Cassini took theon Saturday (March 9) during its fourth and final planned encounter with the Saturn moon. During the encounter, the probe flew within just 620 miles (997 kilometers) of Rhea, which is Saturn's second-largest satellite."Take a good, long, luxurious look at these sights from another world, as they will be the last close-ups you'll ever see of this particular moon," Cassini imaging team lead Carolyn Porco, of the Space Science Institute in Boulder, Colo., said in a statement accompanying the photos.
Saturday's flyby was designed primarily to measure Rhea's gravity field, mission scientists said. But Cassini also managed to take 12 pictures of the frigid moon's battered, pockmarked surface, including one that showcases a mysterious long, curving fracture called a graben.
Rhea is the second-largest of Saturn's 60-odd known moons, with a diameter of 949 miles (1528 km). It's far smaller than the ringed planet's biggest natural satellite, , which at 3,200 miles (5,150 km) across is nearly 50 percent wider than Earth's moon.
Rhea was discovered in 1672 by the mathematician and astronomer Giovanni Domenico Cassini, who gave his name to the NASA mission currently studying the Saturn system.
In 2010, researchers determined that the moon has a wispy atmosphere dominated by oxygen and carbon dioxide. The oxygen likely was blasted free from water ice on Rhea's surface by charged particles streaming from, scientists say, but the origin of the carbon dioxide is more mysterious.
The Cassini mission a joint effort involving NASA, the European Space Agency and the Italian Space Agency launched in 1997 and arrived at Saturn in 2004. It has been studying the ringed planet and its many moons ever since, and will continue to do so on an extended mission until at least 2017.
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