Scientists Say Saturn's Moon Iapetus Was Cryogenically Frozen to Look Like a Giant Walnut

Unlike any other moon in our solar system, Iapetus is in the same shape today as it was when it was just a few hundred million years old; a well-preserved relic from the time when the solar system was just a baby.

Scientists were amazed to find a giant ridge girdling at least one-third of the moon's circumference giving it the appearance of being a giant walnut. Now researchers working with the Cassini spacecraft believe they have found an explanation for why Saturn's moon Iapetus has this strange distinctive ridge.

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Iapetus spun fast, froze young, and left behind a body with lasting curves," said Julie Castillo, Cassini scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. They go on to say that radioactive isotopes imply that Iapetus's age is 4.564 billion years old.

"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 figuring out how it ever became warm enough to form a bulge in the first place, and then what made Iapetus freeze.

The heat source had to have a limited life span, which would allow the moon's crust to rapidly become cold and retain its immature shape. The scientists concluded that the heat came from its rocks, which contain short-lived radioactive isotopes aluminum-26 and iron-60, which decay relatively quickly. 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.

Evidence for these same isotopes has been found in meteorites formed in the inner solar system. Therefore, there is a possibility of comparing the early chronology of the outer solar system with other objects in the inner solar system, such as Earth, Earth's moon and asteroids.

"This is the first direct evidence of the early spin history for a satellite in the outer solar system. It teaches us more about how the speed of a body's rotation influenced its evolution, and broadens our knowledge of the early history of outer planet satellites," said Matson.

On September 10th Cassini will make its closest flyby of the moon yet — just 1,000 kilometers (620 miles) above the surface, and will hopefully reveal more secrets.

Posted by Rebecca Sato

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