Magnetic fields of Uranus and Neptune: Metallic fluid hydrogen

W J Nellis

doi:10.1088/1742-6596/950/4/042046

Abstract

The unusual magnetic fields of Uranus and Neptune (U/N) have been a major scientific issue since their discovery in the 1980s. Based on an extensive database of planetary fluids measured under dynamic compression at pressures and temperatures up to 200 GPa and a few 1000 K, the complex magnetic fields of U/N are made primarily by dynamo convection of metallic fluid hydrogen (MFH) near crossovers from their H-He envelopes to “Ice” cores at ~100 GPa pressures at ~90% of radii of U/N. Because those fields are made close to outer surfaces, non-dipolar magnetic fields can be expected. “Ice” cores are heterogeneous fluid mixtures of nebular Ice and Rock that accreted, sank below the H-He envelopes into the cores, decomposed at high pressures and temperatures and formed new chemical species. Such complex mixtures are expected to have many nucleation sites for convection. Rotational and effective dipolar axes probably have large relative tilts because rotational motions of U/N are weakly coupled to convective motions that make their magnetic fields. “Polar wander” is probably a better descriptor for variations of magnetic field over time than “polar reversal” as for Earth. There is probably little “Ice” in the Ice Giants.

Highlights

Chemical compositionUranus and Neptune (U/N) have similar sizes, densities, magnetic and gravitational fields and probably chemical compositions, exact chemical compositions of U/N are unknown
The cause of the enormous difference between planetary dipolar fields and nondipolar/non-axisymmetric magnetic fields has been a major scientific question since the discovery of the magnetic fields of Uranus and Neptune (U/N)
The cause of the unusual magnetic fields of U/N lies in measured physical properties of fluids that generate those magnetic fields

Summary

Chemical composition

U/N have similar sizes, densities, magnetic and gravitational fields and probably chemical compositions, exact chemical compositions of U/N are unknown. Total mass of the Giant Planets Jupiter, Saturn, Uranus and Neptune is 445 ME. Most of the mass of the solar system is hydrogen and most of that is metallic fluid H at high internal P and T in the Giant Planets. The probable reason little nebular “ice” exists in the Ice Giants is based on measured electrical conductivities, pressure-density equation-of-state data, emission temperatures, and pulsed Raman spectroscopy of a large number of small molecular fluids under dynamic compression. Those experiments began in ~1980 when Voyager 2 departed Saturn and headed for Uranus [12,13]

Planetary fluids: dynamic compression experiments

Conclusions