The fires seen by the Pioneer probes on or near the surface of Venus are chemical fires being fed by no fewer than five compounds of sulfur in the lower atmosphere of the planet.

An instrument known as the mass spectrometer, placed aboard the 700-pound Pioneer that was parachuted to the surface of Venus earlier this month, found evidence in the lower atmosphere of free sulfur, sulfur monoxide, sulfur dioxide, carbonyl sulfide and hydrogen sulfide.

With those chemicals present near a seething surface heated to temperatures of almost 900 degrees Fahrenheit, there is almost no way to avoid combustion.

"If you had all that head and all those sulfur compounds together on Earth," said the University of Michigan's Dr. Thomas Donahue, on eof the chief scientists on Pioneer, "you'd get a fiery glow, no question about it."

Instruments aboard the two Pioneer probes that descended Dec. 9 to the surface of Venus on the night side of the planet saw a glow that grew brighter and brighter as the two probes neared the surface. A mystery at the time, the glows were later described by scientists as fires that were lighting up the surface and lower atmosphere of the planet.

The finding by the mass spectrometer aboard the largest of the four probes that the lower atmosphere is teeming with sulfur compounds reinforces the notion of fire near the surface, on it or both. In particular, the discovery of hydrogen sulfide in the lower atmosphere supports that hypothesis.

"One of the big surprises on Pioneer is that apparently large amounts of hydrogen sulfide have been found in the lower region of the atmosphere," Harvard University's Dr. Milchael McElroy said. "Now the question is, what is the source of the hydrogen sulfide. Where is it coming from?"

McElroy said that one possibility is that the hydrogen sulfide is venting into the Venusian atmosphere through cracks in the surface in much the same way this inflammable gas escapes from volcanoes on earth.

"If you released hydrogen sulfide into an atmosphere that contained a little oxygen," McElroy said, "you'd almost surely get a combustible reaction."

Oddly, the amount of hydrogen sulfide increases closer to the surface of Venus. The sulfur compounds higher up tend to be sulfur dioxide and sulfur monoxide and what appears to be free sulfur.

"There is an abrupt change in the sulfur chemistry at 31 kilometers [18.9 miles] that is quit mysterious," McElroy said, "At that level, the compounds become more oxidized, as if there was a photochemistry at work even though the clouds of Venus are at a higher level and presumably keeping out the sunlight."

The origin of the sulfuric acid clouds, which extend from a height of 30 miles to 40 miles, is still something of a mystery.Dr. John Hoffman of the University of Texas says he thinks the sulfur dioxide and sulfur monoxide just below the cloud deck may be the source of most of the sulfuric acid.

"But if they are and they produce the cloud droplets," Hoffman said, "we're not at all sure what excites them to do so. The mechanism for forming the acid is stilla mystery."