BOSTON — For Earth's meteorologists, weather on Jupiter is more than a curiosity. Its mysteries test their basic understanding of how planetary weather systems work. Thanks to the Galileo spacecraft's scrutiny of Jupiter and recent laboratory experiments, those mysteries have begun to yield their secrets.
Andrew Ingersoll of the California Institute of Technology - lead Galileo meteorologist - explains that, in essence, Jupiter is a just a big ball of hydrogen and helium with decades-old storms. Alternating bands of jet-stream winds blow at speeds averaging around 250 miles an hour. Dr. Ingersoll notes that meteorologists stare at this Jovian weather and "would like to know what's going on." But poorly understood features like the Great Red Spot, which persists for centuries, stare back, mocking their ignorance.
Meteorologists need to know what supplies the system's energy, how that energy flows through the weather machine, and what determines the spectacular displays.
The relative importance of solar heating versus internal heating is a fundamental issue. Jupiter is cooling down from its formation 4.5 billion years ago. It radiates 70 percent more energy than it receives from the sun.
The Galileo project's probe that plunged into Jupiter last December found the planet's raging winds blowing at full intensity deep beneath the visible surface. That implies that heating from below is important.
Peter Olson at Johns Hopkins University and Jean-Baptiste Manneville, a recent Hopkins visiting scientist, put that suggestion to a laboratory test. They filled a plastic model of Jupiter with colored water. They spun the sphere fast enough for the outward directed centrifugal force to simulate the strength of Jupiter's inward directed gravity. Then, as Dr. Olson explains, "since we've reversed gravity, we've also reversed the temperature gradient." They chilled the sphere's core and heated its exterior.
As the sphere spun, a banded weather pattern similar to that on Jupiter appeared in this upside-down experiment. This reinforces the notion that a temperature difference between the planet's core and surface maintained by interior heat drives Jovian weather. This is quite different from terrestrial weather where solar energy is all important.
Computer simulations at Columbia University reported in Science magazine in July, however, raise a caveat. In these experiments, James Cho and Lorenzo Polvani ignored heating. They modeled the planet as a smooth sphere overlain by a shallow layers of turbulent fluid. For basic data, they used only the observed values of Jupiter's radius, rotation rate, average wind velocity, and average thickness of its weather-generating fluid layers. As the simulations ran, Jupiter-like atmospheric bands and jet stream winds appeared spontaneously. In an accompanying commentary, Peter Gierasch of Cornell University observed that, if verified, the experiments have the "striking" implication that fluid dynamics alone "control the gross structure and the visual appearance" of the giant outer planet's weather. He adds that Galileo's Jupiter surveys should supply crucial data to test such computer simulations.
Meanwhile, the survey reported at the project's Aug. 13 press briefing found a clue to the energy in Jupiter's weather - thunderstorms. Lightning had been detected previously. Now, Ingersoll says, "we think we've found ... thunderstorms [themselves.]" They show up as white clouds north of the Great Red Spot. He notes that on Earth, much of the heat transfer from ocean to atmosphere occurs through thunderstorms where water vapor condenses and releases so-called latent heat.
On Jupiter, too, water is raining out and releasing heat in thunderstorms that come and go through rapid life cycles. They're big storms - 50 to 60 miles in diameter and 30 to 50 miles tall. That befits Jovian weather, where Ingersoll says scientists expect all atmospheric structures to be scaled up by a factor of three compared with Earth.
Positively locating these energy-transferring thunderstorms is an important piece in the weather puzzle that planetary scientists are trying to put together.
"Right now," Ingersoll says, "we're sorting out what's what and what are the terrestrial analogues."