The temperature of the upper atmosphere of Venus is warmer over the north and south poles than it is over the planet's equator.

This surprising fact was returned to Earth today by the Pioneer space craft orbiting Venus and was one of at least three surprises revealed about the planet at the National Aeronautics and Space Administration's Ames Research Center, where the Pioneer mission to Venus is being directed.

The four probes that descended through the planet's atmosphere to the surface Saturday found the thick Venusian clouds rich in the rare gas argon, meaning Venus may still be as hot on the in side as it is on the outside.

Why is that? Because argon is a product of the decay of radioactive potassium in a planet's crust, and radioactive potassium is one of the minerals that produces a planet's interior heat.

More important, the probes found high concentrations of an isotope of the element known as argon-36, which is not a decay product of potassium but is one of the gases found in a planet's primordial atmosphere as it would have existed more than 4 billion years ago.

"What we see on Venus is what we see in some meteorites but not on Earth or on Mars," said Dr. Michael McElroy of Harvard University. "The atmosphere of Venus contains as much argon-36 as you would expect from a planet's original atmosphere. Either Venus never lost its primordial atmosphere or it contained a lot more rare argon than Earth or Mars did, and we don't know why."

Still circling the planet, where it will make measurements for a year, the Pioneer orbiter discovered the upper atmosphere over the planet's polar regions to be 40 degrees warmer than the same parts of the atmosphere over the equator.

"This is the first time we've been able to look at the poles of Venus, since you can't see them head-on from Earth, and it was a surprise," said Dr. Fredric Yaylor of the Jet Propulsion Laboratory in Pasadena, where the infrared radiometer was built that measured the temperatures above the clouds of Venus. "In fact, the highest temperatures we found anywhere in the planet's upper atmosphere were at the poles themselves."

The temperature of the atmosphere 50 miles above the planet's poles was found to be 40 degrees below zero Fahrenheit, which compared to an average of 80 degrees below zero above the Venusian equator at the same altitude.

Why the atmosphere is warmer at the poles was a subject of intense speculation among the scientists working on the Pioneer mission. The mostaccepted speculation is that the lower atmosphere is so warm at the equator that it rises to greater heights than it does elsewhere on the planet and then cools abruptly on reaching those heights.

"The upper level of the air at the poles is about 2 kilometers (1.24 miles) below the upper level of the air at the equator," Taylor said. "We see this on our instruments, so it suggests there is rising and cooling air at the equator and warming, descending air at the poles."

Supporting that speculation is the fact that the sun's heat generates higher winds in the upper atmosphere at the planet's equator. These winds produce their own cooling effect at a height of 50 miles.

"It could be that energy is transported into heat at the poles and transformed into motion at the equator," Taylor said, "but it's all speculation. We don't know the reasons for this yet."

Far more intriguing to the scientists here was the finding that the Venusian atmosphere has 100 times more rare argon-36 than Earth and 10,000 times more than Mars. Like many discoveries about planets, it raises as many questions as it answers.

Scientists have long assumed that Earth, Venus and Mars were all created equally and that they're still similar in many respects. But if that were ture, the three planets should still have more or less the same amounts of primitive gases like argon-36 in their atmospheres.

They do not. That certainly means that the primeval Venusian atmosphere was different than Earth's and it probably means that Earth, Venus and Mars had different beginnings.

"It sends us back to the drawing board," Dr. Thomas Donahue of the University of Michigan said today. "We can only speculate about what the reasons for the argon-36 are."

Of less importance but just as much interest, the probes found that the clouds of Venus end at an altitude of 30 miles. From there down to the surface, the planeths atmosphere is essentially clear.

The one probe that survived for more than an hour on the surface Saturday also raised a cloud of dust whem it hit the surface, a finding confirmed today by a tiny optical instrument on the probe called a nephelometer. The dust cloud lasted for four minutes, which tells scientists that winds on the surface of Venus are almost nonexistent.