Venus was described today as a planet that has been so hot for so long that its surface it literally on fire.

Instruments on board the two Pioneer space probes that descended to the surface on the night side of the planet last Saturday began picking up a glow at an altitide of 7.5 miles. The glow got brighter and brighter as the two probes fell toward the surface.

"I think we were seeing the red glow of the surface rocks, which got brighter as the probes neared the surface," said Dr. Donald Hunten of the University of Arizona. "It doesn't make any difference what the composition of the rocks might be, it's hot enough on the surface of Venus to set them afire."

"I think we were looking at the fires of hell," said Dr. Thomas Donahue of the University of Michigan after a four-hour press conference describing Pioneerhs exploration of Venus.

The atmosphere over Venus is no more hospitable than the surface, according to the measurements made by Pioneer's probes - there were four altogether and that the single Pioneer spacecraft still in orbit around the planet.

Even the upper atmosphere is so dense that it absorbs half the light reaching Venus from the sun. This enormous concentration of heat in the upper atmosphere triggers winds that reach more than 200 miles an hour and carry the heat from the equator to the poles in less than four days.

"Balloons we've released in the Earth's upper atmosphere over the equator," said Dr. Verner Soumi of the University of Wisconsin, "take six weeks to get to the polar regions."

The same winds carry heat as rapidly from the day to the night side making the atmosphere on the dark side of the planet only 20 degrees cooler than it is on the sunlit side. This is even more remarkable when one considers that Venus rotates so slowly that night is 58 days long.

So dense are the sulfuric acid clouds blanketing the entire planet that a traveler would lose sight of the sun moments after descending into the clouds. A traveler would not know where the sun was on the horizon, even though it might be at high noon and even though the sun is one-third closer to Venus than it is to Earth.

Even if there were no clouds, the carbon dioxide atmosphere above and below the clouds is so dense that a traveler would not see the surface from above the atmosphere. The atmosphere was described today as having an "optical depth" four times that of Earth's atmosphere, meaning it's four times as hard to see through.

As long suspected but now confirmed, the surface of Venus is as hot as it is - 850 degrees Fahrenheit - not because it's closer to the sun but because its atmosphere results in a "run away greenhouse effect" that lets heat in but allows almost none to escape.

The four Pioneer probes that reached the surface last Saturday found that only 2 percent of the light from the sun reaches all the way to the surface compared with 30 percent on Earth. Why is the temperature so much higher?

Scientists have long knoen that the atmosphere of Venus is made up mostly of carbon dioxide, which traps heat in any atmosphere. But that alone is not enough to make Venus as hot as it is. The key is water vapor, which the four Pioneer probes found in sufficient quantity in the atmosphere below the clouds to trap still more heat.

"One of the major findings of Pioneer is that one-tenth to several tenths of a percent of the lower atmosphere is water vapor," said Dr. James Pollack of the National Aeronautics and Space Administration's Ames Research Center. "While the amounts are small, they are enough to keep the heat trapped at the surface."

Pioneer also found that the surface pressure at Venus is 90 times what it is on Earth, the equivalent of the pressure more than 3,000 feet down in the ocean.

"These kinds of pressure," Pollack went on, "help the carbon dioxide and the water vapor in the lower atmosphere absorb that heat as it rises off the surface, aiding and abetting the greenhouse effect."

Above the lower atmosphere, the 10-mule-thick clouds of sulfuric acid provide still another roof for the Venusian greenhouse.

"We find that the particles in the clouds," Pollack said, "are big enough to block thermal readiation from escaping."

How long has Venus been a "hell in the heavens"? Pollack says that nobody knows but it has had to be this way for at least 2 billion years and possibly longer. "Most of the life of the planet is my guess," he said.

What does it mean? One reason for the Pioneer's mission is that Venus is so much like Earth and yet so different. The two planets are the same size and are as close to each other as any on the solar system. Both have massive atmospheres but one is covered with oceans and the other has none. One supports 2 million species of life, the other presumably has none.

"Clearly, some of the processes suggested for Venus might still occur on Earth," the University of Michigan's Donahue said. "Man himself is causing the amount of carbon dioxide to increase by burning fossil fuels. He also seems to be devising various methods for depleting the blanket of ozone in the stratosphere.

"A conservative man might shrink from suggesting that Earth could follow the evolutionary path of Venus but a cautious man might recognize the possibility and do all he can to understand the path of evolution followed by each planet."

Scientists have focused their major interest on the atmosphere of Venus because they are convinced it can tell them much about weather and climate on Earth.

Five things produce climate and weather on Earth. One is the temperature on the surface, another the distribution of the continents, a third the extent of the oceans, the fourth the amount of heat from the sun, and the last the rotation of the Earth which moves weather from place to place.

That's too much to measure and that's why weather forecasters can't tell you if next winter is going to be cold or warm.

To measure things precisely on Earth, scientists say, we's have to stop the Earth from rotating and drain the world's oceans as a starter.

"Since you can't do that, the next best thing is to measure how weather happens on our sister planet, Venus," said NASA's Dr. I. S. Rasool. "It doesn't have any water so it doesn't have any oceans. It rotates so slowly that it might as well not rotate at all.

"By measuring the winds at Venus from the tops of clouds to the bottom," Rasool said, "we get half the information we need to identify how weather and climate take place on our sister planet. The other half is temperature, which the probes of Pioneer have told us in the last week."