On the rotation of Jupiter

Abstract

Summary. Jupiter's non-uniform rotation is reflected in the use by observers of three systems for measuring longitude: system I which rotates in the period 9 h 50 m 30.003 s, system II in 9 h 55 m 40.632 s and system III in 9 h 55 m 29.710 s. Disturbances of Jupiter's visible surface of dense ammonia-cirrus cloud within a narrow equatorial zone of about 7° latitudinal half-width show comparatively little motion relative to system I, and most (but not all) higher-latitude atmospheric disturbances including the Great Red Spot and the three white ovals move only slowly in system II. Finding neither system I nor system II particularly convenient for monitoring sources of intense non-thermal radio-emission from Jupiter, radio-astronomers introduced system III with respect to which the radio sources show little systematic movement. The interpretation of Jupiter's non-uniform rotation involves considerations of the hydrodynamics of the atmosphere of the planet and the hydromagnetics of the deep interior. Jovian radio sources are extensive regions of plasma tied to the lines of force of the magnetic field B surrounding the planet. These in turn are intimately linked to material at the outer surface of Jupiter's electrically-conducting fluid core, mean radius rc, which consists largely but not entirely of metallic hydrogen. A method for determining rc from observations of secular changes in B in the accessible region above the visible surface of the planet, mean radius rs= 69 700 km, has recently been proposed and applied using the few available data, giving rc greater than about 50 000 km and possibly as large as 63 000 km. The difference in period between system I and system II reflects the presence at the upper cloud level of a sharply-bounded equatorial jet stream within which the atmosphere moves at about 100 ms−1 in a westerly (eastward) sense relative to higher latitude parts of the atmosphere and to the deep interior. The width of the equatorial jet can be accounted for satisfactorily by fairly general dynamical arguments, which predict that the apparently much stronger equatorial jet (400 m s−1) on Saturn will be found to be about twice as wide as that on Jupiter.