Planetary Magnetic Fields: Achievements and Prospects

Abstract

The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of planetary interior properties and evolutions. Most planetary magnetic fields arise from dynamos, past or present, and planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or "snow"). Although planetary dynamos mostly appear to operate with an internal field that is not very different from (2ρ /σ ) 1/2 in SI units where ρ is the fluid density, is the planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of planetary

The lunar dynamo

“…A dynamo requires organized advection of conducting fluid, which generates magnetic fields by the process of electromagnetic induction (36). For the terrestrial planets, this is thought to occur in their liquid metallic cores.…”

“…The buoyancy flux in Earth's core is provided by secular heat flow, as well as by latent heat release and compositional differences arising from crystallization of the core (42). The field's intensity (22) and longevity (36) are thought to depend on the heat flux at the core-mantle boundary (Fig. 4).…”

Fluctuation dynamo amplified by intermittent shear bursts in convectively driven magnetohydrodynamic turbulence

“…Measurements also show that planetary magnetic fields can change in magnitude and orientation (McFadden & Merrill 1995;Christensen et al 2009;Stevenson 2010;Olson et al 2011). Dynamo action driven by turbulent convection is accepted as the origin of solar and planetary magnetic fields.…”

Planetary Magnetic Field Measurements: Missions and Instrumentation

“…As reviewed by Stevenson (2010), there is a good basic understanding and detailed knowledge of the magnetic properties of all the planets, despite their considerable diversity. The degree of similarity is small among the usual grouping of the planets (terrestrial planets, gas giants, ice giants).…”