Phase Separation of Planetary Ices Explains Why Uranus and Neptune Have Disordered Magnetic Fields

Abstract
  It was a major surprise when the Voyager 2 spacecraft discovered that Uranus and Neptune have disordered, nondipolar magnetic fields. For many years, it remained a mystery what these planets are lacking to generate the typical dipolar fields observed on Earth, Jupiter, and Saturn.
  New ab initio simulations have recently offered an attractive solution to the magnetic field puzzle [1]. The simulations demonstrated that a homogenous mixture of planetary ices – H₂O, CH₄, and NH₃ – spontaneously phase separates at high pressure into a water-rich and into a C–N–H dominated fluid. Based on these predictions, novel models for the interior structure of Uranus and Neptune were constructed that have four layers : (1) an outer hydrogen–helium layer, (2) a water-rich layer, (3) a hydrocarbon layer and (4) rocky core.
  This talk reviews alternate explanations for the disordered fields including diamond rain and superionic water [2].

[1] B. Militzer, "Phase Separation of Planetary Ices Explains Nondipolar Magnetic Fields of Uranus and Neptune", PNAS (2024) DOI: 10.1073/pnas.240398112.
[2] B. Militzer, "Ab Initio Entropy Calculations of Water Predict the Interiors of Uranus and Neptune to be 15-30% Colder than Previous Models", Astrophysical Journal 990 (2025) 20.

Bio
Burkhard Militzer is professor of planetary science at the University of California, Berkeley. He is the director of the Center for Integrative Planetary Science. Since 2007, he has been on the faculty of the Department of Earth and Planetary Science and the Department of Astronomy. He has a background in con-densed matter physics and received his PhD from the University of Illinois at Urbana-Champaign in 2000. Today he works on understanding the interiors of giant planets with NASA missions Juno and Cassini.