Scientists studying new photographs from the Voyager spacecraft near the planet Saturn today described the surface of Titan, Saturn's largest moon, as an ocean of supercold liquid nitrogen covered by a glassy layer of frozen tar.

"This is a picture of a bizarre, murkey swamp," said Stanford University's Dr. Von Eshelman, one of the scientists gathered flight is directed. "The swamp is liquid nitrogen, and the murk is frozen nitrogen and hydrocarbon muck."

Scientists analyzing Voyager's first measurements of Titan have had a series of surprises this week from the only moon in the solar system with an atmosphere. It is a deep freeze version of a primitive Earth, it is at least three times warmer than scientists expected it to be and there are so many hydrocarbons (compounds containing only hydrogen and carbon, as in members of the petroleum family) being made in its upper atmosphere that on Titan it might rain frozen gasoline.

"The ultraviolet light of the sun would work on the methane in the upper atmosphere of Titan to produce octane, which is after all, the main component of gasoline," said Dr. Donald Hunten of the University of Arizona. "I can imagine it raining frozen gasoline on Titan."

Now more than 1 billion miles from Earth, Voyager sent back a photograph of the moon of Saturn called Rhea that was so close and so clear in its detail that it might have been a picture of Earth's moon, taken with a zoom lens from Earth.

One-third the size of Earth's moon, Rhea filled the screen of television monitors at the Jet Propulsion Laboratory. The moon was splotched with browns and tans and covered with craters. Said Dr. Laurence Soderblom of the U.S. Geological Survey: "Rhea is a battered, frozen moon."

Two big surprises of the day were the findings that Titan's atmosphere is at least as thick as the Earth's and is composed mainly of nitrogen, which is the principal gas in the Earth's atmosphere. Scientists had known Titan had a thick atmosphere, but they thought it was made of methane (natural gas) and was not more than one-tenth as dense as that of Earth.

"We had hesitated to think we'd find nitrogen because we were wrong twice before [on Mars and Venus] in thinking we'd find nitrogen," Eshelman said. b"We also hesitated because we think of nitrogen on Earth as a life source."

Infrared and ultraviolet spectrometers on Voyager measured the nitrogen in Titan's atmosphere. The infrared device also found methane, ethane, acetylene, ethylene and hydrogen cyanide, most of the same gases that scientists believed were present in the Earth's atmosphere before life formed more than three billion years ago.

"On Titan, we may have a snapshot of the atmospheric evolution that took place on Earth eons ago, with one major differenct," said Dr. Rudolf Hanel of the Goddard Space Flight Center. "It's so cold on Titan that nobody can imagine any life having formed there." "

Voyager measured Titan's temperature at 90 degrees below absolute zero in the upper atmosphere, rising to as much as 200 degrees above absolute zero further down (that translates to 256 degrees below zero Fahrenheit). The temperature then fell again, to 65 degrees above absolute zero in the middle reaches of Titan's atmosphere. At the same point, Voyager's instruments measureed a pressure of 380 millibars, much higher than the atmospheric pressure on the surface of Mars, and "38 percent of the air pressure in this room," Eshelman said.

"We have not analyzed our measurements all the way to the surface but we're convinced the pressure on the surface is at least one bar, the same as the pressure on the surface of the Earth," he said.

Scientists sketched this profile of Titan's atmosphere: the clouds in the upper reaches are made of methane mixed with nitrogen, which is worked over by the ultraviolet light of the sun to produce a broad range of heavier organic chemicals that rain out into the lower reaches of the atmosphere of nitrogen, some of it gas but much of it in liquid form, as droplets.

"You might say the humidity on Titan is 100 percent," Voyager project scientist Edward C. Stone said. "Liquidity might be a better word than humidity."

Mixed in with the methane in the upper atmosphere are solid particles of what scientists call "Axel dust," a mixture of soot and hydrocarbon aerosols (particles suspended in gas) named for the Princeton University professor who first postulated it 10 years ago. The "Axel dust" is believed to be the source of the rich red color astronomers have seen in the atmosphere of Titan since they first began investigating it almost 20 years ago.

While surprised at the discovery that Titan has a thick nitrogen atmosphere, scientists were quick to point out that nitrogen is almost invisible to telescopes on Earth and to certain instruments carried by the Pioneer spacecraft that flew by Titan a year ago.

"Methane is an easy gas to detect -- that's why we found it and that's why we thought it was the main constituent of Titan's atmosphere," said Dr. Andrew Ingersoll of the California Institute of Technology. "What we see on Titan is an evolving atmosphere that long ago lost its hydrogen and ammonia and began to form complex hydrocarbons, smog and a nitrogen ocean."