A speculative search for a belt of debris stemming from one of Saturn's outer moons has turned up what appears to be the largest known planetary ring in the solar system.
The newfound ring, associated with the far-flung moon Phoebe, stretches to roughly 12.5 million kilometers from Saturn, if not more, according to a paper announcing the finding in this week's Nature. (Scientific American is part of the Nature Publishing Group.) For comparison, the outer bound of Saturn's next largest known ring, the E ring, is less than half a million kilometers from the planet. The gas giant planet has seven main rings (named A to G, in order of discovery) made of ice, rock and dust, with even more gaps and divisions among them.
The researchers also presented their discovery this week at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Fajardo, Puerto Rico.
Using an infrared instrument on NASA's Spitzer Space Telescope, planetary scientist Anne Verbiscer of the University of Virginia (U.V.A.), along with U.V.A. astronomer Michael Skrutskie and University of Maryland College Park astronomer Douglas Hamilton, scanned part of the region between Saturn and Phoebe, which orbits some 13 million kilometers from its planet. The researchers detected thermal emission from a tremendously broad ring of debris, spanning nearly five million kilometers in the Spitzer data. (With incomplete coverage, Verbiscer and her collaborators also drew on observations taken by other researchers to expand the scope of the survey, but the ring could stretch beyond the limits of their available images.)
"This is huge," Verbiscer says of the ring. If it were bright enough to be visible from Earth, it would dwarf the size of the other objects in the night sky. "You could put one full moon on either side of Saturn," she says. "That's how big it is."
Two experts on planetary ring systems who attended the session in Puerto Rico in which Verbiscer and her colleagues presented their research found it convincing. "Seeing is believing, and there's really no doubt when you look at the pictures that there's this big fat ring there," says Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, Calif. "It's an absolutely perfect detection."
"I'm convinced by what I see," agrees Linda Spilker, a planetary ring scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif., adding that the alignment between the Spitzer imagery and the numerical modeling of what the ring should look like is compelling. As deputy project scientist for the Saturn-exploring Cassini probe, Spilker notes that it is not surprising that Cassini, which is armed with a cosmic dust analyzer, did not detect the diffuse ring during its encounter with Phoebe in 2004. "It's not a whole lot of particles," she says of the ring, noting that it would be easy to pass through without noticing.
The study's authors speculate that meteoric impacts on Phoebe's dark, heavily cratered surface liberate the particles that form the ring. That assertion might explain the anomalously two-toned surface of Iapetus, a Saturnian moon inside Phoebe's orbit. The smaller particles of the Phoebe-generated ring should migrate inward, where they would eventually be swept up by Iapetus, coating the inner moon's leading face with dark material—a prediction knocked about for decades that jibes with observation. The presence of the debris ring implies that this process is ongoing.
Faint rings such as that stemming from Phoebe could be relatively common, if difficult to detect, because most small satellites are subject to impacts that would kick up dust and debris. Similar rings populated by moon dust have been found elsewhere in the solar system. "This is a pretty common phenomenon. We've just never quite seen it on the scale of the Phoebe ring," Showalter says. Spilker notes that "wherever you find a moon and you have micrometeorite bombardment, you have a source of dust. And it's very possible that the dust could go on to form a ring."
Verbiscer's team nearly missed out on conducting the search near Phoebe: the Spitzer spacecraft ran out of cryogenic coolant in May, just three months after the Phoebe survey was taken. In the telescope's new "warm" state (still well below –200 degrees Celsius), many of Spitzer's instruments no longer work at full sensitivity, including the 24-micron infrared scanner on the Multiband Imaging Photometer that was used to detect the newfound ring. That mission shift means that follow-up observations will have to be conducted with other instruments or spacecraft that are likely less well-suited to the task.
The proposal for Saturnian ring-seeking time on Spitzer was submitted years ago but was not given high priority, Verbiscer says—perhaps because, as she notes, "it was completely speculative" to go looking for a possible ring.
"You can only observe Saturn with Spitzer every six months, and every six months there's about a 20-day observation window," she says. "We just kept missing the boat on the schedule during those observability windows, and this really was put into the last observability window for Saturn while the telescope still had cryogen left."