Iapetus: Old Moon has Some New Tricks

No matter what it turns out to be, the ridge is just weird-looking
Check out the square craters

NOAA has a cool movie of Iapetus. If you watch, you can see "Hadrian's Wall" on the dark side.

PhysOrg is running a story that Iapetus might be one of the oldest moons in the Solar System. The story goes that Iapetus was made out of radioactive isotopes found only in the early solar system. When the moon finished being so hot, as it contracted the wall was formed.

The Saturn moon system is full of great oddballs, but my fave is Iapetus.
Who wouldn't love a moon that looks like Pac-Man?

Scientists now think the moon's bulging midriff and slow spin rate point to heating from long-extinct radioactive elements present when the solar system was born.

"We've modeled how Iapetus formed its big, spin-generated bulge and why its rotation slowed down to its present nearly 80-day period. As an unexpected bonus, Iapetus also told us how old it was," said Dennis Matson, Cassini project scientist at JPL. "You would expect a very fast-spinning moon to have this bulge, but not a slow-spinning moon, because the bulge would have been much flatter."

Scientists calculate Iapetus originally rotated much faster--at least five hours, but less than 16 hours per revolution. The fast spin gave the moon an oblate shape that increased the surface area (in the same way the surface area of a round balloon stretches when the balloon is pressed into an oblate shape). By the time the rotation slowed down to a period of 16 hours, the outer shell of the moon had frozen. Furthermore, the surface area of the cold moon was now smaller. The excess surface material was too rigid to go back smoothly into the moon. Instead, it piled up in a chain of mountains at the equator.

"Iapetus' development literally stopped in its tracks," said Castillo. "In order for tidal forces to slow Iapetus to its current spin rate, its interior had to be much warmer, close to the melting point for water ice." The challenge in developing a model of how Iapetus came to be "frozen in time" has been in deducing how it ever became warm enough to form a bulge in the first place, and figuring out what caused the heat source to turn off, leaving Iapetus to freeze.

The heat source had to have a limited life span, to allow the moon's crust to rapidly become cold and retain its immature shape. After looking at several models, scientists concluded that the heat came from its rocks, which contain short-lived radioactive isotopes aluminum-26 and iron-60 (which decay very rapidly on a geologic timescale). Since these elements decay at a known rate, this allowed scientists to "carbon date" Iapetus by using aluminum-26 instead of carbon. Scientists calculate the age of Iapetus to be roughly 4.564 billion years old.

Another Saturn odd-ball moon: giant bath-sponge Hyperion.
Some scientists think Hyperion may have given Iapetus all of its dark material

The jury is still out on Iapetus, however, and as I pointed out in my last article, there's just a lot of weirdness there. NewScientist caught up with a scientists who points out that there are still a lot of uncertainties.

In order to get enough heat from this process to explain its early softening, Iapetus would have had to have formed during a narrow window in the solar system's early history, when these short-lived radioactive materials are thought to have been abundant.

Based on this reasoning, Castillo's team has dated the formation of Iapetus to a 2.5 million-year period between about 4.565 and 4.562 billion years ago.

David Stevenson of Caltech in Pasadena, US, who is not a member of the team, says the model can explain the broad bulge at Iapetus's equator. But he says it does not adequately account for what caused the narrow ridge there.

He says Saturn took millions of years to form and suspects Iapetus coalesced after Saturn did. If so, that might have been too late to benefit from heating due to radioactive materials with a short half-life, such as aluminium-26. "I am not saying their model is wrong – it might be right," he told New Scientist. "I am simply saying that it is a lot to swallow."

Another hypothesis says the ridge formed when material from a ring orbiting the moon collapsed onto the surface.

So still nobody knows. You guys keep playing with your slide rules and get back to us with theory #2. Cassini's next close encounter with Iapetus will occur on Sept. 10, 2007, at 1,000 kilometers (620 miles) from the surface. Let's hope for some more cool pics!