When the four Pioneer spacecraft descended to the surface of Venus last Saturday, they fell through clouds of sulfur and sulfuric acid so thick it was like passing through a blizzard.

"It would look like what you'd see," said Dr. Robert Knollenberg of Particle Measuring Systems Inc., "if you were flying through a snowstorm," but a lot warmer.

A laser instrument built into all four probes found that temperatures built at the boiling point of water (212 degree Fahrenheit) and bottoms of the clouds of Venus were close to the near the melting point of sulfur.

"So what we think the probes saw near the base of the clouds was swarms of particles of liquid and solid sulfur," Knollenberg said. "Certainly, the solid particles we saw were most consistent with solid sulfur of an irregular shape."

More than anything else, the mysterious clouds of Venus that perpetually obscure the surface of the planet have been the topic of conversation among the 114 scientists gathered here at the National Aeronautics and Space Administration's Ames Research center gathered here to analyze the Pioneer mission to Venus. The evidence continued to build today that the 10-mile-thick clouds are made up of sulfuric acid and free sulfur.

The clouds start at an altitude of about 40 miles above Venus and reach down to the 30-mile altitude, where they end as abruptly as they begin. The instruments on board the probes saw an "immaculate" atmosphere from 30 miles down to the surface.

Incredibly, the layer of the clouds seems to be same thickness all the way around the planet. None of the four probes saw clouds below 30 miles or above 40 miles.

Knollenberg's instrument shined laser light into the clouds in the probe's descent and then measured the shadows of the cloud particles as they passed through the beam. The shadows should have been the same size as the particles casting the shadows.

Knollenberg said that the clouds appeared to be divided into three zones, the uppermost being composed of the smallest-sized particles and the lowest of the largest. The size of the largest particles is between 15 and 20 microns, or about one-thousandth of an inch in size, Knollenberg said.

There could all be liquid sulfur particles at the base of the clouds," Knollenberg said, "or they could be a mix of solid and liquid sulfur particles."

Scientists speculate that most of the sulfuric acid lies high in the clouds.

The scientists are also offering theories of where the sulfur and sulfuric acid come from. Dr. Donald Hunten of the University of Arizona believes that an insoluble, inert gas in the Venusian atmosphere called carbonyl sulfide rises above the clouds and into the upper atmosphere where it is broken down by the ultraviolet light of the sun into free sulfur and sulfur dioxide. It is the sulfur dioxide that turns into sulfuric acid.