NASA's Cassini spacecraft has been tracking large clumps of ice particles in Saturn's bizarre F ring that break apart in slow motion.Saturn's extensive ring system was first observed by Galileo in the early 17th century, though he didn't realize what it was. Ever since, astronomers have developed an interest in the rings' origin, composition, and behavior. Now, nearly 400 years later, scientists are closer than ever to understanding the bizarre behavior of Saturn's highly active F ring, knowledge that could also advance models of how planets form.
Of all of Saturn's rings, the F ring — though thin and tenuous — is the most dynamic. Its structure is constantly changing due to wavelike kinks and ripples caused by the gravitational influences of two tiny moons, Prometheus and Pandora, that orbit just outside and inside the ring.
Adding to the ring's multiple features, never-before-seen elongated streaks — some stretching more than 90 miles (150 km) out from the ring — were recently identified in photos from the Cassini spacecraft.
Scientists are keenly interested in what causes the streaks, or "mini-jets". The team of scientists at Queen Mary University, London, that first identified these features suggests that they are composed of small, dispersed ice particles dragged along by much larger snowballs, roughly 1 km in diameter, that are moving within the F ring.
"The F ring is sort of in a boundary region between where Saturn's tidal forces are so strong that they prevent objects from growing through accretion and a little bit farther out, where everything accretes and forms moons," says Carl Murray (Queen Mary University of London), who presented the observations at the European Geosciences Union meeting in Vienna on April 24th.
The discovery of these showy snowballs argues that the F ring must contain a host of miniature moonlets. "It's direct photographic evidence of these otherwise unseen largish objects punching through the F ring," says Joshua Colwell (University of Central Florida), who was not involved with the study. These objects had been inferred from other observations, Colwell notes, "but this gives us a much better handle on their abundance."
The team found 570 mini-jet features in a catalog of 20,000 images. One was followed by Cassini for 7½ hours, during which it lengthened from 50 to 160 miles (75 to 250 km). By analyzing the time and distance, Murray and his colleagues calculated the relative speed of the objects to be just 1 or 2 meters per second.
These sluggish speeds might be the catalyst for formation of much larger satellites, says Bonnie Meinke (University of Colorado), who was not involved with the research. "A slow speed means that the material impacted [by a snowball that] moves radially through the ring has the ability to stick," Meinke explains, "and this is possibly how moons are created."
Although these homegrown moonlets consist of water ice, their collective motion can act as a proxy for how planets form from disks of gas and dust around stars. Despite the difference in composition, the physics controlling clumping and moon formation in Saturn's rings is the same as the physics responsible for forming planets, Murray says.
"We have the advantage of having an astrophysical disk on our doorstep that we can study in detail with Cassini and try and understand the physical parameters that we're seeing," he says. "And there are applications of that to understanding astrophysical discs in general."