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Moon’s Ridges May Have Formed As Spinning Slowed

March 27, 2009

Astronomers have theorized that the unexplained terrain on one of Saturn’s icy moons might have happened when the moon went from a relatively fast-spinning body to one spinning more slowly, BBC News reported.

The bulging ridge of Iapetus, which encircles the moon’s equator and reaches an altitude of over 12 miles in certain areas, was discussed in detail during a presentation at the Lunar and Planetary Science Conference in Texas.

James Roberts and colleagues at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) used computer models of the interior of Iapetus to produce the results.

“It looks like somebody screwed two halves of the moon together and did a very bad job soldering the joint,” Roberts said of the ridge on Iapetus.

Among the geology of Solar System bodies, experts say the ridge of Iapetus is unlike anything astronomers have ever seen.

Some of the ridges are larger than Mount Everest, which stands just under 5 miles. On Mars, the Olympus Mons volcano reaches over 15 miles high in places.

However, Roberts said 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.

Early in its history, data suggest Iapetus could have been rotating as fast as once every 16 hours. But Roberts said that today the 932 mile-wide Saturnian moon spins at a rate of about once every 79 days.

He added that tidal interactions with Saturn caused the moon’s rotation to slow down over time until it matched the time taken to complete one orbit around Saturn.

Astronomers call this process “de-spinning”, a condition where the moon always keeps the same face towards its host planet.

When celestial bodies spin, they tend 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.

Compression along the equator is often a result of the de-spinning process, but experts say it cannot have formed the ridge on Iapetus, since the compression is acting in the wrong direction.

But the JHUAPL computer modeling work shows that de-spinning also dissipates more heat at the equator than anywhere else.

That warm, buoyant ice likely rose to the surface from Iapetus’ interior and pushed the brittle surface ice outward, forming a ridge around the equator, Roberts said.

Experts estimated it probably took around 100 million years or so for Iapetus to slow its spin, at which point the heating stopped. The ridge was then frozen in place as the moon cooled down.

Other theories suggest that Iapetus once had its own ring system, just as Saturn does today, and that these rings collapsed, falling on to the equator.

Or it is possible that as the moon de-spun, forces created so-called thrust faults to appear in Iapetus, causing the ridge to form along the moon’s equator.

Heat driving upwelling currents in the ice is another theory that has been proposed as a mechanism for pushing the surface up at the equator.

However, heat generated at the equator by de-spinning can result in the pattern of convection required to cause these currents in the ice, according to Roberts’ theory.

The de-spinning process was likely a part of the formation of most of the moons astronomers know about.

But Roberts told BBC News it still remains unknown as to why this process would have caused a ridge around Iapetus, and not around the equatorial regions of other satellites.

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

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