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