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Venus volcanism: another hint.

Venus volcanism: Another hint

Add another item to the list of evidence some scientists cite as indicating that cloud-engulfed Venus remains a volcanically active planet.

In 1983, Larry W. Esposito of the University of Colorado in Boulder reported that the ultraviolet spectrometer aboard the Pioneer Venus spacecraft had measured a steady drop in the amount of sulfur dioxide in the planet's cloud tops (SN: 10/1/83, p.213). He interpreted this as a possible sign that a large volcanic eruption had injected the sulfur compound into the atmosphere shortly before the Venus-orbiting craft reached its target five years earlier, and that the amount of the gas had gradually declined ever since. Now, Paul G. Steffes of the Georgia Institute of Technology in Atlanta reports that, in addition to the cloud-top measurements, "we have evidence for the first time that levels of sulfur dioxide have been dropping off in the atmosphere below the clouds."

Venus' major cloud layer lies about 48 kilometers above the planet's surface, Steffes notes. The evidence for a declining amount of sulfur dioxide beneath it was detected by Steffes and Jon M. Jenkins of Georgia Tech, together with Michael J. Klein of Jet Propulsion Laboratory in Pasadena, Calif. The data consist of radio emissions from Venus with wavelengths of 1.3 centimeters, detected with the 43-meter radiotelescope of the National Radio Astronomy Observatory in Green Bank, W.Va., and the 70-meter NASA radiotelescope at Goldstone, Calif. The emissions pass through Venus' atmosphere on the way to Earth, so an increase in their intensity indicates a decrease in the amount of Venusian sulfur dioxide available to absorb them.

Jenkins and Steffes also report a decline in the amount of sulfuric acid vapor in the Venusian atmosphere, based on the weakening of 13-cm transmissions from the spacecraft. The Earthward radio beam reaches different depths in Venus' atmosphere as the spacecraft moves, so researchers can use it to calculate differences with altitude. The sulfuric acid and sulfur dioxide almost surely have a common source, Steffes says, even without evidence confirming that volcanoes continue erupting on Venus.

Other scientists have reported additional possible signs of volcanic activity there: radar measurements suggesting rough edges on some surface features, indicating they are too young geologically for erosion to have smoothed them; and radio emissions interpreted by some researchers (though others disagree) as evidence of lightning, similar to that observed over some eruptions on Earth. None of these findings provides sufficient evidence to prove the existence of volcanoes, however, and those of Steffes' group remain inconclusive as well.

Part of the problem is scientists' inability to link either the sulfur dioxide or the sulfuric acid measurements with a particular location on the planet surface. Sulfur dioxide is widely distributed, and although the sulfuric acid does seem more abundant near the Venusian equator than over the polar regions (suggesting upward atmospheric circulation at the poles and "downwelling" at the equator), the data do not pin down longitudes with any more accuracy than about 40[degrees].

NASA's Magellan spacecraft, due to reach Venus next summer with a more powerful transmitter and a much larger antenna than those of Pioneer Venus, should provide the most detailed radar views yet of that planet. Though even Magellan may not locate a surface source for the presently declining atmospheric sulfur dioxide supply, there is a small chance--if volcanoes are indeed still erupting--that it will detect topographic features that differ measurably from those appearing in past radar maps, perhaps indicating fresh lava flows. Magellan scientists hope the craft will last long enough to cover the planet twice, taking about eight months to complete each mapping and perhaps showing differences between them.
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Publication:Science News
Date:Dec 9, 1989
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