Stunning New View of Saturn’s Moon, Rhea


Newly released images of Saturn's second largest moon Rhea obtained by NASA's Cassini spacecraft show dramatic views of fractures cutting through craters on the moon's surface, revealing a history of tectonic rumbling. The images are among the highest-resolution views ever obtained of Rhea.

"These recent, high-resolution Cassini images help us put Saturn's moon in the context of the moons' geological family tree," said Paul Helfenstein, Cassini imaging team associate, based at Cornell University, Ithaca, N.Y. "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."

Cassini scientists designed the March 2010 and November 2009 flybys in part to search for a ring thought to encircle the moon. During the March flyby, Cassini made its closest- approach to Rhea's surface so far, swooping within 100 kilometers (62 miles) of the moon. Based on these observations, however, scientists have since discounted the possibility that Rhea might currently have a faint ring above its equator.

These flybys nonetheless yielded unique views of other features on the moon, including ones that are among the best ever obtained of the side of Rhea that always faces away from Saturn. Other views show a web of bright, "wispy" fractures resembling some that were first spotted on another part of Rhea by the two Voyager spacecraft in 1980 and 1981.

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

The low resolution of Voyager images prevented a closer inspection of these regions. Since July 2004, Cassini's imaging cameras have captured pictures the trailing hemispheres of both satellites several times at much higher resolution. The images have shown that the wispy markings are actually exposures of bright ice along the steep walls of long scarps, or lines of cliffs, that indicate tectonic activity produced the features rather than cryovolcanism.

The NASA Cassini spacecraft flew only 97 kilometers (60 miles) above the surface of  Rhea and found that it has an atmosphere composed of oxygen and carbon dioxide. This is the first time a moon of Saturn has been discovered to have an atmosphere. The spacecraft actually flew through the "tenuous" atmosphere held together by high-energy particles from Saturn that constantly hit the surface of Rhea.

The NASA spacecraft found that the atmosphere is very thin, with the oxygen in the atmosphere reported to be about five trillion times less than the oxygen within Earth’s atmosphere.

They report in the abstract to the paper the finding of a“tenuous oxygen-carbon dioxide atmosphere” that is “sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma.”

Dr. Ben Teolis of the Southwest Research Institute, Space Science & Engineering Division, who led the study says "As the magnetic field rotates around Saturn, particles carried in the field slam into the hemisphere of Rhea that's facing their flow. They hit that hemisphere and break water molecules on the surface. The atoms are then rearranging themselves to make O2 molecules, which are sputtered from the surface by additional impacting particles."

The Cassini researchers added that “The presence of CO2 suggests radiolysis reactions between surface oxidants and organics, or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.”

The research team state that such very thin (tenuous) atmospheres on such moons (and planets) could be very common in the Milky Way galaxy and other such galaxies within the universe.

After completing its mission in June 2008, the mission was renamed Cassini Equinox.
When it ended in September 2010, the spacecraft was renamed a second time to Cassini Solstice. The mission’s extension, which goes through September 2017, is named for the Saturnian summer solstice occurring in May 2017. The northern summer solstice marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere. Since Cassini arrived at Saturn just after the planet's northern winter solstice, the extension will allow for the first study of a complete seasonal period.

The new images have also helped to enhance maps of Rhea, including the first cartographic atlas of features on the moon complete with names approved by the International Astronomical Union. Thanks to the recent mission extension, Cassini will continue to chart the terrain of this and other Saturnian moons with ever-improving resolution, especially for terrain at high northern latitudes, until 2017.

"The 11th of January 2011 will be especially exciting, when Cassini flies just 76 kilometers [47 miles] 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."

Casey Kazan via NASA/JPL

Image: Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft. This image shows the side of the moon that always faces the planet. Credit: NASA/JPL/SSI

More information: The images, captured on flybys on Nov. 21, 2009 and March 2, 2010, can be found at , and .


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