The sulfuric acid clouds that cover the planet of Venus apparently ate through an instrument carried by all four Pioneer probes that descended to the surface of Venus last Saturday.

The instrument was the thermocouple, placed on the outside of the four probes to take the temperature of the atmosphere as the probes descended toward the surface. All four instruments failed at an altitude of nine miles when the atmospheric temperature had climbed to 680 degrees Fahrenheit.

"The instrument failures certainly suggest that sulfuric acid at that elevated temperature could have caused at least a partial short-circuit," said Dr. Alvin Seiff of the National Aeronautics and Space Administration's Ames Research Center, where the Pioneer mission to Venus is being directed. "The fact that all four instruments were lost at the same altitude was certainly not a coincidence."

The instruments were built to measure atmospheric temperatures on Venus to within one-tenth of a degree and were doing just that as the four probes fell at different spots through the clouds and into the clear portions of the lower atmosphere. Temperature readings had climbed to almost 700 degrees at an altitude of 8.7 miles when the instruments suddenly ceased operating.

The failure of the four outside thermometers was the first and only loss in the Pioneer mission to Venus so far, but it was a serious one. Scientists had hoped to get a precise temperature profile of the Venusian atmosphere right down to the surface, where temperatures of almost 1,000 degrees have been measured by Soviet spacecraft.

Precise temperature measurements are crucial to understanding the "greenhouse" effect that traps heat in the lower atmosphere of Venus. It is the only planet in the solar system that keeps its heat from escaping into space, a phenomenon that scientists fear could take place to some degree on Earth from an atmospheric buildup of carbon dioxide.

While the loss of the lower atmospheric temperatures was viewed as a blow to the mission, scientists said it inadvertently provides a new insight into the sulfuric acid clouds that blanket the planet between 30 and 40 miles above the surface. So thick are these clouds that no telescopes on Earth have ever seen through them.

The sulfur content of these clouds is equivalent to the water and ice content of similar clouds above Earth which are at most a tenth as thick.

Since the discovery less than 10 years ago that the clouds were made at least in part of sulfuric acid, scientists have tried to understand where the acid comes from. Tiny traces of sulfuric acid have been found in the thin clouds of the Earth's stratosphere but scientists still don't understand their origins.

"That discovery was made 20 years ago and today we don't know where the sulfuric acid comes from," said Dr. Michael McElroy of Harvard University. "It is a problem larger in magnitude than understanding the smog in Los Amgeles."

There is no doubt there is more sulfur in the atmosphere over Venus than over any other planet in the solar system. Not only are the clouds made up mostly of sulfuric acid, but at least two other sulfur compounds were identified in abundance by the four probes that descended to the surface on Saturday.

One instrument saw high concentrations of carbonyl sulfide in the atmosphere and a second instrument measured large amounts of sulfur dioxide. Both are essential to the formation of sulfuric acid, the sulfur dioxide at low altitudes and the carbonyl sulfide at upper altitudes.

More surprising, the ultraviolet telescope aboard the Pioneer spacecraft still in orbit around Venus has seen a dark cloud circling the planet far above the top deck of the sulfuric acid clouds. Many scientists believe the dark cloud is nothing more than free sulfur that fell out in the upper reaches of the atmosphere when the colder temperatures froze certain other sulfur compounds.

Where does all the sulfur come from? Dr. Donald Prinn of the Massachussetts Institute of Technology believes temperatures are so high on the surface of Venus that sulfur compounds are cooked right out of minerals, like irom sulfide, which are present on Venus in great abundance. The sulfur compounds then react with the carbon dioxide and other gases in the atmosphere to produce sulfur dioxide and carbonyl sulfide -- the precursors of sulfuric acid.

"There is almost no water in the atmosphere of Venus to rain out the sulfur like there is on Earth," Prinn said. "We have rain scavenging sulfur out of the Earth's atmosphere but we have no such phenomenon happening on Venus. Therefore, it should come as no great surprise that there is so much sulfur in the atmosphere of Venus."