From the November 2004 issue

I recently read about the high wind speeds on Saturn and the other gas-giant planets. What is wind speed on a gas giant measured relative to?

By | Published: November 1, 2004
Saturn's Jet Stream
The deep rotation of the gas giants serves as the yardstick for determining how fast their winds blow.

At the equator, Saturn rotates in 10 hours, 14 minutes. Planetary scientists determine this rotation period by looking at radio waves (mostly in the decimeter wavelength range) generated deep within Saturn’s interior. As the planet rotates, the radio waves oscillate in a periodic fashion, and the pattern can be analyzed to determine how fast the planet’s equator rotates. Saturn’s giant magnetosphere also makes one complete sweep around the planet in the same period. This structure is created by Saturn’s magnetic field, generated by a layer of electrically conductive liquid hydrogen deep inside the planet.

Atmospheric winds form as counterflowing east-west streams. Planetary scientists determine wind speeds in all the giant-planet atmospheres by tracking visible cloud features and measuring their speeds relative to the planet’s internal rotation rate as seen in radio waves. At its equator, Saturn has wind speeds about 1,100 miles (1,800 kilometers) per hour faster than the internal rotational period. Saturn’s internal heat, welling up from below, drives this faster motion. Recent observations with the Hubble Space Telescope indicate Saturn’s fastest equatorial winds have slowed to about 690 miles (1,100 km) per hour.

Jupiter, which spins once in about 10 hours, has much slower winds than Saturn — typically about 335 miles (540 km) per hour faster than the internal rotation rate. The situation is more complex for Uranus and Neptune, which respectively have magnetic fields offset by 60° and 47° from their rotational axes. — RICHARD CROWE, UNIVERSITY OF HAWAII AT HILO

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