Did Iapetus Have Its Own Mini Moon?

Article written: 14 Dec , 2010
Updated: 24 Dec , 2015
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There’s a new theory for why Saturn’s moon Iapetus looks like a walnut. The moon has a mysterious large ridge that covers more than 75 percent of the moon’s equator. Figuring out the reason for the ridge, say researchers from Washington University in St. Louis, has been a tough nut to crack. But they propose that at one time Iapetus itself had its very own moon, and the orbit of this mini-moon-around-another-moon would have decayed because of tidal interactions with Iapetus, and those forces would have torn the sub-satellite apart, forming a ring of debris around Iapetus that would eventually slam into the moon near its equator.

This is not the nuttiest proposal ever…

A closeup of Iapetus' ridge. In 2007, Cassini flew within a few thousand kilometers of Iapetus' surface to take this dramatic image. Credit: NASA/JPL/SSI

The ridge on Iapetus is 100 kilometers (62 miles) wide and at place, 20 kilometers (12 miles) high. (The peak of Mount Everest, by comparison, is 8.8 km (5.5 miles) above sea level.) Iapetus itself is 1,470 km across, and is the 11th largest moon in the Solar System.

Professor William McKinnon and his former doctoral student, Andrew Dombard — now from the University of Illinois Chicago — came up with this idea.

“Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet, one after the other, like frozen baseballs,” said McKinnon. “Particles would impact one by one, over and over again on the equatorial line. At first the debris would have made holes to form a groove that eventually filled up.”

“When you have a debris ring around a body, the collisional interactions steal energy out of the orbit,” Dombard said. “And the lowest energy state that a body can be in is right over the rotational bulge of a planetary body — the equator. That’s why the rings of Jupiter, Saturn, Uranus and Neptune are over the equator.”

“We have a lot of corroborating calculations that demonstrate that this is a plausible idea,” added Dombard, “but we don’t yet have any rigorous simulations to show the process in action. Hopefully, that’s next.”

Other ideas for how the ridge was created are volcanism or mountain-building forces.

“Some people have proposed that the ridge might have been caused by a string of volcanic eruptions, or maybe it’s a set of faults,” said McKinnon. “But to align it all perfectly like that — there is just no similar example in the solar system to point to such a thing.”

Dombard said there are three critical observations that any model for the formation of the ridge has to satisfy: Why the feature is sitting on the equator; why only on the equator, and why only on Iapetus.
Dombard says that Iapetus’s Hill sphere — the zone close to an astronomical body where the body’s gravity dominates satellites — is far bigger than that of any other major satellite in the outer solar system, accounting for why Iapetus is the only body known to have such a ridge.

“Only Iapetus could have had the orbital space for the sub-satellite to then evolve and come down toward its surface and break up and supply the ridge,” he says.

Dombard will make a presentation on the preliminary findings Wed., Dec. 15, 2010, at the fall meeting of the American Geophysical Union in San Francisco. The team also included Andrew F. Cheng of the Johns Hopkins Applied Physics Laboratory, and Jonathan P. Kay, a graduate student at UIC.

Source: Wash U

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4 Responses

  1. ricktheruler says

    Richardo Brown, A.A A.S
    I sent a message in space on CD Rom attached to the Casinni/Huygens satelitte. I would like to know the geollogy aspect of this world Lapetus. did it get it’s origin from a gas giant, did it develop from the bombardment of astroids from the asteroid belt which magma fused together to creat this moon.

  2. Torbjorn Larsson OM says

    Why the feature is sitting on the equator;

    Not a test if the feature is (results in) a mass above the geoid, I think; wouldn’t it _become_ the equator eventually?

  3. JimmerSD says

    I think it looks like Audrey II. “Feed me Seymour!”

  4. Run says

    I wonder if the mini-moon had a mini-mini-moon. Is there a ridge on the ridge?

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