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

Dec 06, 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: NASA's reliance on outsourcing launches causes a dilemma for the space agency

Related Stories

What is the habitable zone?

Jun 30, 2015

The weather in your hometown is downright uninhabitable. There's scorching heatwaves, annual tyhpoonic deluges, and snow deep enough to bury a corn silo.

What is the biggest planet in the solar system?

Jun 25, 2015

Ever since the invention of the telescope four hundred years ago, astronomers have been fascinated by the gas giant of Jupiter. Between it's constant, swirling clouds, its many, many moons, and its Giant ...

Saturn spacecraft to buzz icy moon Dione June 16

Jun 16, 2015

NASA's Cassini spacecraft will make a close flyby of Saturn's moon Dione on June 16, coming within 321 miles (516 kilometers) of the moon's surface. The spacecraft will make its closest approach to Dione ...

Saturn's outer ring much bigger than thought

Jun 11, 2015

(Phys.org)—A small team of researchers with members affiliated with the University of Maryland, the University of Virginia and Caltech, has found that the outermost ring of Saturn is much bigger than had ...

The moons of Jupiter

Jun 10, 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 ...

Of course space exploration is worth the money

Jun 09, 2015

I don't suppose that readers would expect me to contradict the title of this article – that would be tantamount to signing my own redundancy notice. But, given an environment where phrases such as "pay restraint", "austerity package" and "falling service provision" are bandied around, it m ...

Recommended for you

New Horizons spacecraft experiences anomaly

5 hours ago

The New Horizons spacecraft experienced an anomaly the afternoon of July 4 that led to a loss of communication with Earth. Communication has since been reestablished and the spacecraft is healthy.

Dwarf planet Ceres offers big surprises for scientists

5 hours ago

The closer we get to Ceres, the more perplexing the dwarf planet grows. NASA's Dawn spacecraft has found several more bright spots as well as a pyramid-like peak jutting out of the frigid world's surface.

User comments : 0

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.