Saturn May Have Millions of Mini-Moons, Images Show

Richard A. Lovett
for National Geographic News
March 31, 2006
We've long known that Saturn has dozens of moons, ranging from gigantic
Titan (about as big as the planet Mercury) to asteroid-size chunks only
a few kilometers in diameter.

There are also uncounted ring particles, varying from marble size to the size of a small house.

Now scientists have found the first evidence of millions of "missing link" moonlets—bigger than ring particles but small enough to fit inside a football stadium.

The international team found signs of four such moonlets in one small segment of Saturn's brightest ring, called the A ring, according to a report in this week's issue of the journal Nature.

(Photo: Saturn's "Greatest Portrait Yet.")

When they made the discovery, the scientists were examining the high-resolution photos taken by NASA's Cassini probe shortly after it reached Saturn in 2004.

"Since we hadn't ever seen the rings in this kind of detail, we were looking for phenomena we'd never seen before," said Carolyn Porco, head of the Cassini imaging team at the Space Science Institute in Boulder, Colorado.

Propeller Blades

What the scientists found were pairs of bright streaks shaped like two-bladed propellers. The streaks weren't big, only about 3 miles (4.8 kilometers) wide.

But they looked familiar.

Computer simulations produce similar structures, Porco said, when the motions of ring particles are simulated in the presence of small moonlets within the ring.

The presence of two Saturn moons, Pan and Daphnis, has created gaps that circle the entire ring system—empty bands in the striped ring pattern. But Pan is 19 miles (30 kilometers) wide and Daphnis is 4 miles (7 kilometers).

The propeller-blade streaks indicate moonlets, probably only 330 feet (100 meters) in diameter.

That's too small to be seen even the Cassini photos, which are the highest resolution images ever taken of Saturn's rings. But it's large enough for the moonlets' gravitational pulls to visibly affect the particles around them.

Amazingly, the Cassini team found four of these propeller-like structures, even though only a few photos were detailed enough to depict such small features.

Since these photos covered only a tiny fraction of the ring, there may potentially be millions of moonlets yet to be found in the A ring alone.

Origin of the Rings

The finding supports the theory that Saturn's rings are formed of debris from one or more large objects that broke into pieces, rather than from matter that never quite managed to fuse together to form a moon.

It's unlikely that moonlets this large were formed through gradual buildup of space material, says Matthew Tiscareno of Cornell University in Ithaca, New York, who led the international study team.

"It's much easier if you start with a solid icy core, like a shard from a breakup [of a large object], which can then grow by sweeping up additional particles," Tiscareno said.

The discovery also helps explain how all of Saturn's moons and ring particles interact—useful not only for understanding the rings but for understanding how planets form from matter circling young stars.

"Everything we do in studying the rings gives us insight into how planets form, because the processes we see occurring today in Saturn's rings are the same as those occurring in the disk of material that eventually formed the solar system and the planets within it," Porco, of the Space Science Institute, said.

The discovery also increases understanding of how Pan and Daphnis create gaps in the ring system.

The propeller-like streaks, Porco says, are wannabe gaps.

If the bodies that created the streaks got bigger and bigger, she said, the "blades" would get longer and longer.

"Finally, they would meet, all the way around the ring," she said.

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