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Mercury, Jan/Feb 2002 Table of Contents

Image courtesy of NASA/JPL/University of Arizona

by Caroline Seydel

Lightning, raging winds, and planet-sized oval storms give Jupiter- researchers a grand spectacle.

Jupiter's wild weather and swirling clouds of gas have fascinated observers for centuries. Since 1995, the Galileo spacecraft has given scientists an unprecedented close-up of the planet and its equally intriguing large moons. And in December 2000, the Cassini spacecraft flew by Jupiter on its way to Saturn, adding a new dimension to Jupiter observations. Using Galileo and Cassini data, planetary scientists are revealing the nature of the processes that drive Jupiter's atmospheric dynamics, adding more detail to the sketch provided by Voyager in 1979.

In just the past three years, observers have watched two white oval storms merge into one, collected evidence that thunderstorms drive larger weather patterns, recorded the first known patch of pure ammonia ice, and discovered that Jupiter's winds may swoop and soar like a giant roller coaster. Each of these discoveries contributes to an understanding of how energy, air currents, and chemistry interact to produce an atmosphere quite unlike Earth's, yet driven by the same basic processes: convection, evaporation, and condensation.

More than twice as massive as all the other planets combined, and voluminous enough to hold 1,400 Earths, Jupiter is about three-quarters hydrogen and one-quarter helium by mass, with traces of methane, water, ammonia, and probably a rocky core. Like a giant pressure cooker, the tremendous mass heats Jupiter's core to a sweltering 30,000° C. As this heat flows outward, it supplies abundant energy to sustain violent weather.

Jupiter's speedy 9.9-hour rotation separates the planet's winds into the colored horizontal bands that make Jupiter so visually distinctive. Chemical and temperature differences in the gases color the bands reddish-brown, yellow, blue, and white. Each band flows at a different speed (and sometimes opposite directions), generating turbulence between adjacent bands that evolves into oval-shaped storms. Smaller vortices sometimes develop into giant hurricane-like storms that can persist for centuries, such as the 24,000-kilometer-wide Great Red Spot. These colorful features make Jupiter a prime telescopic target, especially in times like the present when Jupiter is near opposition.