Saturn's saucer moons
Science & Exploration

Images of Saturn’s small moons tell the story of their origins

07/12/2007 579 views 0 likes
ESA / Science & Exploration / Space Science / Cassini-Huygens

Imaging scientists on the 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 by 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 disc surrounding the planet. And the small, irregularly shaped, ring-region moons were believed to be the pieces left over 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 today’s 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 Colorado, USA. Porco is the lead author of the first of two related articles published week, in Science.

Saturn's saucer moons, annotated
Saturn's saucer moons, annotated

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-km moon requires a seed of about 10 km.

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, USA “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-m 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 ‘fossilised’ accretion disks, fundamental structures seen at all scales in the universe, from planetary rings to galaxies.”

Notes for editors:

The Cassini-Huygens mission is a cooperative project between NASA, ESA and the Italian Space Agency.

The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, manages the Cassini-Huygens mission for NASA's Science Mission Directorate. JPL designed and assembled the Cassini orbiter. ESA developed the Huygens Titan probe, while ASI managed the development of the high-gain antenna and the other instruments of its participation. The imaging team is based at the Space Science Institute, USA.

Related Links

Saturn’s elusive radio rotation
Science & Exploration

Planetary scientists close in on Saturn’s elusive rotation

12/12/2007 1948 views 9 likes
Open item
Saturn's saucer moons
Science & Exploration

Images of Saturn’s small moons tell the story of their orig…

07/12/2007 579 views 0 likes
Open item
Titan’s hazy secrets
Science & Exploration

Organic ‘building blocks’ discovered in Titan’s atmosphere

29/11/2007 1338 views 3 likes
Open item
Titan IVB with Cassini-Huygens on board blasts off from Cape Canaveral
Science & Exploration

Cassini-Huygens - celebrating 10 years since launch

12/10/2007 903 views 1 likes
Open item
Titan's north polar region
Science & Exploration

Cassini’s new view of land of lakes and seas

11/10/2007 1164 views 1 likes
Open item
The Other Side of Iapetus
Science & Exploration

Cassini on the trail of a runaway mystery

08/10/2007 846 views 2 likes
Open item
Towering Peaks of Iapetus
Science & Exploration

Cassini on the trail of a runaway mystery - more images

08/10/2007 492 views 0 likes
Open item
Bright and Dark mountains on Iapetus
Science & Exploration

Saturn’s moon Iapetus is the Yin-Yang of the Solar System

13/09/2007 1186 views 5 likes
Open item
Huygens' descent and landing
Science & Exploration

Fasten your seat belts, turbulence ahead - lessons from Tit…

28/08/2007 1042 views 0 likes
Open item
The ring arc and a site of concentrated ring particles
Science & Exploration

Possible origin of Saturn's mysterious G ring

03/08/2007 1046 views 1 likes
Open item
Tethys and Dione juxtaposed
Science & Exploration

Two more active moons around Saturn

13/06/2007 1297 views 0 likes
Open item
Saturn’s active north pole
Science & Exploration

Cassini images bizarre hexagon on Saturn

27/03/2007 3878 views 6 likes
Open item
Saturn's moon Enceladus
Science & Exploration

Enceladus geysers mask the length of Saturn’s day

22/03/2007 2535 views 0 likes
Open item