Constraining Planetary Interior Structure and Evolution Using Magnetic Fields and Rotational Dynamics

Abstract

The present-day interior structure of a planet is an important reflection of the planet’s formation and subsequent thermal evolution. However, despite decades of spacecraft missions to a variety of target bodies, the interiors of most planets in our Solar System remain poorly understood. In this thesis, we discuss how two methods of geophysical analysis—planetary magnetic fields (dynamos) and rotational dynamics— can provide important insights into the interior properties and evolution of planets. Here we consider Jupiter and Mars as case studies. For Jupiter, we present new analysis of in situ spacecraft magnetometer data from the NASA Juno Mission (currently in orbit about Jupiter). The spatial morphology of Jupiter’s magnetic field shows surprising hemispheric asymmetry, which may be linked to the dissolution of Jupiter’s rocky core in metallic hydrogen. We also report the first definitive detection of time-variation (secular variation) in a planetary dynamo beyond Earth. This time-variation can be explained by the advection of Jupiter’s magnetic field by the zonal winds, which places a lower bound on the velocity of Jupiter’s winds at depth. In contrast, we observe no significant time-variation in the magnetic field of Saturn. Overall, these results provide an important complement to other analysis techniques, as gravitational measurements are currently unable to uniquely distinguish between deep and shallow wind scenarios, and between solid and dilute core scenarios. Future analysis will continue to resolve Jupiter’s interior, providing broader insight into the physics of giant planets, with implications for the formation of our Solar System. We also consider the past thermal evolution of the planet Mars. The planet’s strong, remnant crustal magnetization records the history of its now extinct dynamo magnetic field. While NASA spacecraft such as Mars Global Surveyor and MAVEN have mapped Mars’ field in high resolution, most mathematical analyses of this data use techniques that yield smooth solutions. However, there is no reason to assume a crustal magnetic field should be smooth. Here we use L1 regularized inversions to generate sparse maps of the planet’s magnetic field, and show that only 15% of the planet’s crust needs to be magnetized to explain the magnetic field measurements from Mars Global Surveyor. We also consider the rotational dynamics of terrestrial planets. Previous theories of polar reorientation (true polar wander) consider only elastic or viscous lithospheric rheologies. Here we derive a new theory for viscoelastic lithospheres, to bridge these end-member cases, and apply this theory to Mars as a case study. This theory has important applications to understanding the emplacement timing of the Tharsis region on Mars.