The Origin of Mercury’s Internal Magnetic Field

Abstract

Mariner 10 measurements proved the existence of a large-scale internal magnetic field on Mercury. The observed field amplitude, however, is too weak to be compatible with typical convective planetary dynamos. The Lorentz force based on an extrapolation of Mariner 10 data to the dynamo region is 10 −4 times smaller than the Coriolis force. This is at odds with the idea that planetary dynamos are thought to work in the so-called magnetostrophic regime, where Coriolis force and Lorentz force should be of comparable magnitude. Recent convective dynamo simulations reviewed here seem to resolve this caveat. We show that the available convective power indeed suffices to drive a magnetostrophic dynamo even when the heat flow though Mercury's core-mantle boundary is subadiabatic, as suggested by thermal evolution models. Two possible causes are analyzed that could explain why the observations do not reflect a stronger internal field. First, toroidal magnetic fields can be strong but are confined to the conductive core, and second, the observations do not resolve potentially strong small-scale contributions. We review different dynamo simulations that promote either or both effects by (1) strongly driving convection, (2) assuming a particularly small inner core, or (3) assuming a very large inner core. These models still fall somewhat short of explaining the low amplitude of Mariner 10 observations, but the incorporation of an additional effect helps to reach this goal: The subadiabatic heat flow through Mercury's core-mantle boundary may cause the outer part of the core to be stably stratified, which would largely exclude convective motions in this region. The magnetic field, which is small scale, strong, and very time dependent in the lower convective part of the core, must diffuse through the stagnant layer. Here, the electromagnetic skin effect filters out the more rapidly varying high-order contributions and mainly leaves behind the weaker and slower varying dipole and quadrupole components (Christensen in Nature 444: [1056][1057][1058] 2006). Messenger and BepiColombo data will allow us to discriminate between the various models in terms of the magnetic fields spatial structure, its degree of axisymmetry, and its secular variation.

Mercury’s Interior Structure, Rotation, and Tides

“…If the core consists mainly of iron, the core radius will be about 3/4 of the radius of the planet, resulting in a core-to-mantle size ratio that is larger than for the three other terrestrial planets. If created by a dynamo, the magnetic field observed by Mariner 10 is evidence for a liquid outer core and a strong indication for a solid inner core, which is also predicted by thermal evolution models (Schubert et al 1988;Hauck et al 2004;Breuer et al 2007;Wicht et al 2007).…”

“…In such an evolution stage, compositional buoyancy is absent in the outer fluid core, and it seems unlikely that a dynamo generating a global magnetic field can be effective. The magnetic field observation then suggests that Mercury's inner core radius should be smaller than that at which the core fluid becomes eutectic (see, e.g., Christensen 2006; for a detailed description in this issue, see Breuer et al 2007 andWicht et al 2007).…”

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

“…Estimates from scaling laws such as Eq. 9 above suggest a much stronger field than observed (Stevenson et al 1983;Schubert et al 1988;Wicht et al 2007). Other models that predict a strong quadrupole component (e.g., Christensen 2006;Christensen and Wicht 2008) imply a large dipole tilt, which has not been observed.…”

Earth-Based Visible and Near-IR Imaging of Mercury

“…The measured global magnetic asymmetry of the planet (Connerney and Ness 1988) can be related to the existence of the S Basin according to this hypothesis. We demonstrate that available knowledge about Hermean magnetic fields (Wicht et al 2007) does not contradict this hypothesis. We indicate possible directions of search for correlations between magnetic fields and surface properties in complex multidisciplinary experiments onboard future spacecraft missions.…”

“…Radio measurements of the planetary rotation, using highly accurate radar speckle methods, are intended to find signatures of the existence of the molten interiors (Holin 1988;Van Hoolst et al 2007). For an up-to-date review of the internal, dynamo-generated magnetic field of Mercury, see Wicht et al (2007).…”