Speaker: Burkhard Militzer, UC Berkeley
Title: Phase Separation At High Pressure Explains Why Uranus and Neptune Generate so Unusual Magnetic Fields
Time: Friday, Oct 31 at 12:00pm PST
Location: EMS B210
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. Uranus and Neptune are assumed to contain large amounts of planetary ices at high pressures and temperatures. So this talk will review results from laboratory experiments and computer simulations of different ices under such extreme conditions.
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. With rising pressure, the C–N–H fluid is predicted to expel a growing amount of hydrogen as more C and N bonds form, which stabilizes the fluid against convection. 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. The disordered magnetic fields of Uranus and Neptune are predicted to originate primarily from their water-rich layers.
The talk concludes with discussing the interior temperatures of Uranus and Neptune [2] and what we learned from the recent ring occultation observation of Uranus.
