Saturn propellers reflect solar system origins

It marks the first time scientists have been able to track the orbits of individual objects in a debris disk. The research gives scientists an opportunity to time-travel back into the history of our solar system to reveal clues about disks around other stars in our universe that are too far away to observe directly.

"Observing the motions of these disk-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disk of material surrounding the early sun," said Carolyn Porco, Cassini imaging team lead based at the Space Science Institute in Boulder, Colo., and a co-author on the paper. "It allows us a glimpse into how the solar system ended up looking the way it does."

The results are published in a new study in the July 8, 2010, issue of the journal Astrophysical Journal Letters.

Cassini scientists first discovered double-armed propeller features in 2006 in an area now known as the "propeller belts" in the middle of Saturn's outermost dense ring, known as the A ring. The spaces were created by a new class of moonlets - smaller than known moons, but larger than the particles in the rings - that could clear the space immediately around them. Those moonlets, which were estimated to number in the millions, were not large enough to clear out their entire path around Saturn, as do the moons Pan and Daphnis.

The new paper, led by Matthew Tiscareno, a Cassini imaging team associate based at Cornell University, Ithaca, N.Y., reports on a new cohort of larger and rarer moons in another part of the A ring farther out from Saturn. With propellers as much as hundreds of times as large as those previously described, these new objects have been tracked for as long as four years.

High resolution image

Taken in 2006, this image is part of a growing catalogue of "propeller" moons that, despite being too small to be seen, enhance their visibility by creating larger disturbances in the surrounding fabric of Saturn's rings. Cassini scientists now have tracked several of these individual propeller moons embedded in Saturn's disk over several years.

This image has been re-projected so that orbiting material moves to the right and Saturn is down. The propeller was seen at the edge of the camera's field of view when the image was taken, so some data were missing; the blank space at the top of the image was filled in with a gray color. Scale in the original image was 2 kilometers (1 mile) per pixel. Image scale in this re-projected view is about 1 kilometer (half a mile) per pixel.

This view looks toward the southern, sunlit side of the rings from about 30 degrees below the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 15, 2006. The view was acquired at a distance of approximately 463,000 kilometers (288,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 15 degrees.

Credit: NASA/JPL/Space Science Institute




The moon, which is too small to be seen, is at the center of the propeller structure visible in the upper left of the image, near the Encke Gap of the A ring (marked with a red arrow). The A ring is the outermost of Saturn's main rings. The image has been rotated and contrast-enhanced to aid visibility. The cropped inset of the propeller included here was magnified by a factor of four. This view looks toward the southern, sunlit side of the rings from about 81 degrees below the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2008. The view was obtained at a distance of approximately 364,000 kilometers (226,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 82 degrees. Credit: NASA/JPL/Space Science Institute






The propeller is marked with a red arrow in the top left of the annotated version of the image. The Encke Gap of Saturn's A ring is in the lower right of the image. The A ring is the outermost of Saturn's main rings.

The moon, likely about a kilometer (half a mile) across, can't be seen at this resolution. However, it is larger than other "propeller" moons and has cleared ring material from the bright (because they are less opaque) wing-like regions to its left and right in this image. Disturbed ring material close to the moon blocks more sunlight and appears like a dark airplane propeller.

Several density waves are visible in the ring, particularly in the upper left. A spiral density wave is a spiral-shaped accumulation of particles that tightly winds many times around the planet. It is the result of gravitational tugs by individual moons whose orbits are in resonance with the particles' orbits at a specific distance from Saturn.

The image has been rotated and contrast-enhanced to aid visibility. The cropped inset of the propeller included here was magnified by a factor of four. This view looks toward the northern, unilluminated side of the rings from about 45 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Aug. 19, 2008. The view was acquired at a distance of approximately 310,000 kilometers (193,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 121 degrees.
Credit: NASA/JPL/Space Science Institute

The propeller features are up to several thousand kilometers (miles) long and several kilometers (miles) wide. The moons embedded in the ring appear to kick up ring material as high as 0.5 kilometers (1,600 feet) above and below the ring plane, which is well beyond the typical ring thickness of about 10 meters (30 feet). Cassini is too far away to see the moons amid the swirling ring material around them, but scientists estimate that they are about a kilometer (half a mile) in diameter because of the size of the propellers.

Tiscareno and colleagues estimate that there are dozens of these giant propellers, and 11 of them were imaged multiple times between 2005 to 2009. One of them, nicknamed Bleriot after the famous aviator Louis Bleriot, has been a veritable Forrest Gump, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation over this time.

"Scientists have never tracked disk-embedded objects anywhere in the universe before now," Tiscareno said. "All the moons and planets we knew about before orbit in empty space. In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects. With this new discovery, we can now track disk-embedded moons individually over many years."

Over the four years, the giant propellers have shifted their orbits, but scientists are not yet sure what is causing the disturbances in their travels around Saturn. Their path may be upset by bumping into other smaller ring particles, or responding to their gravity, but the gravitational attraction of large moons outside the rings may also be a factor. Scientists will continue monitoring the moons to see if the disk itself is driving the changes, similar to the interactions that occur in young solar systems. If it is, Tiscareno said, this would be the first time such a measurement has been made directly.

"Propellers give us unexpected insight into the larger objects in the rings," said Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Over the next seven years, Cassini will have the opportunity to watch the evolution of these objects and to figure out why their orbits are changing."

Source: Jet Propulsion Laboratory
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