This mosaic of the Saturn system, taken by Cassini, glows with scattered light from tiny dust grains. The sun is obscured by the planet in this unusual geometry.
The rings of Saturn have been known of since telescopes began peering at the heavens. Galileo first spotted them in 1610. Since that time, astronomers have learned more and more about Saturn's most striking feature, from the material that makes up the rings to the forces that jostle that material around.
But two of the most basic-sounding pieces of information about Saturn's rings ? their mass and age ? remain something of a mystery.
These questions are "the big elephant in the room," said Jeff Cuzzi, interdisciplinary scientist for rings and dust for NASA?s Cassini spacecraft. Cuzzi is based at NASA Ames Research Center in Moffett Field, Calif.
Astronomers hope that this mystery will be solved, or at least better understood, with the help of NASA's Cassini spacecraft, which has spent the last six years exploring the Saturnian system. Cassini is now in its extended mission, and has several maneuvers and observations planned that scientists hope will help settle the questions of just how old Saturn's rings are and how much material is in them.
Young or old?
For decades, astronomers had thought that the rings of Saturn, like the planet they orbited, had formed when the solar system did some 4.6 billion years ago, and so were as old as the solar system was.
This notion was challenged when NASA's Voyager spacecraft flew by the ringed giant in the early 1980s and gathered data on the planet, its many moons and its ring system. Through the Voyager observations, scientists found that complex processes were going on in the rings "that make it very hard to understand how [the rings] could be that old," Cuzzi told SPACE.com.
These processes involve the gravitational pushes and pulls, called resonances, that Saturn's moons and rings exert on each other and the cascading effects that those resonances have on other physical processes in the rings. Essentially, scientists worked out, these processes should push Saturn's small moons out of the ring system and pull the rings in toward the planet. But if the ring system was as old as the solar system, this should have happened long ago.
"The processes are going so fast that they would be all finished ? the rings would be gone," said Larry Esposito of the University of Colorado at Boulder, the principal investigator of Cassini's Ultraviolet Imaging Spectrograph (UVIS) instrument.
Given the rate at which these processes operate, and the current position of Saturn's rings and moonlets, scientists calculated that the rings would have to be one-tenth as old as old as the solar system, or only a couple hundred million years old, Cuzzi said.
But the Voyager flyby data were just snapshots of the Saturn system. When Cassini arrived at Saturn in 2004, its observations showed that the processes churning up the rings were going at an even faster rate than was indicated by the Voyager data, Esposito told SPACE.com. Adjusting the age of the rings accordingly would make them younger than the Voyager exploration era, clearly an impossibility.
So the idea that the rings are much younger than the solar system "seems to me to be suspect at this moment," Esposito said.
Though there is another, independent line of evidence that suggests that the rings are a young feature ? the amount of pollution in the mostly water-ice rings that comes from incoming meteorite debris.
Rock and ice
The rings of Saturn are at least 90 percent water ice, observations have found. But, like other things in the solar system, Saturn's rings are under constant bombardment by interplanetary debris, which is about 60 percent carbon and rock, Cuzzi said.
Over time, the meteorite debris raining down on the rings would "pollute" the water ice, making it increasingly less pure. "Over time, [the rings] are getting dirtier," Cuzzi said. But the bulk of the rings are still largely water ice, suggesting that the rings haven't been around long enough to be highly polluted, which in turn suggests they're rather young.
Of course, how dirty the rings get doesn't depend only on how long they've been around to absorb all that debris; it also depends on how much ring material there is. The pollution wouldn't show up as much in denser, more massive rings as it would in thinner rings. So knowing the mass of Saturn's rings could help scientists figure out how long debris has been bombarding them, and therefore how long they've been around.
"We thought we knew the mass of the rings pretty well," Cuzzi said. Scientists, including Esposito, had determined this mass by measuring how much starlight gets through the rings. But Cassini discovered that the material in the rings exerts a gravitational force on itself that causes it to clump up, and almost no starlight penetrates these clumps.
"So there's really no way for us to know how much matter is in these clumps," Cuzzi said. The mass that scientists were actually measuring was just the mass of the thinner patches between the more massive clumps.
Scientists hope that Cassini can get around this clump problem by measuring the mass of Saturn's rings in a different way.
What Cassini will do
Toward the end of its mission, Cassini will be put into an orbit between the planet and the inner edge of the rings.
"It's going to go in a very, very close orbit," Cuzzi said. "And in these orbits, even the mass of the rings has an effect on the orbit of the spacecraft."
So based on how Cassini's course deviates, scientists hope to be able to calculate how massive the rings are.
The other part of the equation that would give scientists a constraint on the age of the rings is the rate at which meteorite debris is hitting the rings. Combining the mass of the rings, the amount of pollution seen in them and the rate at which that pollution falls would tell scientists how long the debris had been falling on the rings and therefore how long the rings have been around.
Cassini is also trying to answer that second part of the equation. Measuring the flux of meteorite debris into the rings turns out "to be very hard to measure because the system is a very dusty system and a lot of this is just lost to the background," Cuzzi explained. "So we're going to use a bit of trick."
The same meteorite debris that falls onto the rings also falls onto Saturn's moons. When that debris hits the small, icy moons, it kicks up dust that can be measured by Cassini. That data in turn can tell scientists how much meteorite debris is falling on the system.
The spacecraft recently flew by Saturn's moon Rhea, and scientists are awaiting the data to figure out the meteorite flux, Esposito said.
Once all the data from the Cassini mission is in, they hope to combine the calculated mass and meteorite hit rate "and really be able to say whether the rings are as old as the solar system or not," Cuzzi said.
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