INTRINSIC MAGNETIC-FIELDS OF THE PLANETS - MERCURY TO NEPTUNE

In the past three decades, studies of the magnetic fields of Earth's M oon and all the planets, except for Pluto, have been conducted by spac ecraft of the U.S.A. and of Venus and Mars by the former U.S.S.R. Amon g the terrestrial planets, only Mercury (Mariner 10: 1974 and 1975) is globally magnetized while the Moon and Venus are unmagnetized. The si tuation at Mars is still unclear, but if any global field exists, it i s quite small. In 1979, Pioneer 11 discovered a magnetic field and rad iation belt at Saturn, further elaborated on by Voyagers 1 (1980) and 2 (1981). Pioneers 10 (1974) and 11 (1975) and Voyagers 1 (1979) and 2 (1979) examined in detail the magnetic field of Jupiter, which had be en inferred initially and studied remotely due to its non-thermal radi o emissions in the late 1950s. Jupiter's magnetic field is much strong er than Earth's and distinctly nondipolar close to the planet. Saturn has a much weaker field than Jupiter, and it is surprisingly axisymmet ric (to degree n = 3) with respect to its rotation axis. The Voyager f ly-bys of Uranus and Neptune in 1986 and 1989 discovered global magnet ic fields and trapped energetic particle radiation belts. Both Uranus and Neptune display remarkably similar magnetic fields (quite differen t from Jupiter, Saturn and Earth). In an astrophysical sense, Uranus a nd Neptune are described as oblique rotators because of the large angu lar offset of their magnetic axes from their rotation axes (59 degrees and 47 degrees). Additionally, their magnetic 'centres' are displaced by substantial fractions of a planetary radius (0.31 R(U) and 0.55 R( N)) This paper summarizes our present knowledge of the quantitative ch aracteristics of the magnetic fields of these planets.