Cassini Spacecraft Photos Show Saturn's 'Blue Moon' in All Its Glory
A false-color view of Saturn's moon Rhea captured by NASA's Cassini spacecraft on March 2, 2010. This image shows the side of the moon that always faces the planet. Ultraviolet, green and infrared images were combined into a single picture that isolates and maps regional color differences. This "color map" was then superimposed over a clear-filter image that preserves the relative brightness across the body.
Credit: NASA/JPL/SSI

A NASA spacecraft has snapped some of the best-ever pictures of Saturn's moon Rhea, yielding clues about the satellite's recent tectonic rumblings.

NASA's Cassini probe captured the images — some of which shine bright in blue-and-green false color —on two recent flybys of Rhea, Saturn's second-largest moon. The photos show dramatic fractures cutting through craters on Rhea's surface, suggesting the satellite's interior churned and rumbled not too long ago, scientists said. [New Cassini photo of Rhea]

The pictures also reveal that Rhea bears a closer resemblance to another Saturn moon, Dione, than previously thought, researchers said.

"These recent, high-resolution Cassini images help us put Saturn's moon in the context of the moons' geological family tree," Cassini team member Paul Helfenstein of Cornell University said in a statement. "Since NASA's Voyager mission visited Saturn, scientists have thought of Rhea and Dione as close cousins, with some differences in size and density. The new images show us they're more like fraternal twins, where the resemblance is more than skin deep. This probably comes from their nearness to each other in orbit."

Looking for rings

Cassini scientists designed the two flybys — which took place in November 2009 and March 2010 — in part to search for a ring thought to encircle Rhea, researchers said. The photos were released this week.

During the March flyby, the Cassini spacecraft made its closest approach to Rhea's surface so far, swooping within 62 miles (100 kilometers) of the moon. Based on the probe's observations, scientists have now concluded that Rhea — which is 949 miles (1,528 km) across — doesn't currently have a faint ring above its equator.

The two recent close encounters nonetheless yielded unique views of other features on the moon, researchers said. Some images, for example, show a web of bright, wispy fractures resembling features that were first spotted on another part of Rhea by NASA's two Voyager spacecraft in 1980 and 1981.

At that time, scientists thought the wispy markings on the trailing hemispheres of Rhea and its neighboring moon Dione were possible cryovolcanic deposits, or the residue of icy material erupting from volcanoes. Trailing hemispheres are the sides of moons that face backward in their orbit around a planet.

The low resolution of Voyager's images prevented a closer inspection of these regions, however. But since July 2004, Cassini's cameras have captured pictures of the trailing hemispheres of both satellites several times, at much higher resolution.

These images have shown that the wispy markings are actually exposures of bright ice along the steep walls of scarps, or long cliff lines. The features actually result from tectonic activity, not cryovolcanism, researchers said.

Recent tectonic rumblings

In November 2009, Cassini's cameras captured Rhea's trailing hemisphere with unprecedented resolution. Scientists combined images taken about one hour apart to create a 3-D image of the moon's terrain, revealing a set of closely spaced troughs.

The 3-D image also shows uplifted blocks interspersed through the terrain that cut through older, densely cratered plains. While the cratered plains imply that Rhea has not experienced much internal activity since its early history — as that would have repaved the moon — the new photos suggest that some regions have ruptured in response to tectonic stress more recently, researchers said.

Troughs and other fault topography cut through several large craters, for example. These big craters are not too scarred with smaller craters, indicating that they are comparatively young. In some places, material has moved downslope along the scarps and accumulated on the flatter floors, researchers said.

A mosaic of the March 2010 flyby images shows bright, icy fractures cutting across the surface of the moon, sometimes at right angles to each other. A false-color view of the entire disk of the moon's Saturn-facing side reveals a slightly bluer area, likely related to different surface compositions or to different sizes and fine-scale textures of the grains making up the moon's icy soil, researchers said.

Making better maps

The new images have also helped scientists make better maps of Rhea, including the first cartographic atlas of features on the moon complete with names approved by the International Astronomical Union.

And the mapping work will go on. Cassini will continue to chart the terrain of Rhea and other Saturnian moons with ever-improving resolution, especially for terrain at high northern latitudes, until 2017, researchers said.

"The 11th of January 2011 will be especially exciting, when Cassini flies just 47 miles (76 km) above the surface of Rhea," said Thomas Roatsch, a Cassini imaging team scientist based at the German Aerospace Center Institute of Planetary Research in Berlin. "These will be by far the best images we've ever had of Rhea's surface — details down to just a few meters will become recognizable."