It Pays to Nail Down Lawnchairs on Jupiter

On any given day, the winds on Jupiter blow 400 miles per hour - four times the speed of an average earthly hurricane. Nor is that all: Storms on the swirl-shrouded planet can last 300 years.

For more than a century, scientists have puzzled over what drives Jupiter's headstrong weather. Now an hour's worth of signals from a tiny space probe is solving what has been one of the solar system's great meteorological mysteries.

The latest results from the study of data sent back by the Galileo probe, which plunged into the planet's atmosphere on Dec. 7, show that Jupiter's spectacular wind stream and colorful circulation features such as the Great Red Spot are deeply rooted in the planet's interior.

Some astronomers have speculated that solar heating of a thin atmospheric region powered Jupiter's weather just as it powers weather on Earth. But the probe's 10,000-mile plunge took it well below the level of any possible solar influence. And the deeper it went, the stronger the winds blew.

"This shifts our whole thinking toward the idea that ... Jupiter's whole fluid interior is in motion just as rapidly as the winds at the surface," says meteorologist Andrew Ingersoll of the California Institute of Technology in Pasadena.

This means that, unlike earthly weather, Jupiter's meteorology is driven by the planet's own internal heat. Jupiter radiates 1.7 times more energy into space than it absorbs from the sun. That extra 70 percent is heat released as the planet continues the long cool-down from its formation 4.5 billion years ago.

Much of that primordial heat is concentrated in the planet's core. The deep fluid layers overlying the core are like a pot of water with a flame underneath. This heating from below powers a system of deep convention currents that, in turn, drive the planet's winds and storms.

Commenting on this finding as reported during the spring meeting of the American Geophysical Union, Dr. Ingersoll says it helps explain why Jupiter has 300-year-old storms, jet streams that last for centuries, and very stable weather generally. "You've got so much inertia behind it, because the interior is involved ... that it's just like a giant flywheel spinning forever," he says.

Galileo project scientist Torrence Johnson notes that the finding is partly a tribute to technological progress. Unanticipated progress in tracking technology made it possible for Earth-based antennas to track the probe's radio signal as it drifted with the wind. Most of the probe's communications went through the Galileo orbiter, the mother ship that released the probe before reaching Jupiter last Dec. 7. But when the orbiter's line-of-sight angle was obscured, the Earth antennas took over. Such Earth-based tracking was ruled "impossible" when the Galileo mission was planned in the late 1970s, since the signal coming directly from the probe would be too weak.

The Galileo team is continuing to study probe data, all of which now are on the ground. Meanwhile, the team is preparing for the Galileo orbiter to begin its two-year, 11-orbit dance among Jupiter's four major moons. On June 27, it will fly by Ganymede, passing within 560 miles of the icy surface. Images of Ganymede, which is roughly the size of the planet Mercury, should be coming in from early to mid-July.

Galileo scientists also continue to study the probe's data on Jupiter's composition. An early look at those data suggested Jupiter lacks the water scientists expected to find. But Tobias Owen of the University of Hawaii at Honolulu says the region sampled by the probe may not be representative of Jupiter, just as a probe dropped into Earth's atmosphere over the Sahara Desert would provided a skewed view of Earth.