Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields

Abstract

The discovery of Uranus' and Neptune's non-dipolar, non-axisymmetric magnetic fields destroyed the picture--established by Earth, Jupiter and Saturn--that planetary magnetic fields are dominated by axial dipoles. Although various explanations for these unusual fields have been proposed, the cause of such field morphologies remains unexplained. Planetary magnetic fields are generated by complex fluid motions in electrically conducting regions of the planets (a process known as dynamo action), and so are intimately linked to the structure and evolution of planetary interiors. Determining why Uranus and Neptune have different field morphologies is not only critical for studying the interiors of these planets, but also essential for understanding the dynamics of magnetic-field generation in all planets. Here we present three-dimensional numerical dynamo simulations that model the dynamo source region as a convecting thin shell surrounding a stably stratified fluid interior. We show that this convective-region geometry produces magnetic fields similar in morphology to those of Uranus and Neptune. The fields are non-dipolar and non-axisymmetric, and result from a combination of the stable fluid's response to electromagnetic stress and the small length scales imposed by the thin shell.

Mixtures of planetary ices at extreme conditions

“…At more extreme conditions, we find that the mixture becomes electronically conducting at pressures below 120 GPa, where pure water is reported to be still an opaque semiconductor 17 . Because electronic conductivity generally dominates over the ionic contribution, the source of the planetary magnetic fields observed at Uranus and Neptune may be shallower than currently inferred from conductivity data on pure substances, in agreement with revised planetary models 24 . Our simulations also indicate that ionized water is, not surprisingly, an exceptionally aggressive chemical solvent, capable of dissociating methane already at 50 GPa.…”

“…These studies yield conductivities typical of an ionic fluid throughout most of the planetary radius, with evidence for metallic behaviour only in the deepest regions of the planets, close to the core. However, recent planetary models would be more compatible with metallic-like conductivities in the icy layer 24 , suggesting qualitative differences in the electrical properties of the mixture, with respect to pure water. From a theoretical standpoint, water-methane mixtures have been studied up to 2,500 K and 18 GPa by atomistic simulations based on empirical force fields 7 .…”

Convection in rotating non-uniformly stratified spherical fluid shells in dependence on Ekman and Prandtl numbers

“…It is an important dynamical factor in the processes acting in planetary atmospheres (e.g., [10]), planetary interiors (e.g., [1,19,20,24,21,6,16,17]), etc. A systematical parameter study of rotating convection in uniformly and non-uniformly stratified spherical shells in dependence on Prandtl, Ekman and Rayleigh numbers was done e.g., in [24,2,3].…”

“…However, in the other planets the ratio of the thickness of the appropriate sublayers (e.g., of the stably stratified to unstably stratified sublayers) and their geometric configuration vary. This is noticeable especially with the Giant planets [19,20,24,21].…”

“…For a different geometric configuration the influence of a non-uniform stratification is fundamental [19,20]. They assumed reverse stratification, i.e.…”