Jovian Jolt : Moon Io Heads for Collision With Planet--in 2 Billion Years

The active volcanoes that set Jupiter’s mysterious moon Io apart from all other satellites in the solar system are caused by internal heating as it slowly spirals in toward the planet on a collision course, scientists disclosed here Tuesday.

They said that, unlike Earth’s moon and nearly all of the other satellites in the solar system, Io is gaining speed--and thus is getting inexorably closer to Jupiter.

“It’s a shocking thing,” said Samuel J. Goldstein Jr., a University of Virginia astronomer, during the annual meeting of the American Astronomical Society here.

“Everybody thought Io was bound to be moving out,” because that is the pattern throughout the solar system, including the Earth’s moon, he said.

Io will eventually plunge into the Jovian atmosphere in about 2 billion years. Nobody is quite sure what will happen then.

Goldstein and astronomer Kenneth C. Jacobs of Hollins College in Roanoke, Va., said they arrived at their conclusions about the cause of Io’s unique volcanism indirectly.

After discovering that Io is drawing closer to its planet--rather than drifting away--they had to explain what happened to the loss in energy resulting from the smaller orbit. They said most of it apparently became thermal energy, heating up the satellite and causing the volcanic activity.

Io has fascinated astronomers for centuries, and that fascination intensified greatly after two American spacecraft, Voyagers 1 and 2, sent back photos of Io several years ago showing intense volcanic activity not found on any other satellites and not rivaled even on any of the planets. Io, which is about the size of the Earth’s moon, has many active volcanoes that spew plumes of materials to more than 150 miles above its surface.

Io has been studied ever since it was discovered by Galileo in 1610. But it was only recently that scientists were able to determine that the satellite is speeding up and thus closing in on Jupiter at the rate of about five inches per year.

“We’re convinced it’s real,” Goldstein said.

According to Goldstein, two astronomers from the court of France’s King Louis XIV made 50 observations of Io during the decade beginning in 1668. Two centuries later, astronomers in South Africa and at Harvard University made a total of 501 observations.

By comparing current observations with earlier ones, Goldstein and Jacobs concluded that Io is accelerating, albeit very slowly. The increase is so slight that the time it takes Io to travel around Jupiter is decreased by only 71 millionths of a second per year, they said in a paper delivered here.