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Atmosphere of Venus yields evidence of volcanic activity on Earth's sister planet

By David F. SalisburyStaff writer of The Christian Science Monitor / February 15, 1984

Mountain View, Calif.

Beneath its thick veil of clouds, Venus may be staging some spectacular volcanic shows. To a number of scientists, a strong string of circumstantial evidence has transformed active vulcanism on Earth's sister planet from intriguing speculation to a strong likelihood.

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Researchers involved agree that the study of Venusian volcanoes would likely shed valuable light on how volcanoes work on Earth.

Evidence that has tipped the scientific scales toward active volcanic activity on Earth's sunward neighbor comes from the work of Larry Esposito at the University of Colorado at Boulder. He leads a team of scientists who have an instrument aboard the Pioneer Venus spacecraft, which has been orbiting the planet since December 1978. Their device measures particles in the planet's stratosphere. He presented his findings Monday at a press conference at the National Aeronautics and Space Administration's Ames Research Center here.

''When we arrived at the planet, we were surprised to see large concentrations of sulfur dioxide,'' Dr. Esposito explains.

Sulfur dioxide, along with water and carbon dioxide, is a major constituent in the eruption plumes of many volcanoes on Earth. But the Colorado scientist admits that the team's first reaction was not to credit these concentrations - 50 times those measured previously - to a volcanic eruption. Rather, he thought that earlier, earth-based measurements might simply have been wrong.

It was not until 1982, following the eruption of Mexico's El Chichon volcano, that this possibility was suggested. Esposito and his colleagues were studying data from a similar instrument on a satellite orbiting Earth and were struck by the similarities between these sulfur dioxide measurements and those from Venus.

On Earth, major eruptions can inject large amounts of sulfur dioxide into the stratosphere, where the particles float for a long time. It takes several years for these particles to sift back down to the earth and for the stratosphere to return to normal. This is precisely the sort of thing that Esposito's instrument recorded on Venus. Over the five years since the satellite has been in orbit, the sulfur dioxide level has fallen steadily.

''So, it seems to us that the most likely explanation for this is a large ejection of sulfur dioxide into the atmosphere. And, certainly if it were Earth, the most natural way to interpret this would be a volcanic eruption which occurred a matter of months before we went into orbit,'' Esposito concludes.

There could be other explanations, he cautions. But this seems to be the simplist and it dovetails nicely with several other, independent lines of evidence.

Because of the planet's perpetual cloud cover, it is impossible to photograph its surface with ordinary cameras. However, radar can be used to map its surface terrain in considerable detail. The first efforts to do this, using ground-based radar, revealed surface structure that looked distinctly volcanic in nature. Since then, this has been confirmed by radar-mapping efforts of the Pioneer Venus spacecraft as well as by Soviet Venera missions. ''The (geologic) information makes a compelling case that Venus has a number of large and small isolated volcanoes,'' says Harold Masursky of the US Geological Survey.