Jupiter and Earth: something in the air.

Diminutive Earth and giant Jupiter appear about as twin-like as Danny DeVito and Arnold Schwarzenegger. But the upper atmosphere of these seemingly disparate planets may have more in common than meets the eye.

When Glenn S. Orton and A. James Friedson began studying heat emissions from Jupiter's stratosphere 11 years ago, the weren't looking for similarities in the Jovian and terrestrial atmospheres. Rather, these scientists at the Jet Propulsion Laboratory in Pasadena, Calif., and their colleagues wanted to learn more about storms deep within the Jovian atmosphere by studying temperature fluctuations at higher altitudes.

Using NASA's Infrared Telescope Facility atop Mauna Kea in Hawaii, the researchers found that heat emissions from methane over Jupiter's equator wax and wane in a four-to-six-year cycle. Since Jupiter's stratospheric methane levels remain relatively constant, the researchers interpreted their results as direct evidence that the planet's equatorial stratosphere alternates between warm and cold periods. Orton and Friedson's team unveiled these findings in the April 26, 1991 SCIENCE.

That report captured the interest of Conway B. Leovy at the University of Washington in Seattle. He recognized that the Jovian findings resemble periodically alternating wind patterns over Earth's equator. Upper stratospheric winds switch direction about once every six months; winds in the lower stratosphere reverse roughly every two years, an effect called the Quasi-Biennial Oscillation (QBO).

Scientists trace the semi-annual wind switch above Earth to seasonal sunlight-related differences that have no apparent parallel on Jupiter. But the detailed pattern and location of QBOs bear a strong similarity to the periodic temperature fluctuations in Jupiter's atmosphere -- and might also resemble features of other planets' atmospheres, Leovy asserts.

Indeed, Leovy suspects that temperature changes in Jupiter's stratosphere may create a longer-period, QBO-like wind system. Moreover, notes Leovy, the phenomenon underlying the shift in Jovian winds may resemble the terrestrial one: Storms that arise in lower layers of the atmosphere and transport heat upward act like a hammer on the stable stratosphere, generating atmospheric waves that alter wind patterns. The high rotation rates of both planets confine such waves to the equator.

While the sun drives QBOs on Earth, Leovy suspects that the small amount of heat left over from Jupiter's formation drives the fluctuations on the giant planet -- possibly accounting for their longer period. Leovy, Friedson and Orton detail their comparative study in the Dec. 5 NATURE.

Further studies of Jupiter, Leovy notes, may help elucidate how different layers of Earth's atmosphere interact. The work, he says, also suggests that other planets that rotate rapidly and vigorously transport heat upward may exhibit similar variations in temperature and wind speed.