Click on each moon for quick facts

New revelations, riddles about solar system's most intriguing satellites

At top, a volcanic eruption on Io; below, a false color image of cracks and ridges on Europa

In this story: Hellish world of poison gas Ocean of slush or liquid? Unexpected magnetic attraction An impenetrable hydrocarbon haze 'Knocking on the door' of life RELATED STORIES, SITES

By Richard Stenger
CNN.com Writer

(CNN) -- Does Titan have continents of frozen methane? Does the surface of Europa fold into ice valleys and ridges? Could large moons in the solar system possess life? As scientists unravel the mysteries of these distant worlds of fire and ice, they are finding them both unpredictably bizarre and surprisingly familiar.

Scientists only became aware of the basic physical properties of major satellites around Jupiter and Saturn when a pioneering NASA probe visited the two systems two decades ago.

"Our understanding of the different characteristics of the outer planet moons has undergone a revolution since the Voyager I reconnaissance in 1979," said Torrence Johnson, a NASA scientist at the Jet Propulsion Laboratory in Pasadena, California.

"Prior to getting the Voyager data back, most astronomers thought they would find frozen dirt balls with craters on them."

Voyager created a sensation when it beamed back images of material spewing from the surface of Io, the first evidence of volcanoes other than those on Earth.

But its fuzzy pictures and primitive instrumentation left scientists hungry for more information.

"There were only tantalizing hints of what might be there. Voyager raised many questions. It just didn't have the answers," said Paul Geissler, a planetary geophysicist at the University of Arizona in Tucson.

Its more powerful successor has shed light on some of the mysteries since it entered the jovian system in 1995. Snapping infrared and color images and measuring magnetic fields, "Galileo immediately started getting answers when it got there," said Geissler. But it also raised many new tantalizing questions.

	IO
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Galileo continues to beam back close-up images of Io showing a hellish world of poison gas plumes, erupting molten rock and giant mountain ranges.

Lava lakes sizzle as hot as 3,000 degrees F (1,650 C) and in other places the thermometer plummets to minus 250 degrees F (-160 C), the greatest temperature range in the solar system.

Io orbits closer to Jupiter than the planet's three other major moons, Europa, Ganymede and Callisto. The gravitational forces of its mother planet and sister moons constantly torture and bend Io, heating it up in the same way repeated twisting heats up a metal coat hangar.

Scientists once speculated the boiling cauldrons on Io ejected mostly sulfur material. Closer examination of Galileo data proved otherwise.

"Maybe a few are, but we realize now that most have a rocky composition. The temperature is too hot for sulfur," Geissler said.

Callisto's pockmarked surface is the most cratered in the solar system

But serious puzzles remain hidden near the two largest planets. Jupiter has a magnetosphere twice the size physicists would expect. Scientists theorize charged particles from Io are the culprit.

Every second Jupiter's magnetic field strips from Io about one ton of material, which becomes ionized and forms a thick ring of intense radiation. Ions escaping the ring create aurora storms in Jupiter's atmosphere and inflate the planet's magnetic field, according to NASA scientists.

A joint robot expedition later this year should shed light on the matter. Cassini, a NASA probe headed to Saturn, is expected to team up with Galileo in December to study the intense, interconnected magnetic fields in the planet's system.

	EUROPA
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Galileo images revealed unknown surface features on Europa, another large moon around Jupiter.

"There are a system of highways on Europa, ridges with valleys crisscrossing one another. These ridge sets would have been impossible to see with Voyager," Geissler said.

But Europa remains an enigma. Recent Galileo images show ice floes the size of cities that seem to drift along its frozen and cracked surface. Scientists are confident the moon harbors a salty ocean beneath its crust containing more water than found on Earth. They remain unsure whether it is in a liquid or slurry state or how close it is to the surface.

"We're certain there's water. We're not certain it's liquid. The evidence that there might be liquid water is compelling," said Geissler. "I think that liquid water is just below the surface. But a different school of thought says the ice layer is much thicker."

NASA's Europa Orbiter, scheduled to launch in 2003 and begin orbiting the moon in 2009, will use radar to measure the ice layer and detect any underlying water.

	GANYMEDE
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Europa's neighbor Ganymede is the largest moon in the solar system. Scientists hope data from Galileo, which recently flew by the satellite, will help explain features that might be volcanoes.

Ganymede is also the only known planetary satellite with its own magnetic field, a trait that took planetary geologists by surprise.

"How did it get so hot that liquid iron in its core moves around enough to make a magnetic field? Either something's wrong with our theory or our understanding of Ganymede's history," said Johnson.

	CALLISTO
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Callisto orbits outside the main radiation belts of Jupiter, protected from the intense forces that contort its neighbors. It has the oldest and most cratered landscape in the solar system, with a nearly complete absence of geologic activity on its surface. Long thought a dead world, Callisto has nevertheless weathered some unknown type of erosion, according to Johnson.

"When we got close, we did not see as many small craters as expected, which means the surface has been modified," said the Galileo project scientist, adding that the evaporation of carbon dioxide ice could be the cause.

	TITAN
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Voyager I aimed a camera at Titan during a visit to the Saturn system in 1980, but failed to penetrate a thick smog of nitrogen and hydrocarbons that blankets the moon.

Yet scientists using Earth-based observatories and the Hubble Space Telescope have managed to glimpse the surface of the moon, which has an atmosphere 1.5 times as dense as that on Earth.

A bright patch near the equator the size of Australia emerged from the fuzzy radar and infrared images, stirring lively speculation among astronomers at an international conference in England this month.

One camp suggested the shiny spot is a continent of frozen methane, drifting in a methane ocean; another that it is a mountain range of water ice, continuously eroding under a methane rain.

Cassini, set for a 2004 rendezvous with Titan, will map the surface with a high-resolution radar system similar to the one that recorded the topography of Venus. The orbiter will then send the Huygens probe to the surface to snap pictures and measure atmospheric gases. No one knows exactly what to expect.

"There could be mountains, craters, lakes of hydrocarbons, long fracture patterns like those across Europa," Johnson said.

Despite their peculiarities, the moons in many ways resemble the Earth. Most have hot metal cores and interiors laden with rocks, carbon compounds and water. Most have atmospheres, some with oxygen, carbon dioxide and nitrogen.

Many have geologically active interiors and surfaces, replete with volcanoes, mountains and tectonics. Some have water on or near the surface. Scientists think that their conditions in some ways mimic conditions on our planet early in its youth.

Could they harbor life? The Cassini and Europa orbiters and Huygens lander could help answer such questions. Johnson cautions against such speculation, given our limited knowledge. Yet he points out that life forms have been discovered in extremely harsh conditions all over Earth, whether buried below Arctic ice or thriving alongside undersea heat vents.

"They have opened people's eyes about the range of places we might look for biologically interesting things," he said.

Some scientists consider the major satellites among the most promising places to search in the solar system.

"Do you have the right materials and do you the right conditions? The materials are certainly there," said Johnson. He and others consider Europa a good place to start.

"I think there is liquid water and there are local heat sources, probably volcanoes. To me it seems like it has all the ingredients needed for life to be there," Geissler said.

"Within our lifetime it will be a possibility for us to knock on the door and see if anyone is home."

Galileo spots new volcanoes on Io
June 1, 2000
Galileo findings boost possibility of Europa ocean
January 11, 2000
Galileo, Cassini to study Jupiter in joint expedition
March 9, 2000
Chandra's X-ray vision of universe awes, puzzles
June 8, 2000
Dusty Jupiter moon gives clues to interplanetary rings
June 16, 1999
Looking for life in all the weird places
April 1, 1999

NASA Jet Propulsion Laboratory
Galileo Project
Voyager Project
Europa Orbiter
Cassini: Voyage to Saturn
Huygens Information
NASA
The University of Arizona

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