Planetary Magnetic Fields Presentation by Craig Malamut September 18, 2012 Stevenson (2003) Illustration courtesy of NASA (

Why Study Planetary Magnetic Fields? Planetary interiors History of magnetic field Planet’s conductivity structure (if it has an induced field) Informs dynamo theory Impacts life

Heat Transfer Image courtesy of NC State University

Magnetic Fields Permanent magnets Electric currents

Large planetary magnetic fields require energy sources without fluid motion, field undergoes free decay for each body, free decay time < age of solar system therefore, body must be generating field

Dynamos rely on induction – motion of conducting fluid across magnetic field lines – creates electromotive force magnetic Reynolds number (R m ) must exceed ~ (R m = vL/λ) large core radius = good high fluid velocity = good convection + Coriolis force ase.tufts.edu/cosmos

A dynamo depends on heat flow. Planet or satellite needs to be cooling efficiently. If you have more heat flow than can be carried by conduction, then you get convection. So, high conductivity can actually prevent convection and hence, the dynamo.

Stevenson (2003) Conductivity too high, convection not needed to carry heat, no dynamo Conductivity too low, cannot exceed critical magnetic Reynolds number Body needs to be of a certain size for sufficient pressure and temperature

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001)

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001) Gas giant planets – large heat flows compared to conductive transport – hydrogen has low conductivity

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001) Ice planets – large heat flows compared to conductive transport – possibly quadrupolar as well as dipolar

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001) Terrestrial “planets” – depends on thermal history and energy sources – needs efficient cooling – growing inner solid core (compositional convection may help)

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001) May have had ancient dynamo (before 4 Ga) 3 reasons why dynamo may have died: 1.Core cooling slowed down 2.Lost efficient plate tectonics 3.Core of Mars froze too much, making fluid layer too thin to sustain dynamo

Observed Magnetic Fields Russell (1993), Connerney (1993), Showman & Malhotra (1999), Acuna et al. (2001) Induced magnetic fields – varying external magnetic field from Jupiter – most likely salty water

Conclusions Planetary dynamos arise from thermal or compositional convection in fluid regions of large radial extent. Cooling rate of planet must be rapid so that total heat flow exceeds conductive heat flow Composition can be a predictor of field Induced fields strongly suggest water oceans in bodies

References D.J. Stevenson, Planetary Magnetic Fields, Earth Planet Sci. Lett. 208 (2003) C.T. Russell, Magnetic fields of the terrestrial planets, J. Geophys. Res. Planet. 98 (1993) J.E.P. Connerney, Magnetic fields of the outer planets, J. Geophys. Res. Planet. 98 (1993) A.P. Showmann, R. Malhotra, The Galilean satellites, Science 286 (1999) M.H. Acuna, J.E.O. Connerney, P. Wasilewski et al. Magnetic field of Mars: Summary of results from the aerobraking and mapping orbits, J. Geophys. Res. Planet. 106 (2001)