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Loony Moons: Chaos, Order and Strange Behavior

About the only thing the moons of our solar system have in common is a penchant for strange behavior. A pair of new studies shows that while a number of the more than 100 known satellites take predictable, orderly paths that hint at their origins, other moons are governed by total chaos. In between are all kinds of crazy antics.

Much of the lunacy is rooted in the method of a moon’s origin. If you are a planet, there are many ways to make a moon.

You can capture a small orbiting companion, as Jupiter and Saturn are suspected of doing more than once. Some of these objects orbit at odd angles and even backward compared to the larger, "normal" moons. You can wait for something to hit you, as Earth most likely did, then assemble the wreckage into a new object that will inspire legend and love.

Or you can craft the leftovers of your own formation into a few round objects and get them to synchronize their movements.

The latter approach might be what led to Io, Europa, and Ganymede, three of Jupiter’s biggest moons, developing the odd habit of lining up every now and then.

The end result is significant: The gravitational interactions during the alignments force the moons into non-circular orbits; the moons’ varying distances from the planet lead to so-called tidal distortions of the moons caused by Jupiter’s gravity. It’s much the same as Earth’s tides, which are distortions caused by the gravity of the Moon and Sun.

However, on the Jovian moons, the distortions are much greater. They heat the satellites the way a paper clip is heated when bent back and forth rapidly, explains Stanton Peale of the University of California at Santa Barbara.

The heating, in turn, fuels extreme volcanism on the inner moon Io and helps maintain a liquid sea -- with the chance for life -- beneath the frozen crust of Europa, the second moon out.

Order from chaos

Jupiter’s moons are able to line up because they all travel in the same plane of space, defined by Jupiter's equator. This alone suggests they probably formed out of material leftover from Jupiter’s formation, stuff that swirled in a disk around the giant planet’s midsection about 4.5 billion years ago.

But there’s more to the configuration.

Ganymede’s orbital period -- the time it takes to go around Jupiter -- is twice that of Europa. And Europa’s period is twice that of Io. These simple ratios allow the satellites to line up every few days or so, Peale said.

He and other scientists figure the orbits were random at first, and that they migrated into the present stable setup. But what caused the migration?

A new model, developed by Peale and his colleague Man Hoi Lee, suggests the changes involved gravitational interactions between the moons and the disk of planetary debris from which they were, presumably, born.

"This special arrangement of the inner three Galilean satellites is very unlikely to have occurred by chance," Peale said in an e-mail interview.

The new model, detailed last week in the journal Science, means that the synchronization must have taken place shortly after Jupiter’s formation and did not involve gravitational interactions with Jupiter itself, as other models have suggested. And it lends further support to the near certainty that the Galilean moons, named for the man who discovered them in the early 1600s, indeed formed along with the planet and were not captured later.

The model is based on schemes first proposed by another researcher more than a decade ago. But that initial work offered no means of forming the orbital setup, Peale said.

"Our paper showed that there is a way," he said, "and established the credibility of a primordial origin, which was generally believed by few scientists before."

More strange behavior

Moon mannerisms get stranger.

Earth’s moon, for example, is moving away from the planet. Every year, it shifts another inch-and-a-half (4 centimeters) into space. One day, a total eclipse of the Sun will no longer be possible.

The drifting is caused by tides. As the Moon orbits Earth, its gravity helps generate an ever-shifting tide (the Sun contributes to this, too) that goes around the planet. If you could draw a line from the center of Earth to the center of the Moon, you’d see that the high tide facing the Moon is actually just a bit ahead of the Moon along this line. Why? Because Earth spins on its axis (once a day) faster than the Moon is able to orbit the planet (once a month). Friction pushes the high tide ahead.

The tide, in turn, has a tiny gravitational effect on the Moon. It pulls the satellite forward and, according to the complex mechanics of orbiting objects, into a higher orbit.

Some moons go the other way.

Mars has two moons, Phobos and Deimos. Phobos is getting closer to Mars every day. In about 50 million years it will either crash into Mars or be torn apart by gravity. Shreds of Phobos might then form a ring around Mars, similar to the rings of Saturn.

Saturn’s shifty satellites

Around Saturn, there are some truly loony moons whose capers -- including a propensity to herd rings like sheep -- have puzzled astronomers for decades. More recently, two of these moons have been found in places they were not supposed to be.

One of Saturn’s rings, called the F-ring, is confined by two shepherding moons. Pandora rides herd on the outside, and Prometheus takes charge from within. The moons tug on the ring’s material, confining the dust and small rocks to a narrow band.

The Voyager spacecraft discovered the moons in the early 1980s, and observations allowed scientists to project the satellites’ orbits into the future.

When the future arrived, however, the Hubble Space Telescope found that the moons were not in the predicted locations. Pandora is about 100,000 miles (160,000 kilometers) farther around in its orbit than expected, and Prometheus lags behind by about the same amount.

The reason: total chaos, according to a new study that will be published in the journal Icarus. The research builds on several other studies done recently that have verified the positions of the moons.

All other moons in the solar system are on predictable orbits, so far as astronomers have been able to determine. But two decades ago, Nicole Rappaport of NASA's Jet Propulsion Laboratory and colleague Peter Goldreich of Caltech predicted that the orbits of Pandora and Prometheus might be chaotic.

Completely different course

Chaos is a mathematical theory that suggests small changes in initial conditions of any phenomenon can bring about large changes down the road. It’s been popularized by the notion that a butterfly flapping its wings on one continent might ultimately bring rain to another part of the world.

Rappaport explains chaos this way: Some people might map out their lives from early childhood and do exactly as planned. Others might lay similar plans but take a completely different course because of a comment made by an elementary school teacher.

With Saturn’s two little moons, chaos plays a role because their orbits are not perfect circles. The initial conditions observed by Voyager have been altered, by very tiny amounts, in the intervening years.

Here’s what happens:

Because the moons orbits are of different lengths, they gather on one side of Saturn now and then and give each other a gravitational kick. That much is predictable. But the elliptical orbits dictate that the distance between the moons is slightly different on each pass. The strength of the gravitational kick varies each time, the researchers say. Chaos ensues.

Rappaport said it’s rather convenient this chaos could be observed on a human time scale.

"As far as we know, all other moons are in predictable orbits," she told SPACE.com. "We might find theoretically that other moons are on chaotic orbits, but this might not be observable because the time scale of the chaos might be very long, or the extent of the chaos might be too small."