Images of Saturn's Small Moons Tell the Story of Their Origins

December 6, 2007
The highest resolution images of Pan (right) and Atlas (left) reveal distinctive "flying saucer" shapes created by prominent equatorial ridges not seen on the other small moons of Saturn. Image credit: NASA/JPL/Space Science Institute

Imaging scientists on NASA's Cassini mission are telling a tale of how the small moons orbiting near the outer rings of Saturn came to be. The moons began as leftover shards from larger bodies that broke apart and filled out their "figures" with the debris that made the rings.

It has long been suspected that Saturn's rings formed in the disintegration of one or several large icy bodies, perhaps pre-existing moons, by giant impacts. The resulting debris quickly spread and settled into the equatorial plane to form a thin disk surrounding the planet. And the small, irregularly shaped ring-region moons were believed to be the leftover pieces from this breakup.

Now, several years' worth of cosmic images of Saturn's 14 known small moons have been used to derive the sizes and shapes of most of them, and in about half the cases, even masses and densities. This information, published in the Dec. 7 issue of the journal Science, has led to new insights into how some of these moons may have formed.

The tip-off was the very low density of the inner moons, about half that of pure water ice, and sizes and shapes that suggested they have grown by the accumulation of ring material. The trouble was, these moons are within and near the rings, where it is not possible for small particles to fuse together gravitationally. So how did they do it? They got a jump start.

"We think the only way these moons could have reached the sizes they are now, in the ring environment as we now know it to be, was to start off with a massive core to which the smaller, more porous ring particles could easily become bound," said Carolyn Porco, Cassini imaging team leader from the Space Science Institute in Boulder, Colo. Porco is the lead author of the first of two related articles published in this week's issue of Science.

Simple calculations and more complicated computer simulations have shown that ring particles will readily become bound to a larger seed having the density of water ice. By this process, a moon will grow even if it is relatively close to Saturn. The result is a ring-region moon about two to three times the size of its dense ice core, covered with a thick shell of porous, icy ring material. To make a 30-kilometer moon (19 miles) requires a seed of about 10 kilometers (6 miles).

Where did such large cores come from? And when did this all take place?

"The core may in fact be one of the remnants from the original ring-forming event," said co-author Derek Richardson, professor of astronomy at the University of Maryland, College Park, "which might have been left intact all this time and protected from additional collisional breakup by the mantle of ring particles around it."

Just exactly when the rings formed is not known. "But it is not out of the question that the moons date back to the time of ring formation," said Porco.

The researchers show that the cores of Pan and Daphnis, which orbit within gaps in the outer A ring, were large enough to open narrow gaps. Accretion, or accumulation of material, they say, probably occurred quickly. The moons grew and their gaps widened, achieving their present sizes before the gaps were completely emptied of material, and probably before the local rings reached their present thickness.

So how did Pan in the main rings, and Atlas, which orbits just beyond the outer edge of the main rings, get the prominent equatorial ridges that make them look like flying saucers? The second paper reports evidence for a secondary stage of accretion that occurred after the moons' growth was completed and after the rings flattened to their present 20-meter (66 feet) thickness.

"Our computer simulations show that the ridges must have accreted rapidly when Saturn's rings were thin, forming small accretion disks around the equators of Pan and Atlas," said Sebastien Charnoz, lead author and an associate of imaging team member Andre Brahic at the University Paris-Diderot and CEA Saclay, in France. "The ridges might be the remains of 'fossilized' accretion disks, fundamental structures seen at all scales in the universe, from planetary rings to galaxies."

Source: NASA

Explore further: The moons of Saturn

Related Stories

The moons of Saturn

September 14, 2015

Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has ...

The moons of Jupiter

September 15, 2015

Jupiter was appropriately named by the Romans, who chose to name it after the king of the gods. In addition to being the largest planet in our Solar System – with two and a half times the mass of all the other planets combined ...

Moons in hiding

September 23, 2015

Prometheus and Pandora are almost hidden in Saturn's rings in this image.

The Lords of the Rings among centaurs

September 14, 2015

(—Chariklo, the largest known centaur object, orbiting in a region between Saturn and Uranus, is a very intriguing celestial body that surprised astronomers last year. This remote minor planet has unveiled the ...

Jupiter's moon Europa

September 30, 2015

Jupiter's four largest moons – aka. the Galilean moons, consisting of Io, Europa, Ganymede and Callisto – are nothing if not fascinating. Ever since their discovery over four centuries ago, these moons have been a source ...

Recommended for you

Dead comets and near-earth encounters

October 13, 2015

Near Earth Objects (NEOs) are asteroids or comets whose orbits sometimes bring them close to the Earth, thereby posing a potentially threat. The asteroid that struck Chelyabinsk last year was an NEO about 40 meters in diameter. ...

What happens when your brain can't tell which way is up?

October 13, 2015

In space, there is no "up" or "down." That can mess with the human brain and affect the way people move and think in space. An investigation on the International Space Station seeks to understand how the brain changes in ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.