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The Sponge Moon

by Perry Pezzolanella

Until the dawn of space flight, the moons of the planets were nothing more than points of light. Only a few gave hints that they were different, but there was one that was so strange that it defied explanation for several years. Hyperion is the strange moon that orbits Saturn just outside of the orbit of its largest moon, Titan. When Cassini flew within 310 miles of Hyperion on September 26, 2005 it revealed a battered, irregularly shaped moon that looked like a giant sponge.

Hyperion is the most unusual moon of all as the rotational period is chaotic and the axial tilt is variable. If someone were standing on Hyperion, each day would be a different length that would be unpredictable and the sun angle would always be different due to the unstable axial tilt. This is a clue that Hyperion is probably a fragment of a larger body that was broken apart by a large impact a long time ago. The shape is like a badly damaged hockey puck or poorly made hamburger patty with dimensions of 203x161x133 miles. To add to its strangeness, it has numerous craters that are so sharp and steep that it looks like a giant sponge.

The bottoms of its craters have deposits of dark organic material that was thought to absorb heat, which made it melt deeper into the icy moon, but a more logical theory has evolved. Cassini flew close enough to Hyperion to determine that it is made mostly of water ice and carbon dioxide ice (dry ice) with a coating of hydrocarbons that darken it. The hydrocarbons are similar to what covers the dark areas of Iapetus and probably were swept up by Hyperion having originated from further out Phoebe. This debris has darkened it enough so that it reflects only 30% of the sunlight, much less than the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea closer in towards Saturn. The big discovery from the flyby was that it found Hyperion to be about 40% empty space. This porosity allows craters to remain nearly unchanged through time.

The critical factor in creating the sponge-like appearance of Hyperion is the low density or porosity. It is hardly more than half as dense as water, so the gravity is unusually low for a moon this size. Anything impacting Hyperion would actually form craters by compressing the porous outer surface layers rather than blasting out material compared to moons that are denser and have stronger gravity. The low gravity also means that any material that gets ejected from the craters has a good chance of escaping completely and not re-impacting the surface. This makes Hyperionís craters look sharper and deeper as they are not covered up by a blanket of fine debris.

Cassini is one of the most productive missions ever flown. The 2005 flyby is the closest that it will ever get to Hyperion, but there will continue to be several distant encounters until the mission ends in 2017. The superior equipment continues to make stunning discoveries that may seem mysterious, but also aids in understanding the processes behind what shapes the moons of Saturn.