JPL
Careers
Education
Science & Technology
JPL Logo
JPL Logo
Solar System
.

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.+ Full image and caption
Credit: NASA/JPL/Space Science Institute
Janus appears above Saturn's rings and Prometheus, more elongated, is below the rings.+ Full image and caption
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

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."

Images of Saturn's small moons are available at: http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini and http://ciclops.org.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.
+ Cassini home page

News Media Contact

Carolina Martinez

(212) 460-4111

Preston Dyches

818-354-7013

preston.dyches@jpl.nasa.gov

2007-142

Related News

Mars .

NASA’s Perseverance Rover 22 Days From Mars Landing

Solar System .

NASA’s Deep Space Network Welcomes a New Dish to the Family

Mars .

6 Things to Know About NASA’s Mars Helicopter on Its Way to Mars

Mars .

NASA to Host Virtual Briefing on February Perseverance Mars Rover Landing

Mars .

NASA InSight’s ‘Mole’ Ends Its Journey on Mars

Mars .

Mars 2020 Perseverance Rover to Capture Sounds From the Red Planet

Solar System .

NASA’s Juno Mission Expands Into the Future

Mars .

NASA’s Curiosity Rover Reaches Its 3,000th Day on Mars

Mars .

NASA Extends Exploration for Two Planetary Science Missions

Mars .

Celebrate the Perseverance Rover Landing With NASA's Student Challenge

Explore More

Image .

Juno's Mission Goes On

Topic .

Solar System

Image .

A Hot Spot on Jupiter

Image .

Jupiter's Storm Oval BA As Viewed By An Artist

Image .

Two Views of Jupiter Hot Spot

Image .

A Jupiter Circumpolar Cyclone

Image .

Jupiter North Pole Detail

Video .

What's Up - January 2021

Image .

All Eight Northern Circumpolar Cyclones in 2020

Image .

Tracking Clouds on Jupiter

About JPL
Who We Are
Executive Council
Directors of JPL
JPL History
Documentary Series
Virtual Tour
Annual Reports
Missions
All
Current
Past
Future
News
All
Earth
Mars
Solar System
Universe
Technology
Galleries
Images
Videos
Audio
Podcasts
Infographics
Engage
JPL and the Community
Lecture Series
Public Tours
Events
Team Competitions
JPL Speakers Bureau
Topics
Solar System
Mars
Earth
Climate Change
Stars and Galaxies
Exoplanets
Technology
JPL Life
For Media
Contacts and Information
Press Kits
More
Asteroid Watch
Robotics at JPL
Subscribe to Newsletter
Social Media
Get the Latest from JPL
Follow Us

JPL is a federally funded research and development center managed for NASA by Caltech.

More from JPL
Careers Education Science & Technology Acquisitions JPL Store
Careers
Education
Science & Technology
Acquisitions
JPL Store
Related NASA Sites
Basics of Spaceflight
Climate Kids
Earth / Global Climate Change
Exoplanet Exploration
Mars Exploration
Solar System Exploration
Space Place
NASA's Eyes Visualization Project
Voyager Interstellar Mission
NASA
Caltech
Privacy
Image Policy
FAQ
Feedback
Site Manager: Veronica McGregor
Site Editors: Tony Greicius, Randal Jackson, Naomi Hartono