., a research team headed by James Pollack has been able to account "very precisely" for the Venus heating as a result of the greenhouse effect.

The difficulty of doing this in the past has been due to the complexity of the Venusian atmosphere and to lack of detailed data on its composition, temperature distribution, and radiative properties.

American Pioneer Venus probes, which traversed the atmosphere, and Soviet Venera landers have supplied the needed data. Using these data, Dr. Pollack and his colleagues Martin Tomasko of the University of Arizona and Brian Toon of NASA-Ames have been able to make a theoretical model of the planet's atmosphere. Through calculations using this model, "the team was able to produce the actual surface temperature of Venus very precisely," Pollack says.

Besides CO[2], the greenhouse effect is due partly to water vapor (about 50 parts per million [ppm] in the lower atmosphere), sulfur dioxide (about 200 ppm in the lower atmosphere), and various types of cloud particles. A planetary haze layer also accounts for 13 to 16 degrees C. of the heating.

All told, the scientists estimate the various contributions to the greenhouse effect to be 55 percent for CO[2], 25 percent for water vapor, 15 percent for sulfur dioxide, and 5 percent for clouds and haze.

As the NASA-Ames announcement points out, the success in accounting for Venus's heat has implications for energy planning on Earth. "Having calculated atmospheric heating for one planet [which is almost the most extreme case imaginable], calculations of carbon dioxide effects on Earth's atmospheric heat budget should be much easier," it explains.

Many atmospheric scientists are concerned that Earth could warm up as the air's CO[2] content is raised through increased burning of coal. While that warming would be unlikely to exceed a few degrees, this could have damaging climatic effects in some regions. Shifting rainfall patterns, for example, might make drought the rule for North American wheat lands.

The likely impact of CO[2]-induced climatic change still is uncertain. Any advance in scientists' ability to model a planet's atmosphere accurately enough to specify greenhouse effects is valuable. It reduces the uncertainty so that energy planners can better anticipate what climatic impact a given level of increased coal use may have.

Besides helping to solve the Venus temperature puzzle, Pioneer Venus findings also have stripped away some of the mystery about the planet's surface, which can't be seen beneath the clouds, and about the strange circulation of the planet's atmosphere.

Radar mapping by the Pioneer Venus 1 orbiter covers some 93 percent of the surface. It shows a varied topography of mountains, high plateaus, and vast plains.

More than a thousand pictures of Venus's shifting cloudscape have been taken with the aid of ultraviolet "light." These help reveal atmospheric circulations.

At its lower levels, the Venusian atmosphere is sluggish. But at high levels , winds sweep around the planet once in four days. This is a very high circulation rate for a planet that itself rotates only once in 243 Earth days.

The data suggest that the main engine driving the atmosphere is the cloudy layer at intermediate altitudes, which absorbs much of incoming solar energy. This drives a circulation that carries heat from equatorial regions towards the poles at a relatively slow rate. The net result is a more or less uniform temperature over the planet.

Another unusual feature of the Pioneer findings is the fact that the atmosphere at cloud and cloud-top levels seems to rotate as a solid body. This has the natural consequence that wind speeds are much higher at the equator than at the poles, for the distance the wind has to travel in a given amount of time is much larger at the equator.

Venus's thick atmosphere, with surface pressure 90 times greater than that at sea level on Earth, has revealed itself to be a crucial factor in the development of that planet. Although it is only 30 percent closer to the sun than Earth, Venus has a searing environment -- thanks to an exaggerated greenhouse effect.

Now scientists expect to refine these conclusions, and perhaps to encounter new surprises, as the Pioneer spacecraft continues its measurements.

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