Iapetus Fights the Battle of the Bulge

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Among Saturn’s many moons is Iapetus, an icy sphere that looks a lot like the Death Star when viewed through just the right telescopic lens (although sister moon Mimas is better known as the Death Star’s doppelgänger). That’s partly due to the unusual coloration: half of Iapetus is dark, the other half is light. In fact, it’s sometimes called the yin and yang of the Saturnian moons.

That two-toned coloration is just one scientific mystery surrounding Iapetus. Scientists have also puzzled over the moon’s bulging waistline and strangely squashed poles, giving it an odd shape falling somewhere between a sphere and an ellipse — shapes more typical for a moon. It looks a bit like a walnut. There’s also an unusual equatorial ridge, high enough to further distort the moon’s shape, at least when you look at it from a distance.

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Scientists have some good ideas about the unusual coloration of Iapetus, and now researchers at the Southwest Research Institute in Boulder, Colorado, think they’ve hit on an explanation for the bulge and ridge.

Famed Italian/French astronomer Giovanni Cassini discovered Iapetus back in October 1671, first observing it when it was on Saturn’s western side, but he wasn’t able to see the object on the planet’s eastern side — at least not until 1705, when telescope resolution improved sufficiently that he was finally able to see the moon’s lighter side. He correctly surmised that this was due to light and dark hemispheres. And because Iapetus is tidally locked with Saturn, the bright side always appeared to the west and the dark side to the east of Saturn.

The current theory about this unusual coloration favored by planetary geologists is that the moon’s dark hemisphere is due to a chemical residue left over when water ice sublimates — that is, when ice goes directly from a solid to a gas without going through the middle water phase first.

The dark side started sublimating more quickly than the light side some one billion years ago, leaving a dark residue, while ice condensed on the light side. And it’s an ongoing cycle, too, since the dark hemisphere absorbs more sunlight, thereby increasing the temperature and hence the rate of sublimation.

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As for the bulge and ridge, Harold Levison and his SRI colleagues have concluded that it all comes down to the oddities of orbital dynamics. A little rapid spinning early in its history (once every 16 hours or so), a collision with another large moon — hey, it is kinda crowded around Saturn — a huge spray of ejecta shooting into orbit, and voila! you’ve got a squashed-looking walnut-shaped moon with a bulge in its middle and strange ridge jutting out along its equator.

Okay, it’s not quite that straightforward. A key factor is that there is a boundary of sorts known as the Roche radius. At a particular distance (the Roche radius) from an object like Iapetus, any dust or rubble lying about condenses into a solid body, i.e., a moon.

Levison et al. surmise than when Iapetus collided with that other moon, some of the ejecta shot out beyond the Roche radius and escaped its gravitational pull. But the rest of it stayed inside the radius, and instead of forming a solid body, it formed a ring instead. Over time, this ring collapsed onto the moon’s surface, forming the ridge.

One question their hypothesis doesn’t answer: why the ridge follows the moon’s equator so perfectly; it’s pretty much confined to the Cassini Regio region on the dark hemisphere. There is also evidence that the ridge shows tectonic faults, which is inconsistent with a collapsed-ring hypothesis. So as always the case with novel explanations for unusual phenomena, we await further observation and data to confirm (or disprove) this latest hypothesis.

Image credit: NASA

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