Sponge Rock

Hyperion's highly pitted surface was seen up-close for the first time when Cassini flew less than 1,000 kilometers (620 miles) from Saturn's moon on September 26, 2005. The image scale is 362 meters per pixel.
Courtesy NASA/JPL/Space Science Institute.
Some pictures speak a thousand words. Others ask a thousand questions. Take for example this close-up image of Saturn's oblong-shaped moon Hyperion. Last September 26th astronomers flew the Cassini spacecraft within 505 kilometers (314 miles) of the icy, enigmatic object. The images beamed back to Earth had the experts scratching their heads. Cassini's cameras revealed a heavily pitted object that looks more like a sea sponge than a space rock.

Hyperion's density, around 0.6 gram/cm3, suggests that nearly half its volume is empty space. Its appearance, "shoulder to shoulder craters," says Cassini deputy project scientist Linda Spilker (NASA/Jet Propulsion Laboratory), points to a violent past. "It is clear from the Cassini images that Hyperion has been chopped up by some very large impacts," says Cassini team member Paul Helfenstein (Cornell University).

But astronomers know little else. Among the most notable features are dark deposits seen in the center of the pits. A similar effect is seen in high-altitude glaciers on Earth; dark materials can collect in small cavities, absorb sunlight, and melt their surroundings. The result is a sun cup. Over time the rims of the sun cups often intersect to form a honeycomb-like pattern. "Ultimately, a bizarre surface relief can develop where deep pits are surrounded jagged icy walls and pinnacles," says Helfenstein.

Cassini won't revisit Hyperion for a close flyby again in the spacecraft's primary mission, but astronomers will continue to image the moon from afar. "What I really want to know is what the other side looks like," says Spilker. But viewing the rest of the moon will be tricky. Hyperion has a chaotic rotation period thanks to the gravitational influence of Saturn's moon Titan, so you never know what side of the moon you'll see when you take a picture.

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