Mohawk Valley Astronomical Society

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The Airy Moon

by Perry Pezzolanella

Some moons may look ordinary from a distance and even close up they may not appear much different, but flying a spacecraft close to one may reveal its history. Rhea is such a moon and it took a few close flybys of the Cassini spacecraft to make unexpected discoveries that were not visible to the eyes.

Rhea is the second largest moon of Saturn at 949 miles in diameter and is an icy moon consisting of about 25% rock and 75% water ice. Ice behaves like rock at Rhea’s frigid temperature of -300ºF. It is a heavily cratered moon where new craters can only form by destroying old ones. There are a few fractures and two large impact basins. The surface can be divided into two different regions based on crater density and size. The first area contains craters larger than 25 miles across, while the second area, located at parts of the poles and equator, has only craters that are less than 25 miles across. This indicates a major resurfacing event occurred some time during its early history.

Voyager 1 revealed bright, wispy streaks across the surface that were thought to be material ejected from fissures, but a close encounter by Cassini revealed that the wisps were actually fractures in the crust that exposed bright, icy cliffs. The wisps were the cliffs that were just beyond the resolving power of the Voyager cameras. The fractures indicate a warmer history for Rhea and as it cooled, the interior froze and expanded causing the crust to fracture. While Rhea may appear to be a larger version of Dione, two discoveries set them far apart.

The Cassini spacecraft flew close enough to Rhea to detect a disturbance in the flow of electrons trapped by Saturn’s magnetic field. This could be caused by dust and debris orbiting along Rhea’s equator. A cluster of small ultraviolet bright spots along the equator probably where particles de-orbited and impacted the surface strengthened the evidence for a ring. Further close flybys of Cassini at various angles found no evidence of ring material so there is now doubt that a ring exists unless another explanation is found.

The biggest discovery came on November 27, 2010 with the announcement that Rhea has a very thin atmosphere of oxygen and carbon dioxide. The ratio of oxygen to carbon dioxide is 5 to 2 but the density of the atmosphere is hardly a trillionth of Earth’s. The atmosphere was found when Cassini flew only 63 miles above the surface and flew right through it. The oxygen comes from a chemical reaction caused by Saturn’s magnetic field. The particles slam into Rhea’s water-ice surface which breaks down the surface ice and releases oxygen. The release of carbon dioxide is still a mystery, but Cassini will make several flybys of Rhea in the years ahead to try and solve this puzzle.

Rhea may have a water-ocean beneath the surface where the oxygen and carbon dioxide in the atmosphere could potentially be transported. This could enrich the ocean creating complex compounds that could promote life. Cassini is one of the most successful missions ever flown and has revealed the rich diversity of Saturn’s moons.