Imagine a moon with an aluminum core surrounded by ice and covered by nitrogen gas under an ocean of liquid acetylene, the waves of which are whipped by winds of methane.

If you can imagine that moon, you're thinking about Titan, the largest of Saturn's 15 moons, the 12th one out and the most unusual body in the solar system. Its gold color fascinates scientists, who have long known that Titan is the only one of the solar system's 37 moons with an atmosphere. Now, thanks to a silver-and-black spacecraft named Voyager that flew by Titan last month, they know this golden moon, 5/12ths the size of earth, has an atmosphere that's denser and deeper than Earth's atmosphere, and is made up of nitrogen, the basic component of all the living things that we know of, and methane -- natural gas, the same stuff you burn to warm a house.

"We know there's an object out there whose atmosphere has just as much nitrogen as Earth," said Michael McElroy of Harvard University. (The air we breathe is 78 per cent nitrogen.) "Titan is by far the most interesting thing in the outer solar system."

From the still-incomplete data that scientists are poring over, McElroy paints a vivid picture of how he thinks Titan was formed. A star 20 times the size of our sun exploded almost five billion years ago, he suggests, scattering radioactive debris across billions of miles of space and into the vicinity of what is now our solar system. Amidst the debris was a huge hot ball of radioactive aluminum-26, which drew clouds of ice and dust to its searing surface. That was the start of Titan.

"The principal gases accumulating on this body were methane and ammonia, which quickly broke down into nitrogen and hydrogen," McElroy said. "What you finish up with after the aluminum loses its radioactivity and gets colder is a rock of still relatively warm aluminum surrounded by nitrogen gas, which is surrounded by an ocean of acetylene, surrounded by nitrogen in droplet form and methane gas. An intriguing celestial body."

Cornell University's Carl Sagan paints an even more intriguing picture from his reading of the data. Sagan imagines the surface of Titan to be an ocean of ammonia and water, which in combination stays liquid below the freezing points of either ammonia and water on their own. The ocean is also thought to be kept warm by a greenhouse effect that traps heat in the atmosphere.

The atmosphere of nitrogen and methane gets broken down by the sun's ultraviolet light into a broad mixture of almost every organic chemical seen in interstellar space. Formaldehyde, hydrogen cyanide, acetylene, ethane, propane and octane. The last four chemicals, particularly, release tremendous energy when burned, which accounts for the idea that it rains gasoline on Titan. This rain produces a sticky tar called "tholins," which is Greek for tar and which may have covered the surface of the Earth four billion years ago, before life began to form.

"Titan is a planet-sized laboratory of pre-biological chemistry that's been at work for four billion years," Sagan says. "That's a small planet that's worth a visit."

Voyager's trip to Saturn was clearly worth the visit. When it flew beneath the majestic rings of Saturn, the spacecraft found the planet circled by at least 1,000 rings. Voyager discovered three new moons around Saturn and two red spots in its clouds, permanent hurricanes that gave the planet a Jupiter look. It saw winds speeding Saturn's clouds around the planet at 900 miles an hour, faster than the speed of sound in our atmosphere. It saw a crack in a moon of Saturn called Mimas that if it had been any bigger would have split the moon in two.

"There might have been hundreds of moons around Saturn at one time," said Tobias Owen of the State University of New York at Stony Brook. "The moons we don't see may have been all destroyed by impacts over time."

The passage of Voyager under the rings may lead to an explanation of why they're still there. The prevailing theory had long been that Saturn's moons keep the rings together -- that their different positions and orbiting speeds just outside the rings kept the rings fenced in.

Every time Mimas goes around the planet, a moon interior to Mimas goes around twice and a moon exterior to Mimas makes a half swing around Saturn. All moving at different speeds, they pull and tug on the rings in ways that might leave gaps between the rings. Each of Saturn's 15 moons could create 40 or 50 gaps. That may be why there are 1,000 rings.

All but one of the rings showed up red in the Voyager pictures. The innermost ring appeared blue, no matter how the sun lighted it. That's puzzling. Almost every body in the solar system is red, almost nothing is blue. Earth's sky, with its nitrogen content, is one exception. Why is the inner ring of Saturn blue?

"Blue is a color we just don't see," Sagan said. "A blue anything that isn't an atmosphere is very interesting because it is very peculiar."

It's easier to explain the red color of the rest of the rings. Sagan thinks the rings may all be dusted with a thin coating of the tholins that stain the clouds of Titan. Where do the tholins come from? They may all come from Titan, as the top of its atmosphere was boiled off by the moon's passage back and forth through the electrically charged magnetosphere of the planet. That would stain the other moons and the rings of Saturn in a way that doesn't happen anywhere else in the solar system.

Talk as they do about the rings, most scientists still come back to Titan when they speak of Voyager's mission to Saturn. Titan, the golden moon that looks like a frozen, primitive Earth. For years, scientists thought the atmosphere surrounding Titan was methane. Voyager found the methane was only a haze above the real atmosphere, which was almost pure nitrogen.

"We went looking for nitrogen on Venus and didn't find it, and we went looking for nitrogen on Mars and didn't find it," said the University College of London's Garry Hunt. "Now we have it, a major nitrogen atmosphere that means we have another earth-like planet one billion miles from the sun."

Titan looks earth-like in other ways. Voyager found at least two haze layers over Titan, both colored a vivid purple and made of droplets of nitrogen, methane and hydrogen cyanide. These are the same organic chemicals that conspired to bring forth life on Earth more than three billion years ago. t

"In the study of primitive atmospheres, hydrogen cyanide is always a link into amino acids and we all know what that means," Hunt said. Amino acids are the building blocks of life as we know it. "I'm not saying there's life on Titan, but in the coldest regions of Earth there are primtive forms of life. I don't think we can say the same thing can't happen on Titan."

How cold is it on Titan?A body that far from the sun theoretically should be no warmer than 87 degrees above the absolute zero (364 degrees below zero Fahrenheit) but Voyager found a greenhouse effect that warmed it as much as 33 degrees above that. Scientists analyzing the Voyager tapes still don't know Titan's surface temperature. Some think the surface temperature might be as warm as 120 degrees above the absolute zero (minus 331 degrees F.).

The Voyager data shows a temperature of 92 degrees above absolute zero and an atmospheric pressure 1.5 times as dense as earth's. Sagan at one point thought the pressure might have gone as high as 20 times that of earth, which would warm things up considerably at the surface. Stanford University's Von Eshelman never thought it would go any higher than three times what it is on earth.

"Carl and I had a bet that involved a Susan B. Anthony dollar," Eshelman said. "If the pressure was any more than six times what it is on earth, he would have won. I won the dollar."

No matter who won the bet, Titan has now become a prime candidate for a future space mission. One idea already being toyed with is to put a spacecraft in orbit around Titan, then float a balloon in its atmosphere and land a small robot on its frozen ocean surface. If Sagan is correct and the ocean is liquid, the robot lander would radio that fact before it sinks into the ammonia-and-water seas.

Can a mission that difficult be made to work? As a reminder of how good they are, the flight controllers at the Jet Propulsion Laboratory where Voyager was directed on its three-year, one billion three hundred million-mile journey, tell people they flew by Titan at a distance of 2,500 miles. They missed their "aim point," as they call it, by 12 miles.