Water Vapor Discovered on Tiny Moon of Saturn

Cassini, a $3.3 billion behemoth launched in 1997 by NASA and the European and Italian space agencies, has 12 instruments on board.

Cassini has found several new rings surrounding Saturn, has discovered that Saturn's winds blow 170 mph faster at lower altitudes than at high altitudes, and has observed huge thunderstorm columns that produce lightning bolts 10,000 times as strong as any on Earth.

The spacecraft has also provided new evidence that Saturn's moon Phoebe is an interloper that wandered in from deep space and was captured into orbit by the planet, and that Phoebe's surface is more diverse than that of any known body in the solar system except Earth.

The spacecraft discovered Polydeuces, a 3-mile-wide midget moon, orbiting in concert with the larger moon Dione. Saturn is the only planet known to have these companion moons, called "Trojan moons."

During a summer encounter with Saturn's rings, Cassini's infrared spectrometer found sharp temperature variances on the surfaces of both large and small particles in Saturn's three main rings, an indication that the particles are rotating slowly.

And in the A ring, 76,000 miles from Saturn's center, Cassini found that particle clusters, ranging in size from "sedans to moving vans," are constantly combining and being torn asunder by tidal forces caused by Saturn's gravity.

"You have to have enough big particles for gravity to stick them together, and enough distance from Saturn so the tidal forces rip them apart immediately," said University of Colorado planetary scientist Joshua E. Colwell. "Only the A ring has both."

On the heavily cratered moon Iapetus, scientists found an equatorial mountain range twice the height of Mount Everest, but they still do not know how half of Iapetus came to be coated with dark material while the other half shines white.

Iapetus vies with Enceladus among project scientists for the title of "favorite icy moon," Johnson said. "Iapetus is strange on the old end, and Enceladus is strange on the young end."

Known before the Cassini mission as the brightest object in the solar system aside from the sun because of its shroud of crystalline ice, Enceladus is now also known as the smallest body to have active volcanism.

"What we think is happening is that jets of gas are escaping at substantial velocity from fissures to form a large column of gas above the south pole," Brown said. Mass spectrographic analysis showed that the gas is composed of water vapor, carbon dioxide, methane and maybe nitrogen, he added, and though temperatures at the vents are well below zero, liquid water will flash into gas when it is flushed into the vacuum of space.

Scientists were also able to confirm from these observations that Saturn's E ring, as suspected, is made of microscopic icy "smoke" from Enceladus's vents. What they have not been able to figure out is how there could be enough heat to make liquid water.

"Deep down, you have a reservoir of stuff -- liquid water mixed with carbon dioxide and light organics that is hot in a relative sense," Brown said. "Why, and why only at the south pole? Those are the big questions, and none of the explanations advanced so far is satisfactory."