A hint of fresh volcanism on Venus.

What are the odds that a probe that parachuted onto Venus would encounter an active volcano during its one-day descent through the planer's dense atmosphere? Pretty slim, asserts planetary scientist Thomas M. Donahue at the University of Michigan in Ann Arbor. Nonetheless, his new analysis of chemical data .from a Pioneer-Venus instrument that landed on the planet nearly 14 years ago suggests the probe did indeed pass through the plume of an active volcano.

Donahue and his collaborators, including R. Richard Hodges Jr. of the University of Texas at Dallas, characterize the findings as so surprising that they were loath to publicize them. They say the new work may provide some of the first evidence that Venus has undergone very recent volcanism. While several studies of the planet, notably the Magellan spacecraft's ongoing radar mapping mission, indicate that Venus has experienced plenty of volcanic activity in the past, no One has yet found compelling evidence of fresh eruptions.

The researchers base their unlikely conclusion on the abundance and composition of methane detected by a mass spectrometer aboard the Pioneer-Venus probe. Scientists had known for years that the spectrometer had recorded a sharp rise in methane, beginning at about 14 kilometers above the surface of Venus, during the probe's descent. But for nearly a decade, Donahue and his co-workers believed the surge merely reflected methane placed in the spectrometer on Earth in order to calibrate the instrument, not activity on Venus.

But they couldn't explain an unexpected decline in water vapor recorded by the spectrometer at about the same altitude, so the researchers took another look at the data. They discovered that the methane measured by the probe contained very little deuterium, an isotope of hydrogen that has twice hydrogens mass but is chemically identical. In particular, the deuterium concentration in the methane was less than 10 percent of that found in the surrounding Venusian atmosphere.

The amount of deuterium relative to hydrogen in the atmosphere of a planet increases substantially over time, since hydrogen atoms weigh less and can more easily escape a planet's gravity. And in just days, fresh methane on Venus should exchange some of its hydrogen atoms for the relatively plentiful supply of deuterium in the surrounding atmosphere, so that it, too, becomes enriched in deuterium.

Thus, the deuterium-poor methane found by the probe indicated to Donahue's team that the methane must have erupted very recently from apristi ne source within the planet-- no more than 20 days before the instrument landed.

"We concluded that the methane sampled was a primeval methane freshly vented from the planets interior," says Donahue. He presented the findings last month in Pasadena, Calif., at an international colloquium on VenUs.

Donahue estimates that a volcanic eruption spewing out the amount of methane found by the Pioneer-Venus probe would occur only about once every 100 million years. Moreover, it appears that the probe passed through the plume near the top of the atmosphere, where winds would have stretched the vented methane over a wide area, as well as closer to the surface of the planet, where the plume would have been far more localized and less likely to have been detected.

"It is embarrassing to invoke such a wildly unlikely event as a chance encounter between the entry probe and a rare and geographically confined methane plume, but so far we have eliminated all other plausible explanations," Donahue adds.

The probe's landing site - and the location of the proposed plume -lies in a fiat area about 4[degrees] north of Venus' equator, on the edge of a highland region called Phoebe Regio. Donahue's team now plans to analyze Magellan images of the area for further hints of volcanic activity.