Giant "snowballs" have been discovered plunging through Saturn's outermost ring, creating glittering trails of ice dubbed mini-jets, researchers have announced.
The jets were uncovered in new images from NASA's Cassini orbiter, which has been touring the Saturnian system for the past seven years.
The colliding snowballs are formed as material in Saturn's F ring coalesces due to the gravitational pull of the nearby moon Prometheus. Scientists estimate that the icy bodies, including the one seen above, are each about 0.62 mile (a kilometer) wide.
Sometimes a snowball sails back through the F ring at a gentle clip of roughly 4 miles (6.4 kilometers) an hour, dragging along icy particles.
The resulting jets "are about 10 kilometers [6.2 miles] wide and extend out from the rings anywhere from 40 to 180 kilometers [25 to 112 miles], depending on their age, with their lengths increasing dramatically over just a few hours," said Carl Murray, a Cassini imaging team member based at Queen Mary University in London, England.
"At any given time we might expect to see about ten of these if we looked all the way around the F ring."
Image courtesy Caltech/SSI/QMUL/NASA
Cassini scientists had noticed bizarre jet-like trails in images of the F ring ever since the spacecraft arrived at Saturn in 2004. But without continuous coverage of these formations over several hours, their exact origins had been a mystery—until now.
Murray's group saw a tiny trail in a Cassini image from January 2009, and they were able to track the feature for more than eight hours. The footage confirmed that a small object from the F ring had created the trail. (Related Saturn picture: "Mystery Object Pierces a Ring.")
Murray and colleagues then made a systematic search of 20,000 Cassini images taken between 2005 and 2011. The survey turned up more than 500 examples of mini-jets, including the six seen above.
"Even though we don't have orbits for these ones, we have sufficient numbers to look at the statistics and see if these mini-jets are consistent with what we know from the one for which we now have an orbit," Murray said.
Image courtesy Caltech/SSI/QMUL/NASA
Using pictures from Cassini's narrow-angle camera, scientists were able to track the snowballs at the ends of the mini-jets, such as the one above, as they plowed through Saturn's F ring.
"We have yet to obtain resolved images of these objects," Murray said.
"However, when we see an obvious 'head' at the end of the mini-jet trail, it tends to have a fuzzy appearance"—suggestive of the halo that surrounds a comet—"produced, perhaps, from material on its surface or F ring core material."
In the 500 examples of mini-jets seen so far, most appear as isolated trails that are nearly perpendicular to the ring itself. But sometimes, as seen in the above set of images, the jets can appear in groups and can create more distorted trails.
The barb-like appearance seen in the upper right image, for example, was likely caused by a group of snowballs punching through the thin ring.
The upper left image shows disruptions in the ring material that are probably due to shearing as a single body pierces the ring's core multiple times, while the bottom left frame shows shearing only in the ring's outer edge.
The above image shows an exaggerated top-down view of the wiggles in Saturn's F ring, with the planet removed from the center.
Scientists believe that the ring's neighboring 53-mile-wide (85-kilometer-wide) moon Prometheus gravitationally tugs at the ring's icy particles as the moon orbits the planet, creating the ring's characteristic wavy pattern, as well as the giant snowballs.
"The F ring is unique in the extent and variety of phenomena it exhibits, and it has taken a lot of effort to understand them," Murray said. "But there are still many puzzles to solve and challenges to face."
The new analysis of mini-jets in Saturn's F ring was presented this week at the European Geosciences Union meeting in Vienna, Austria.