SAN FRANCISCO – The bizarre ring of mountains encircling Saturn's moon Iapetus may be the fractured remains of an icy sub-moon.
Iapetus's spiky belt, which stretches all the way around the moon's equator, has baffled astronomers for decades.
"In my opinion it's the weirdest thing in the solar system," said planetary scientist Andrew Dombard of the University of Illinois at Chicago here at the American Geophysical Union meeting Dec. 15.
The ridge's mountains stretch up to 12 miles high, more than twice as high as Mount Everest. The mountain range is "ramrod straight, and sits exactly on the equator," Dombard said. "There's nothing like this anywhere else in the solar system."
The usual ways rocky worlds form mountains don't work for Iapetus, he says. If tectonic plates crunched together and folded the landscape above them, the way mountain ranges like the Rockies and Himalayas formed on Earth, then Iapetus should have other mountains north and south of the equator.
Some have suggested the ridge could have come from a ring, much like Saturn's famous rings, that collapsed onto the moon's surface. If that were true, though, other icy moons like Rhea and Jupiter's moon Callisto should also have spiked collars, but they don't. Iapetus also probably lacks the gravitational oomph to pull material from Saturn's rings to form its own.
But the rings could have come from inside Iapetus itself. In a talk at the AGU meeting, Dombard suggested that Iapetus could have once had a sub-moon knocked from its surface by a colossal impact. The Earth's own moon and Pluto's moon Charon are thought to have formed the same way.
Depending on its mass and distance from Iapetus, the sub-moon could have orbited comfortably for 100,000 to millions of years, Dombard said. But the sub-moon would slowly lose energy and spiral in toward Iapetus. Eventually, tidal forces from the larger moon would shred the sub-moon into hundreds of tiny moonlets.
Those chunks of ice and rock would briefly orbit Iapetus's equator as a ring, then rain down on the moon's surface to build the mountain range.
"This is the best explanation so far," said planetary scientist Cynthia Phillips of the SETI Institute in Mountain View, California, who was not involved in the new work. "It seems like a very strange case, but it's a very strange feature."
A similar idea was recently proposed to explain the origin of Saturn's rings: Saturn may once have had a big icy moon that broke up into the gas giant's famous rings and several smaller moons.
"It's a complimentary idea," Dombard said.
Dombard and colleagues haven't done rigorous simulations to prove their idea right, though they hope to soon. But Saturn's moon Rhea provides some supporting evidence. Earlier this year, planetary scientist Paul Schenk of the Lunar and Planetary Institute in Houston and colleagues found that Rhea has a line of splotchy blue streaks around its equator, which they suggest was formed by a collapsing ring. (Schenk also made the movie of flying over Iapetus's ridge, above, using data from the Cassini orbiter.)
"The same process happened on Iapetus, but taken to its ridiculous extreme," Dombard said.
The similarity to Rhea convinced Schenk that Dombard's model is believable. "I became a convert," he said.
Video: Paul Schenk, Lunar and Planetary Institute, Houston
See Also:
- Saturn's Bizarre Moon Iapetus Yielding Black-and-White Answers
- More Saturn Moon Close-Ups: Two-Tone Craters, Mountains Six Miles High
- Dying Moon May Have Spawned Saturn's Rings
- Giant Waves Solve Saturn Ring Mystery
- Video: Moon Builds Snowballs in Saturn Ring
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