On May 31st, the Cassini spacecraft flew by Saturn’s funky moon Hyperion. The resulting images highlight the moon’s unusually pocked surface.

This image was taken with Cassini’s narrow-angle camera. Although Saturn’s other outer moons also have craters on their surfaces, Hyperion’s craters appear to be exceptionally deep. Astronomers have likened the moon’s appearance to a sponge or a wasp’s nest.
Credit: NASA / ESA

Hyperion is one of our solar system’s most intriguing objects. One reason is its unusually low density. Although it’s the largest of Saturn’s potato-shaped moons, with an average diameter of 270 km (170 miles, less than a tenth our Moon’s size), it has a density about half that of water. Due to this low density, and the high reflectivity of its craters’ sides, planetary geologists surmise that the moon is made largely of water ice.

Another interesting tidbit about Hyperion concerns its chaotic rotation. Most moons in the solar system rotate synchronously, which means that they always point the same side at their host planet. Our own Moon exhibits this behavior, which is why we can never see its farside from Earth. Hyperion, however, will face any of its sides toward Saturn at random. Planetary scientists recently discovered that Pluto’s moons Nix and Hydra are also rotating chaotically. Researchers are still searching for the cause of this rotation chaos.

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Hyperion’s porous appearance is also mystifying. Although the sides of the moon’s craters are bright, the bottoms look very dark. Some astronomers argue that the color and depth could be due to waste left over by volatile elements (those elements with a low boiling point, such as carbon dioxide and methane) when they were heated and vaporized. The resulting darkly colored sediment would soak up additional heat from the Sun, continuing the vaporization process and thus deepening the craters even more.

A close-up of Hyperion’s spongy surface. Note the bright crater edges and dark bottoms.
Credit: NASA / ESA

However, others argue that Saturn and its moons are too far from the Sun to explain the necessary heating. Instead (or, maybe, also), Hyperion potentially formed from the rubble left behind by a large impact. The miscellaneous pieces of ice and rock debris stuck together but didn’t have enough gravity to compact themselves. The result would be a porous, low-density body.

Cassini will continue to explore Saturn’s moons until 2017, when it will begin its final mission to fly in and out of Saturn’s ring system.


Image of Bob-dBouncier


June 9, 2015 at 6:47 am

I definitely tend toward seeing the pockmarks as out-gassing vents. That, and taking into account its odd rotation, along with the possibility of water, leads me to think it might be a captured comet. That the rotation remains chaotic, may mean the capture is fairly "recent".

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Image of john-tudek


June 9, 2015 at 9:51 am

Is it me or does this central area look like an old impact crater? Outer red circle would be the crater boundary. Inner red circle would be the central rise. https://flic.kr/p/umPyMm

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Image of Peter Wilson

Peter Wilson

June 9, 2015 at 11:38 am

That's always been my interpretation.

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Image of Tom-Hansen


June 9, 2015 at 1:44 pm

To me this is the most interesting surface on Saturn's moons. Rather than crash Cassini into Saturn when the mission is over, why not maneuver it into orbit around this mysterious little moon, and study it as long as possible, then land on its surface a la NEAR-Shoemaker at Eros?

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