Scientists unlock secrets of Saturn's humongous dust ring

For decades, Saturn's moon Phoebe has been eyed as the imp behind a case of cosmic graffiti: an odd, dark splotch taking up about half the surface of its larger, brighter sibling Iapetus, which orbits much closer to Saturn.

Astronomical detectives had their suspicions about how the deed was done. Now, new measurements involving a gigantic dust ring around Saturn and associated with Phoebe are helping to fill in the picture.

Astronomers have for the first time mapped the full extent of the dust ring and have made an initial estimate of the size distribution of the ring's particles.

The ring is enormous, even by Saturn's standards. The Phoebe ring, mainly composed of tiny dust particles, is about 1.6 million miles thick with an inner edge that begins about 3.7 million miles from Saturn and extends roughly 6.4 million miles farther. This gives the ring an overall diameter of about 20 million miles, according to the new study, set to appear in Thursday's issue of the journal Nature.

The researchers find that the vast majority of particles are small – on the order of 10 millionths of a meter. That's small enough for the radiation pressure from sunlight to gently nudge them inward toward Iapetus, making what University of Maryland astronomer Douglas Hamilton whimsically dubs the "black rain from Phoebe" available to coat half of Iapetus.

Iapetus' orbit is tidally locked to Saturn; it presents the same face to the planet as it orbits. This also means the same hemisphere is always pointed in the direction the moon is traveling as it orbits. The current thinking holds that this black rain built up on the leading hemisphere, absorbing more heat than the moon's naturally icy surface would have. Over time, this triggered a runaway loss of ice on the leading hemisphere, exposing darker material underneath.

Beyond adding evidence that points the finger back at Phoebe for Iapetus' yin-and-yang color scheme, understanding the distribution of grain sizes can open a window on past conditions around Saturn, Dr. Hamilton adds. "Encoded in their distribution is a history of everything that's gone on in the outer Saturnian system for the age of the solar system," he says.

In principle, by looking at the current distribution of Saturn's more distant moons and the distribution of dust in the Phoebe ring, researchers could uncover "what was there when Saturn formed," he adds.

Understanding the properties of large planetary dust rings could help with the hunt for planets orbiting other stars, says Daniel Tamayo, a planetary scientist at the University of Toronto.

In short, the dust reflects more light as dust than it would lumped together as a planet because more surface area is available to reflect light.

The Phoebe ring today is extraordinarily dim. But when it was forming through repeated collisions among so-called irregular satellites with intersecting orbits early in Saturn's history, the ring would have been denser and brighter.

An extrasolar planet's dust ring could shine brightly enough to be detected from Earth, even if the planet it surrounds isn't bright enough.

One example may be Fomalhaut b, Dr. Tamayo says, an extrasolar planet 25 light-years away that was first imaged in 2008 using the Hubble Space Telescope.

"When people figure out how big this planet would have to be to explain the light we see, it would be 1,000 times the area of Jupiter, which makes no sense physically," he says. Instead of a direct image of a planet, Hubble may have spotted an irregular-satellite debris ring surrounding a planet.

Phoebe's ring was discovered in 2009 by a team led by University of Virginia astronomer Anne Verbiscer. Theoretical calculations suggested a dust ring should exist, given Phoebe's distance from Saturn and its vulnerability to collisions from meteors or other irregular satellites orbiting Saturn.

Based on the team's observations with NASA's Spitzer Space Telescope, they estimated that the ring was some 10 times larger than the largest known ring at the time.   

The latest analysis is based on images taken with NASA's Wide-field Infrared Survey Explorer. Its ability to capture relatively large expanses of sky in a single image allowed Hamilton and colleagues, including Dr. Verbiscer, to take the full measure of the ring. It's some 30 percent larger than the team's earlier estimate.

Last year, Tamayo and two colleagues reported detecting portions of the ring at visible wavelengths using an imager on the Cassini spacecraft, currently orbiting Saturn and its moons, and some painstaking processing. Currently the researchers are looking for dust in the region around Iapetus' orbit to see if the moon sweeps it up. 

If so, that would provide strong observational evidence that Phoebe is behind Iapetus' yin-and-yang appearance.