Zenographic longitude systems and Jupiter's differential rotation

Abstract

More than ten times the Earth in diameter and 300 times as massive, Jupiter is by far the largest planet in the Solar System. Fifth in order of distance from the Sun, which it orbits in 11.8626 years, Jupiter spins more rapidly than any other planet, with a rotation period of just under 10 hours. Unlike the Earth and the other ‘terrestrial’ planets, Jupiter's principal chemical constituent is hydrogen, mainly in the molecular gaseous form at the visible cloud levels in the Jovian atmosphere, but liquid at the high pressures prevailing at much deeper levels within the planet. The electrical conductivity increases from negligible values near the cloud levels to metallic values at great depth. Differential rotation between equatorial and higher-latitude regions of the Jovian atmosphere is reflected in the two systems of zenographic longitude introduced by observers monitoring the motions of atmospheric cloud patterns. These are Systems I and II with rotation periods T 1 =35430.003s (9h 50min 30.003s) and T 2 =35740.632s (9h 55min 40.632s) respectively. The most striking feature of the cloud patterns is the durable (but longitudinally mobile) Great Red Spot, originally known as ’Hooke's Spot’, the first observations of which were recorded in the Philosophical Transactions of The Royal Society of London at the very beginning of its first issue, published in 1665. Differential rotation between the atmosphere and underlying electrically conducting regions is reflected in the need for radio astronomers to use yet another zenographic longitude system—System III, with rotation period T 3 =35729.771s (9h 55min 29.711s)––when monitoring powerful Jovian sources of non-thermal electromagnetic radiation at decimeter and decametre wavelengths. These are located in the plasma envelope of Jupiter, which is closely tied to the deep interior of the planet by lines of force of the zenomagnetic field.