NASA scientists think they might have a lead on where to find Saturn's missing moons: near rings recently discovered by the Cassini spacecraft.
In mid-September, the sun was poised behind Saturn, providing Cassini scientists with an unprecedented imaging opportunity.
Under the cover of the planet's shadow, the entire ring system became visible, and never-before-seen microscopic particles began to appear.
The spacecraft discovered a single, faint new ring at the orbits of two moonlets, Janus and Epimetheus. A second, narrow ring that overlies the orbit of the tiny moon Pallene was found a week later. A third and fourth ring are visible in the Cassini Division, the big gap in Saturn's main ring system.
Scientists suspect a moon might be lurking near one of the new rings.
"Just like the old maxim that says where there's smoke, there's fire, at Saturn, where there's a new ring, there's bound to be a moon," said Jeff Cuzzi, a Cassini scientist at NASA's Ames Research Center.
Saturn's smallest moons have weak gravity and cannot retain any loose material on their surfaces. When these moons are struck by rapidly moving interplanetary meteoroids, this loose material is blasted off their surfaces and into Saturn orbit, creating diffuse rings along the moon's orbital path.
Collisions among several moonlets, or boulder-sized rubble, might also lead to debris trails. For instance, Saturn's G ring doesn't seem to have any single moon large enough to see ─ it might have formed from a recent breakup of a moon.
"We are hot on the trail of these possible elusive moonlets," said Joe Burn, Cassini imaging scientist at Cornell University. "Finding the moons and learning about their interactions with the rings will help us understand how the moons formed and perhaps how the Saturn system formed."
The Cassini spacecraft was also able to take advantage of the unusual viewing alignment to note distinct color differences in the rings. These differences could indicate variations in compositions and in microscopic particles in the rings. They could also imply the particles are being sorted by size.
"The main rings show a neutral color, while the C ring is reddish, and the D and E rings are quite blue," said Phil Nicholson, another Cassini scientist at Cornell. "We don't quite understand if these variations are due to differences in particle size or composition, but it's nice to be surprised every once in awhile."