Like midnight taggers, Saturn's moons Dione, Tethys, Mimas and Rhea may be spraying their unique signatures all over Saturn's environment when no one's looking. Or maybe not; they've never been caught in the act, unlike their sibling moon Enceladus, which has been repeatedly observed shooting a dramatic plume of ice vapor high above its surface.

Other than Enceladus, there are just a few active moons in the solar system. Icy geysers shoot from the surface of Neptune's Triton and Jupiter's Io is wildly alive with molten sulfur volcanoes. There is some evidence that Jupiter's Europa may be active, and a future mission is being planned to take a closer look. These rare worlds provide a window on the processes that shape different planetary environments.

Astronomer Bonnie Buratti of JPL is one of many Cassini scientists searching for signs of activity on the mid-size moons of Saturn. Evidence exists for something going on with some of these moons: two distinct streams of charged gas detected in the Saturnian magnetic environment appear to have originated at Tethys and Dione. Signs of past surface melting and gravitational squeezing of these moons point to the possibility that Tethys and Dione could be warmed by friction and therefore be active.

While no activity at these two moons has been directly observed by Cassini instruments, the search continues. Mimas and Rhea, too, are checked for activity when Cassini has an opportunity.

"Just because we haven't found it doesn't mean it's not there," said Buratti, a veteran explorer of the solar system's outermost bodies. She specializes in infrared observations that reveal the surface characteristics and chemistry of the satellites. "Any activity on these satellites might be sporadic, or maybe we just haven't gotten the right viewing geometry," she says.

"We are looking for direct evidence through imaging at very large phase angles, when things are kind of dark, so we can see things around the moon," said Buratti.

But so far, she added, nothing has been observed. A crescent moon, backlit by the Sun, provides one of the best angles for Cassini's imaging instruments to see any material rising above the surface. Solar backlighting illuminates plume or atmospheric particles in the same way dust on a windshield becomes apparent in the glare of sunlight, said Buratti. It's a view that can only be seen from a spacecraft in the Saturnian system.

Scientists hoping to find activity on one of these moons are heartened in their search by the 25-year wait to capture images and other measurements of the plume on Enceladus. The moon had been long-suspected of being an active body since 1980 when NASA's Voyager spacecraft returned the first Enceladus close-ups that showed signs of tectonic movement. Even before that, Enceladus, viewed through telescopes and the Voyager spacecraft, was as bright as new-fallen snow.

"Brightness was the first hint something might be going on with it," said Buratti. The fresh look made Buratti and colleagues wonder in 1988 whether ice particles from an unknown source might be "snowing" onto the surface.

Early in Cassini's mission, Cassini's magnetic field instrument saw signs that Enceladus was deflecting the magnetic field of Saturn differently than could be explained by the presence of Enceladus alone. On the basis of those findings, mission plans were changed to allow closer visits to this moon.

On the first of these closer flybys in 2005, Cassini's Composite Infrared Spectrometer found significantly elevated temperatures on Enceladus's surface along the "tiger stripes" that had been seen in images earlier around the south pole.

Armed with this evidence, Cassini's cameras took advantage of a more distant flyby with a back-lit Enceladus and long exposures and finally saw what Buratti calls "the smoking gun" of the huge plume erupting from Enceladus' south polar region.

Hoping to repeat their success of finding the plume at Enceladus, scientists have pored over Cassini's past observations of other mid-size moons around Saturn in search for subtle signs of activity that might have been missed when the data were first analyzed. Cassini instruments have repeatedly looked at the moons and the space around them in visible, infrared and ultraviolet wavelengths trying to see hotspots, telltale changes or plumes that would prove surface activity occurs.

Mimas appears to be subjected to tidal forces that could lead to activity, but Cassini has few good opportunities to study that moon in the extended mission. Faint rings may have been detected around Rhea, but as for signs of surface activity, Rhea has come up short, said Buratti. "We're pretty sure Rhea is dead."

Dione and Tethys are more promising. Magnetic field and particle-detecting instruments on Cassini are looking for evidence of faint atmospheres around these two moons, where traces of a possible atmosphere were detected by the visible and infrared mapping spectrometer.

Tethys and Dione also show evidence of recent resurfacing. Most interestingly, Cassini's plasma spectrometer found plasma streams of electrons apparently ejected from Tethys and Dione. This is the same type of evidence that was detected from Enceladus before the plume was discovered, said Buratti, so scientists remain optimistic in their hunt for another discovery of an active world. Studying active worlds in the Saturnian system can help astrobiologists determine if such moons around giant planets could harbour habitable environments.

For more information on this subject, materials from Burrati's Oct. 27, 2009, public teleconference on this subject can be found here.