Jupiter and Saturn

Abstract

Jupiter, the largest planet, and Saturn, the second largest, contain nine-tenths of the material of the solar system outside the Sun and most of the angular momentum of the solar system is associated with their orbital motion. Both planets rotate very rapidly (rotation periods ~ 10 h) and possess rich satellite systems. Owing to their strong gravitational fields and low surface temperatures, Jupiter and Saturn may, unlike the ‘terrestrial’ planets, be fairly close in chemical composition to the primordial material out of which the solar system originally formed; they consist mainly of hydrogen, much of which is compressed to a metallic form. Jupiter is the only planet other than Earth showing evidence of a general magnetic field. Absorption of incident solar energy accounts for less than one-half the estimated total thermal (infrared) radiation emitted by Jupiter and Saturn. The balance is probably due to internal heat sources and could be accounted for in terms of a gravitational contraction at about 0.1 cm/year. The outward flow of heat should maintain the atmospheric temperature gradients close to their adiabatic values, which is a significant result for theories of atmospheric motions (see appendix A). These theories are largely concerned with explaining the rough alinement of clouds in bands parallel to the equator, the presence of strong eastward equatorial currents, the occurrence of transient spots and other irregular markings and, in the case of Jupiter, the nature of the enigmatic Great Red Spot. Jupiter, unlike Saturn, is a strong emitter of non-thermal radio noise on decametre and decimetre wavelengths. Plausible theories of this radio emission invoke a strong Jovian dipole magnetic field and an associated system of van Allen-type ‘radiation’ belts of electrically-charged particles extending beyond and interacting with the first Galilean satellite Io. The most likely source of the Jovian magnetic field - which theories of Jupiter’s internal constitution must now take properly into account - is a hydromagnetic dynamo (see appendix B) associated with fluid motions in the electrically-conducting parts of Jupiter’s interior. The absence of a non-thermal component in Saturn’s radio spectrum implies that radiation belts cannot form around that planet, possibly because Saturn is non-magnetic or, if it is magnetic, because charged particles in the vicinity of Saturn are rapidly removed through interactions with Saturn’s rings. Modern research on Jupiter and Saturn is based on a rich variety of data, soon to be augmented by observations from space-craft. Future progress with the theoretical interpretation of these data in terms of improved models of the structure and evolution of the giant planets will involve not only the further application of a wide range of established knowledge but also the development of new ideas in several areas of basic science. The paper ends with two appendices, on the dynamics of rapidly rotating non-homogeneous fluids and on hydromagnetic dynamos.

Typical Velocities and Magnetic Field Strengths in Planetary Interiors

“…Our estimate of the internal field is therefore a factor (r * /r 0 ) 1/2 smaller than that in Hide (1974). For the Earth, for example, using the estimates of Table I, R m ∼ 300 and the Hide (1974) estimate gives (µ 0ρ * V * r 0 ) 1/2 ∼ 0.025 T, compared with our estimate of 0.004 T. On the other hand, the Hide (1974) estimate of the escaping poloidal field is ∼0.025 R −1 m ∼ 0.0001 T, slightly smaller than our estimate, based on 10% of the internal field escaping the core, which is 0.0004 T. The main difference between our approach and that of Hide (1974) is that we envision poloidal and magnetic fields of the same magnitude in the OC, as in the simulations of Glatzmaier and Roberts (1997) and others, rather than having them differ by the large factor R m . Taking this point of view, it is natural to define the Elsasser number as = B 2 * /µ 0ρ * η, and now balance between Coriolis and Lorentz accelerations requires η ≈ V * r * .…”

“…We can compare our prescription of the typical values with previous methods, e.g., that of Hide (1974), based on the Elsasser number = B 2 sca /µ 0ρ * η and R m = V * r 0 /η. This approach is essentially kinematic, since R m is assumed either to be known from observations or to take its critical value for dynamo action, whereas in our work V * is estimated from the heat flux and the balance between Coriolis acceleration and buoyancy.…”

“…This approach is essentially kinematic, since R m is assumed either to be known from observations or to take its critical value for dynamo action, whereas in our work V * is estimated from the heat flux and the balance between Coriolis acceleration and buoyancy. In Hide (1974), the scale B, B sca , is defined by taking = 1, and the toroidal field inside the OC is taken as R…”

Planetary Dynamos

“…Since a common ingredient of planetary dynamos is the existence of a fluid part of the core with a sufficiently high electrical conductivity, the latter parameter together with the core radius and the angular velocity of the planetary rotation usually enters the similarity relationships such as those proposed by Hide (1974), Busse (1976), Jacobs (1979), andDolginov (1977). Since a common ingredient of planetary dynamos is the existence of a fluid part of the core with a sufficiently high electrical conductivity, the latter parameter together with the core radius and the angular velocity of the planetary rotation usually enters the similarity relationships such as those proposed by Hide (1974), Busse (1976), Jacobs (1979), andDolginov (1977).…”

Stability of a Rotating Compressible Stratified Plasma in the Presence of Hall Currents and Magnetic Resistivity

“…The hydromagnetic stability of a magnetized fluid of variable density is of considerable importance in a variety of astrophysical situations such as in the theories of Sunspot magnetic fields, heating of the solar corona, the stability of the stellar atmospheres in magnetic fields and in the spiral arms of the galaxies. KRUSKAL and SCEWARZ-SCEILD [12], HIDE [9] have all shown the stabilizing influence of the magnetic field on the problem of the instability of a conducting fluid. ARIEL [4] investigated the stability of an inviscid compressible fluid of variable density in the presence of a uniform vertically imposed magnetic field BHATIA [6] has extended the problem to the case of a viscous compressible conducting fluid of variable density.…”