NEW ORLEANS, LOUISIANA—The rings of Saturn seem like permanent fixtures in the solar system, firing the imagination of poets and scientists alike. But observations made this year, in the final months of NASA’s Cassini spacecraft, and reported here at a meeting of the American Geophysical Union (AGU) show they are surprisingly youthful: Until a few hundred million years ago, they did not exist. Saturn acquired its jewels relatively late in life. If any astronomers had gazed at the sky in the time of the dinosaurs, they might have seen a bare and boring Saturn.
It was then that some sort of catastrophe struck the gas giant. Perhaps a stray comet or asteroid struck an icy moon, tossing its remnants into orbit. Or maybe the orbits of Saturn’s moons somehow shifted, and the resulting gravitational tug-of-war pulled a moon apart. However it happened, two new lines of evidence from Cassini make it clear that the rings were not around in the early days of the solar system 4.5 billion years ago, as scientists had long believed, says Jeff Cuzzi, a ring specialist at NASA’s Ames Research Center in Mountain View, California. “It rules out the primordial ring story,” Cuzzi says. “That’s what it looks like to me.”
The first line of evidence comes from the mass of the rings. For years, many scientists leaned toward a large mass, greater than that of Saturn’s moon Mimas, because of the opaque, dense appearance of Saturn’s primary ring, the B ring. Enough grist to form a massive ring could have only been supplied billions of years ago, when the early solar system was chock full of planetesimals.
But now, data gathered during five of Cassini’s final 22 passes before it plunged into the planet are bringing the mass of the B ring into focus, Luciano Iess, a planetary scientist at the Sapienza University of Rome, announced at the AGU meeting today. Iess leads Cassini’s radio experiment team, which used tiny Doppler shifts in the spacecraft’s radio signal to determine the mass of objects it orbited. When Cassini began threading the gap between Saturn and its rings during its last passes, the team could pick out the gravitational pull of the rings—and hence their mass. “The central value is consistently 0.4 Mimas’s mass,” Iess said. If theories that link mass to age are correct, he added, “This is a clear indication that the rings did not form together with Saturn.”
That conclusion is buttressed by another line of evidence, which will be presented at the meeting on Wednesday. A constant rain of sooty micrometeorites falls into Saturn from the edge of the solar system, which would be expected to darken the pristine water ice in the rings over time. How quickly they would darken depends on the bombardment rate, which has been uncertain.
After 12 years of painstaking measurements and analysis, the Cosmic Dust Analyzer, a Cassini experiment that measures small particles, has pinned down the micrometeorite flux—and it is “inconsistent with an old ring,” says Sascha Kempf, a space physicist at the University of Colorado (CU) in Boulder who will present the results tomorrow. This flux, which runs about 10 times higher than thought prior to the Cassini, suggests a ring age of between 150 million to 300 million years, or even younger. “Our measurement is the most direct way you can measure it,” Kempf adds. “There’s not much you can do about it. It has to be young.”
Taken together, the two results make a compelling case for young rings, says Larry Esposito, a planetary scientist also at CU Boulder who has long believed the rings to be old. “These two Cassini results really argue strongly that the rings are young, maybe under 200 million years,” he says.
In the early 1980s, Esposito says, the Voyager spacecraft flew past Saturn and returned data that seemed to point toward a low ring mass—and a possible youthful age. But Voyager scientists had a hard time coming up with a compelling scenario to explain it—the notion that a saturnian moon might have exploded at a time when the solar system would have had few potential asteroids or comets to ram into it seemed far-fetched. “The best idea we had then was that we’re just lucky,” Esposito says. “I’m back to square one.”
Scientists have only begun to study how the ring-forming collision could have happened. “Part of the reluctance for everyone to leap off this bridge into the unknown is we haven’t had any kind of feasible explanation,” Cuzzi says. It’s time for new ideas, he adds. “The solar system could be full of surprises like this.”