Cassini sees moon building giant snowballs in Saturn ring
Tue Jul 20, 2010 at 19:21 UTC
While orbiting Saturn for the last six years, NASA's Cassini spacecraft has kept a close eye on the collisions and disturbances in the gas giant's rings. They provide the only nearby natural laboratory for scientists to see the processes that must have occurred in our early solar system, as planets and moons coalesced out of disks of debris.
New images from Cassini show icy particles in Saturn's F ring clumping into giant snowballs as the moon Prometheus makes multiple swings by the ring. The gravitational pull of the moon sloshes ring material around, creating wake channels that trigger the formation of objects as large as 20 km (12 miles) in diameter.
"Scientists have never seen objects actually form before," said Carl Murray, a Cassini imaging team member based at Queen Mary, University of London. "We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris."
Murray discussed the findings today (July 20, 2010) at the Committee on Space Research meeting in Bremen, Germany, and they were published online by the journal Astrophysical Journal Letters on July 14, 2010. A new animation based on imaging data shows how one of the moons interacts with the F ring and creates dense, sticky areas of ring material.
Saturn's thin, kinky F ring was discovered by NASA's Pioneer 11 spacecraft in 1979. Prometheus and Pandora, the small "shepherding" moons on either side of the F ring, were discovered a year later by NASA's Voyager 1. In the years since, the F ring has rarely looked the same twice, and scientists have been watching the impish behavior of the two shepherding moons for clues.
Prometheus, the larger and closer to Saturn of the two moons, appears to be the primary source of the disturbances. At its longest, the potato-shaped moon is 148 km (92 miles) across. It cruises around Saturn at a speed slightly greater than the speed of the much smaller F ring particles, but in an orbit that is just offset. As a result of its faster motion, Prometheus laps the F ring particles and stirs up particles in the same segment once in about every 68 days.
"Some of these objects will get ripped apart the next time Prometheus whips around," Murray said. "But some escape. Every time they survive an encounter, they can grow and become more and more stable."
High resolution image
The bright, discontinuous features seen near the core of the F ring are clumps of ring material that may be created by the passage of the ring's shepherding moon Prometheus. They can be seen casting narrow shadows that extend toward the bottom of the mosaic. The shadows are marked with arrows in the annotated version. From right to left in the mosaic, the blades of the "fan" appear darker than the surrounding material above the bright core of the F ring near the larger, diagonal channels created by the ring's shepherding moon Prometheus. The fans (marked "F" in the annotated version) can be seen as a series of channels within the F ring's particles that appear to have a common origin but that spread outward radially in different directions. Gravitational perturbations on the ring material by a moonlet or clump of material can create these fans. The moonlet or clump orbits more or less elliptically compared to the rest of the F ring. It is probably embedded in the ring and is causing the base of the fan channels to meet.
The diagonal streamer-channels are periodically created by the gravity of the potato-shaped moon Prometheus which is 148 km (92 miles) on its longest side but is on average 86 km (53 miles) across.
The images have been re-projected in this mosaic so that the F ring appears straightened rather than curved and compressed azimuthally (along the ring). This change represents a scale compression in the horizontal direction of about 33 to one which is why Prometheus looks like a bright line. Prometheus is marked "Pr" in the annotated version.
This sequence of 23 images was taken over about an hour. The earliest image is on the right, and time progresses moving left in the mosaic. Each image was re-projected and placed side by side in this montage. Scale in the original images was about 8 km (5 miles) per pixel. The images were contrast enhanced and re-projected to a scale of 33 km (21 miles) per pixel in the mosaic's horizontal direction and one kilometer (0.6 miles) per pixel in the mosaic's vertical direction. The single, cropped inset of the clumps included here was then magnified by a factor of two.
The view in the original images looked toward the northern, unilluminated side of the rings from about 54° above the ring plane.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on April 16, 2009. The view was acquired at a distance of approximately 1.3 million km (808,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 98°.
Credit: NASA/JPL/Space Science Institute
High resolution image
The view in the original images looked toward the northern, unilluminated side of the rings from about 70° above the ring plane.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 8, 2008. The view was obtained at a range of distances from approximately 597,000 km (371,000 miles) to 615,000 km (382,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 77°.
Credit: NASA/JPL/Space Science Institute
High resolution image
The view in the original images looked toward the northern, unilluminated side of the rings from about 26° above the ring plane.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on July 5, 2008. The view was acquired at a distance of approximately 1.1 million km (684,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 34°.
Credit: NASA/JPL/Space Science Institute
Cassini scientists using the ultraviolet imaging spectrograph previously detected thickened blobs near the F ring by noting when starlight was partially blocked. These objects may be related to the clumps seen by Murray and colleagues.
The newly-found F ring objects appear dense enough to have what scientists call "self-gravity." That means they can attract more particles to themselves and snowball in size as ring particles bounce around in Prometheus's wake, Murray said. The objects could be about as dense as Prometheus, though only about one-fourteenth as dense as Earth.
What gives the F ring snowballs a particularly good chance of survival is their special location in the Saturn system. The F ring resides at a balancing point between the tidal force of Saturn trying to break objects apart and self-gravity pulling objects together. One current theory suggests that the F ring may be only a million years old, but gets replenished every few million years by moonlets drifting outward from the main rings. However, the giant snowballs that form and break up probably have lifetimes of only a few months.
The new findings could also help explain the origin of a mysterious object about 5 to 10 km (3 to 6 miles) in diameter that Cassini scientists spotted in 2004 and have provisionally dubbed S/2004 S 6. This object occasionally bumps into the F ring and produces jets of debris.
"The new analysis fills in some blanks in our solar system's history, giving us clues about how it transformed from floating bits of dust to dense bodies," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The F ring peels back some of the mystery and continues to surprise us."
The late Kevin Beurle was made the honorary first author on this paper because of his contributions in developing software and designing observation sequences for this research. He died in 2009.
| Source: CICLOPS/Space Science Institute | |
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