Like The Force, Saturn's moon Iapetus has a light and dark side

Chuck Bednar for redOrbit.com – @BednarChuck

A new image captured earlier this year by the Cassini mission show how one of Saturn’s moons is like the force, the ubiquitous metaphysical power featured in the Star Wars movies, in that it also possesses both a light side and a dark side.

NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California on Monday, the new photo (which was taken in green light using Cassini’s narrow-angle camera on January 4) shows that Iapetus possesses dark/light asymmetry that scientists believe was created by material migrating away from the dark side of the moon.

Dark side of the moon

This asymmetry formed, they explain, due to what they refer to as the Thermal Runaway Model. Using a computer model, they demonstrate how Iapetus was originally uniformly covered in ice with some dark material mixed in. Additional dark material is then slowly added to the leading hemisphere, which is centered at 90 degrees west longitude.

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After a period of roughly 260 million years, the leading side has darkened and become slightly warmer, causing the ice to begin evaporating near the equator, where the temperatures are at their highest. As the ice continued to evaporate, additional dark material was left behind, and thus the surface area became increasingly darker.

By the time 1,200 million years had passed, a large dark region that is completely ice-free had formed on the leading side of Iapetus. After 2,400 million years, the dark region had grown to resemble the size and shape of the dark region depicted on an actual global mosaic map of the moon made from Cassini images captured back in December 2009.

Iapetus has a diameter of 941 miles (1,471 kilometers) and a density of just 1.2 times that of liquid water, leading some experts to suggest that it is only 25 percent rock and 75 percent ice. It orbits Saturn as a distance of over 2.2 million miles (3.5 million kilometers), and because of the distance, it is likely not affected by the tidal forces of Saturn or its other moons.

The face never changes

Despite the great distance, however, Iapetus has been tidally locked by Saturn, meaning that it always presents the same face towards its host planet. The moon also features an equatorial ridge or a chain of six-mile (10 kilometer) high mountains along its equator.

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The mountains were originally discovered by Voyager I and Voyager II, and are thus unofficially known as the Voyager Mountains. There are two theories as to how the ridge may have formed: they may have been formed at a far earlier time when Iapetus rotated faster than it currently does, or the mountains may be made up of remnant material from the collapse of a ring.

Iapetus was discovered by Italian astronomer, engineer, and mathematician Giovanni Cassini on October 25, 1671, but only appeared as a dark, bright dot until the Voyager missions of 1980 and 1981. It was named in honor of the Greek titan Iapetus by John Herschel, who wanted the moons of Saturn to be named after the mythical brothers and sisters of Kronus.

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