Abstract

The solar system offers a diverse group of planetary magnetic fields. While prevailing in all four giant planets, present-day large-scale magnetic fields are not common among terrestrial planets and moons. Mercury is the only terrestrial planet other than the Earth that harbors a planetary-scale magnetic field, while Ganymede is the only moon that manages to maintain a large-scale magnetic field at present. In the first part of this talk, I will present a comparative review of the magnetic fields of Mercury and Ganymede. Three defining features of Mercury’s magnetic field are 1) its relatively weak strength (~1% of the Earth’s surface magnetic field), 2) its strong north-south asymmetry, and 3) its axial dominance. Ganymede possesses a surprisingly strong axial-dipole-dominant magnetic field. However, our understanding of the characteristics of Ganymede’s core field beyond the dipole component is limited due to the sparse coverage of existing observations and the likely electromagnetic induction signal from the subsurface ocean. There remain significant uncertainties in our understanding of core properties of Mercury and Ganymede (e.g., the composition, the existence and extent of iron snow zones, and the electrical conductivity), which directly impact our inference of the dynamo mechanism responsible for their magnetic fields. In the second part of this talk, I will briefly summarize recent observations of the Saturnian magnetic field and the challenges to understand its origin. Saturn’s magnetic field is highly axisymmetric (with a dipole tilt less than 0.007 degrees) yet rich in spatial scales. With smooth transitions in material properties from deep interior to surface, Saturn’s dynamo is expected to be strongly coupled to dynamics in the molecular envelope. Zonal flow magnetic field interaction in the transition region likely plays an important role in the truncation of deep zonal flows and the generation of small-scale axisymmetric magnetic field. Different types of stable stratification have been proposed for the interior of Saturn, e.g., an extensive helium rain layer and a large diffuse core, which leaves not much space for the traditional convective dynamo to operate. I will conclude this presentation with an outlook for upcoming observations and a summary of key open questions concerning the origin of planetary magnetic fields.