The U.S. Cassini spacecraft orbiting Saturn has found evidence that the giant planet's dazzling rings are decaying and may eventually be gone. And the rings are dirtier than thought, too.

Newly revealed Cassini instrument readings taken as the spacecraft approached Saturn from December through March show tons of oxygen dissipating from the outermost "E" ring, causing its erosion.

University of Southern California scientist Donald Shemansky says oxygen is being knocked out of the ice crystals and other particles that make up the ring, possibly by collisions between them, a meteorite impact, or some event on the nearby moon Enceladus. Without replenishment, Mr. Shemansky says the lifetime of the ring is limited.

"We would project a 100 million year lifetime for the "E" ring, based on the way this process is eroding away the oxygen out of that system," he said.

Mr. Shemansky suggests that this erosion is occurring in Saturn's other rings, saying there must be a connection between them.

Scientists estimate the age of the rings at 100 million years and believe they are made of debris from collisions with the planet's moons or from other bodies that were broken apart when they came near Saturn's powerful gravitational field. The U.S. space agency NASA says Saturn might have had several ring systems over the ages, forming and dissipating many times since the solar system coalesced out of a disk of dust and gas more than four billion years ago.

Other new evidence from the Cassini spacecraft shows that the icy rings are contaminated with what U.S. Geological Survey researcher Roger Clark says is a dirt-like substance.

"We're calling it dirt for now because we haven't identified specifically what it is because we haven't had enough time to look at the data in detail, but it is a dirt-like signature. So while most of the rings are very, very pure ice, we're now seeing that in some places in the rings, in particular in the gaps and now this "F" ring, we're seeing more dirt than before," he said.

A major goal of the Cassini mission is to study Saturn's biggest moon, Titan. Titan's atmosphere is thought to be like that of Earth millions of years ago before life appeared. The spacecraft has flown by Titan for the first of many close passages over the next four years. It peered through its murky atmosphere and sent back images showing dark, linear objects on the ground. Project manager Carolyn Porco says this probably means Titan has tectonics, geological processes that on Earth and other solar system bodies gives rise to mountains, volcanoes, and earthquakes.

"Tectonic features would imply internal processes. We're not sure, of course," he said. "If you don't see the surface, you can't read the story of that geology. This is a glimmer of what is going on. It may not turn out to be the case, but that's what we think we're seeing now."

In December, the European Huygens spacecraft attached to Cassini will separate to conduct its own probe of Titan by plunging through its atmosphere and landing on the surface.