Slower spin 'made moon's bulge'

By Paul Rincon Science reporter, BBC News, The Woodlands, Texas

The Cassini flyby returned exquisite pictures of the bulge

Scientists have come up with a novel theory to explain the unexplained terrain on one of Saturn's icy moons.

The most striking feature of Iapetus is a bulging ridge, which encircles the moon's equator and reaches an altitude of 20km in places.

A new theory suggests the ridge formed when the moon went from a relatively fast-spinning body to one spinning more slowly.

Details were presented at the Lunar and Planetary Science Conference in Texas.

The results are based on computer models of the interior of Iapetus created by James Roberts and his colleagues at the Johns Hopkins University Applied Physics Laboratory (JHUAPL).

Referring to the ridge on Iapetus, Mr Roberts explained: "It looks like somebody screwed two halves of the moon together and did a very bad job soldering the joint.

The ridge is unparalleled in the geology of Solar System bodies. At up to 20km high in places, it dwarfs Mount Everest, which stands only 8km high. The Olympus Mons volcano on Mars stands some 25km tall.

But Mr Roberts points out that Mars is over four times the diameter of Iapetus: "Proportionally, this is the tallest feature in the Solar System with respect to its host body," he said.

Spin cycle

Today, the 1,500km-wide Saturnian satellite spins at a rate of about once every 79 days. But Mr Roberts' data suggest Iapetus could have been rotating as fast as once every 16 hours early in its history.

Over time, tidal interactions with Saturn caused the moon's rotation to slow down until it matched the time taken to complete one orbit of its planet.

This process is known as "de-spinning", reaching a condition where the moon always keeps the same face towards its host planet.

Spinning celestial bodies tends to flatten out at the poles and bulge at the equator. The amount of flattening is controlled by the object's rotation rate: if the rotation slows, the flattening decreases.

De-spinning can cause compression along the equator, but it cannot have formed the ridge on Iapetus because this compression is acting in the wrong direction.

The nature of Iapetus' surface colouring has also proved enigmatic

However, the computer modelling work carried out at JHUAPL shows that de-spinning also dissipates more heat at the equator than elsewhere.

Mr Roberts suggested that warm, buoyant ice rose to the surface from Iapetus' interior and pushed the brittle surface ice outward, forming a ridge around the equator.

The slowing down of Iapetus' spin is estimated to have taken 100 million years or so, at which point the heating stopped. As the moon cooled down, the ridge was frozen in place.

Extreme case

A number of other theories have previously been advanced to describe the development of the ridge.

One group has proposed that Iapetus oence had its own ring system, just as Saturn does today, and that these rings collapsed, falling on to the equator.

It has also been theorised that as the moon de-spun, forces created so-called thrust faults to appear in Iapetus, causing a ridge to pop up around the equator.

The idea of heat driving upwelling currents in the ice has already been proposed as a mechanism for pushing the surface up at the equator.

But Mr Roberts' theory shows that heat generated at the equator by de-spinning can result in the pattern of convection required to cause these currents in the ice.

Most moons are thought to have undergone de-spinning. But why this process should have caused a ridge around Iapetus, and not around the equatorial regions of other satellites, remains an open question, Mr Roberts told BBC News.

However, he added, going from 16-hour rotation to 79 days could mean Iapetus presents the most extreme case of de-spinning in the Solar System.