From the January 2016 issue

Couldn’t Hyperion’s low density and spongy texture be better explained by it being a captured comet?

Jose Gonzalez, Bethlehem, Pennsylvania
By | Published: January 25, 2016
NASA’s Cassini Saturn orbiter snapped this image of Hyperion during its final close-up with the odd moon on May 31, 2015. Astronomers have long been puzzled by the satellite’s spongelike appearance.
Saturn’s moon Hyperion presents a puzzle to planetary scientists with its spongelike surface, which looks more like a sea creature than a rocky satellite. It’s also oddly large for such an irregularly shaped moon, with an average diameter of about 170 miles (275 kilometers).

NASA’s Cassini Saturn orbiter was the first to see Hyperion in sufficient detail to make out its strange craters. And over the past decade, Cassini also has helped make the origins of this alien object much less mysterious.

The moon’s minuscule gravitational pull on Cassini shows Hyperion is made mostly of empty space. And like the rest of Saturn’s moons, ice is its primary building block with little rock, so its density is quite low. That helps explain the satellite’s craters. Most impact craters we’re familiar with form after an object smacks into a body’s surface and sends out an explosive shock wave that blasts material in a circular pattern. But on Hyperion, astronomers suspect impactors actually compress the weak surface, and the ejected material is blown off the moon entirely. And because the satellite is so porous, the craters remain pristine.

That faint gravity also helps explain Hyperion’s chaotic rotation, which resembles the bizarre movement New Horizons saw with some of Pluto’s moons.

But could it be a comet? At this point, that seems unlikely. The moon is several times larger than any comet seen before. However, astronomers do see it as cometlike. The dust-covered moon has a similar makeup as comets and has been repeatedly bombarded by debris over the years. It also never had to cope with the Sun’s harsh rays in the inner solar system. Astronomers suspect Hyperion formed in an icy impact, which left the moon as a thin rubble pile without the needed density to completely compact.

Eric Betz
Associate Editor