Cassini finds 'missing link' moonlet evidence in Saturn's rings
Thu Mar 30, 2006 at 09:57 UTC
Scientists with NASA's Cassini mission have found evidence that a new class of small moonlets resides within Saturn's rings. There may be as many as 10 million of these objects within one of Saturn's rings alone.
The moonlets' existence could help answer the question of whether Saturn's rings were formed through the break-up of a larger body or are the remnants of the disk of material from which Saturn and its moons formed.
"These moonlets are likely to be chunks of the ancient body whose break-up produced Saturn's glorious rings," said Joseph Burns of Cornell University, Ithaca, N.Y., a co-author of the report.
Careful analysis of high-resolution images taken by Cassini's cameras revealed four faint, propeller-shaped double streaks. These features were found in an otherwise bland part of the mid-A Ring, a bright section in Saturn's main rings. Cassini imaging scientists reporting in this week's edition of the journal Nature believe the "propellers" provide the first direct observation of how moonlets of this size affect nearby particles. Cassini took the images as it slipped into Saturn orbit on July 1, 2004.
Previous measurements, including those made by NASA's Voyager spacecraft in the early 1980s, have shown that Saturn's rings contain mostly small water-ice particles ranging from less than 1 centimeter (one-half inch) across to the size of a small house. Scientists knew about two larger embedded ring moons such as 30-kilometer-wide (19-mile) Pan and 7-kilometer-wide (4-mile) Daphnis. The latest findings mark the first evidence of objects of about 100 meters (300 feet) in diameter.
From the number of moonlets spotted in the very small fraction of the A ring seen in the images, scientists estimated the total number of moonlets to be about 10 million.
"The discovery of these intermediate-sized bodies tells us that Pan and Daphnis are probably just the largest members of the ring population, rather than interlopers from somewhere else," said Matthew Tiscareno, an imaging team research associate at Cornell and lead author on the Nature paper.
Image Credit: NASA/JPL/Space Science Institute
These figures show four propeller-shaped structures discovered by the Cassini spacecraft in close-up images of Saturn's A ring.
The propellers are about 5 kilometers (3 miles) long from tip to tip, and the radial offset (the "leading" dash is slightly closer to Saturn) is about 300 meters (1,000 feet).
The figures were cropped from two original Cassini spacecraft narrow-angle camera images and magnified for visibility. The images were then re-projected so that orbital motion is to the left and Saturn is up. The unseen moonlets lie in the center of each structure. The figures were cropped from two original Cassini spacecraft narrow-angle camera images, taken during Saturn orbit insertion on July 1, 2004, and magnified for visibility.
Image Credit: NASA/JPL/Space Science Institute
This magnified view illustrates the general orientation of the "propeller" features in Saturn's rings as they orbit the planet. The propellers are features detected in Cassini images that reveal the gravitational influence of moonlets approximately 100 meters (300 feet) in diameter.
The view is from one of the two Saturn orbit insertion images, taken on July 1, 2004, in which the propellers were discovered.
The two dashes of the propeller are oriented in the direction of orbital motion. The "leading" dash is also slightly closer to Saturn; this "radial offset" is about 300 meters (1000 feet). The unseen moonlet lies in the center of the structure.
The grainy appearance of the image is due to magnification and the fact that the propellers are very faint--just visible above the level of background noise. Consequently, the image enhancement procedures used have also enhanced the noise.
This propeller image is identified as "feature 1" in the first image. The original Cassini spacecraft narrow-angle camera image has been magnified from its original scale for presentation.
Image Credit: NASA/JPL/Space Science Institute
This collection of Cassini images provides context for understanding the location and scale of propeller-shaped features observed within Saturn's A ring.
Careful analysis of the highest resolution images taken by Cassini's cameras as the spacecraft slipped into Saturn orbit revealed the four faint, propeller-shaped double-streaks in an otherwise bland part of the mid-A ring. Imaging scientists believe the "propellers" provide the first direct observation of the dynamical effects of moonlets approximately 100 meters (300 feet) in diameter. The propeller moonlets represent a hitherto unseen size-class of particles orbiting within the rings.
The left-hand panel provides broad context within the rings, and shows the B ring, Cassini Division, A ring and F ring. Image scale in the radial, or outward from Saturn, direction is about 45 kilometers (28 miles) per pixel; because the rings are viewed at an angle, the image scale in the longitudinal, or circumferential, direction is several times greater.
The center image is a closer view of the A ring, showing the radial locations where propeller features were spotted. The view is approximately 1,800 kilometers (1,100 miles) across from top to bottom and includes a large density wave at bottom (caused by the moons Janus and Epimetheus), as well as two smaller density waves. The footprints of the propeller discovery images are between density waves, in bland, quiescent regions of the ring.
The propellers appear as double dashes in the two close-up discovery images at the right and are circled. The unseen moonlets, each roughly the size of a football field, lie in the center of each structure. These two images were taken during Saturn orbit insertion on July 1, 2004, and are presented here at one-half scale. Resolution in the original images was 52 meters (171 feet) per pixel. The horizontal lines in the image represent electronic noise and do not correspond to ring features.
The propellers are about 5 kilometers (3 miles) long from tip to tip, and the radial offset (the "leading" dash is slightly closer to Saturn) is about 300 meters (1,000 feet).
The propeller structures are unchanged as they orbit the planet. In that way, they are much like the wave pattern that trails after a speedboat as it skims across a smooth lake. Such a pattern is hard to discern in a choppy sea. In much the same way, scientists think other effects may be preventing Cassini from seeing the propellers except in very bland parts of the rings.
Moons as large as Pan and Daphnis clear large gaps in the ring particles as they orbit Saturn. In contrast, smaller moonlets are not strong enough to clear out the ring, resulting in a partial gap centered on the moonlet and shaped like an airplane propeller. Such features created by moonlets were predicted by computer models, which give scientists confidence in their latest findings.
"We acquired this spectacular, one-of-a-kind set of images immediately after getting into orbit for the express purpose of seeing fine details in the rings that we had not seen previously," said Carolyn Porco, Cassini imaging team leader and co-author. "This will open up a new dimension in our exploration of Saturn's rings and moons, their origin and evolution."
The detection of moonlets embedded in a ring of smaller particles may provide an opportunity to observe the processes by which planets form in disks of material around young stars, including our own early solar system. "The structures we observe with Cassini are strikingly similar to those seen in many numerical models of the early stages of planetary formation, even though the scales are dramatically different," said co-author Carl Murray, an imaging team member at Queen Mary, University of London. "Cassini is giving us a unique insight into the origin of planets."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. 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.
JPL News Release
