Venus's volcanism: present or past?

Venus's volcanism: Present or past?

Are volcanoes erupting on the surface of Venus? The possibility may be the most dramatic and controversial in the ongoing study of the planet, with a growing list of data being cited by various researchers as evidence that Venus is active, not just in the "geologically recent" past, but right now. But according to Harry A. Taylor of the NASA Goddard Spaceflight Center in Greenbelt, Md., the whole case is "a house of cards."

The most widely cited piece of evidence, presented by Frederick L. Scarf of TRW Inc. in Redondo Beach, Calif., has been a large number of radio bursts picked up since 1978 by an electric-field detector aboard the Pioneer Venus Orbiter spacecraft (PVO). The bursts, according to Scarf, appear to be from "whistlers" produced by lightning in the Venusian atmosphere. Lightning sometimes- appears over volcanoes on earth, and on Venus, Scarf says, the bursts appear to be clustered over two highland regions that radar measurements suggest to be the youngest spots on the planet (SN: 12/5/81, p. 362).

Fellow PVO scientist Taylor, however, together with Paul A. Cloutier of Rice University in Houston, Maintains not only that the bursts fail to show any such clustering but that they are not even lightning. Instead, these researchers aver that they are merely "ion acoustic noise" generated in troughlike regions formed by sharp density gradients in the Venusian ionosphere. Maps of the bursts' locations, made by comparing the electric-field detector's readings with PVO's orbital positions at the time, resemble ion trough maps produced by another of its instruments, an ion mass spectrometer.

The "lightning" events identified by Scarf and colleague Christopher Russell of the University of California at Los Angeles indeed appear clustered between about 35[deg.] north and south latitude, but so do the ion troughs -- and according to Taylor and Cloutier, for the same reason. The latitude limitation occurs, they say, because only there is PVO flying low enough to be in the dense portion of the ionosphere where the 100-hertz (Hz) bursts are generated. Part of that latitude range also happens to include two admittedly volcanic-looking regions called Beta and Aphrodite, but it also encom-passes an even larger amount of less-in-triguing-looking territory Taylor and Cloutier note, in fact, that when the terrain is divided into the 260 five-degree-square areas that have each produced at least three bursts, more than 85 percent of them lie outside the highlands. Furthermore, in a paper submitted to SCIENCE--and called "Venus: Dead or alive?"--the authors note that the abrupt onsets and cessations of the trooughs match those of the 100 Hz noise.

There is more to the ion trough case, but there is also more to the case for volcanism. Perhaps the next-best-known item is the finding from PVO's ultraviolet mass spectrometer that the amount of sulfur dioxide (SO2) in the top of the Ventusian atmosphere has decreased markedly since the spacecraft got there in December 1978. Larry W. Esposito of the University of Colorado in Boulder has suggested that this could be evidence that a major volcanic eruption took place on Venus shortly before the spacecraft's arrival, spewing forth a vast concentration of SO2 that has been lessening ever since (SN: 10/1/83, p. 213).

Taylor and Cloutier maintain that Esposito's interpretation of the "claimed SO2 behavior" was "clearly stimulated by the claimed 'lightning signal clustering," part of a "bandwagon effect" that led to over-interpretation of one piece after another of uncertain "evidence." Esposito himself, however, says he was led to his interpretation by observations with a similar instrument of Mexico's El Chichon volcano. "I was always skeptical about the lightning until I saw this in our data," he says. And would volcanism have come to mind if the subject to Venusian lightning had never been raised? Says Esposito, "Absolutely."

Venusian volcanoes are a loaded question--with more answering needed.