Planetary Science and Exploration Seminar, Catherine Johnson - "Mercury’s Enigmatic Magnetic Field"

Mercury’s interior structure and its magnetic field are unique among the terrestrial planets and moons of the Solar System. Its large metallic core (~83% of the planetary radius) formed under highly reducing conditions, requiring a core composition that includes substantial silicon and some, but more limited, sulfur and carbon. Mercury is also the only planet other than Earth to have both core and crustal magnetic fields. The core field constrains the present-day dynamo and crustal magnetization, if remanent, can constrain the dynamo history. The core field is dominantly dipolar, axisymmetric, asymmetric about the planetary equator, and ~100 times weaker than Earth’s field at the planetary surface. Structure beyond this simple picture is weak but exists, in particular a high-northern-latitude regional magnetic field anomaly spatially coincident with the topographic and gravity signature of Mercury’s Northern Rise. Crustal fields in many places can be explained by magnetizations induced in the present field and thus are a probe of the magnetic properties of the crust rather than the dynamo history. However, stronger magnetizations at some craters and basins are consistent with the existence of a dynamo at the time of formation of the Caloris Basin and more recently, with the caveat that impactor-delivered iron could also contribute to enhanced induced magnetizations. These magnetic field constraints must collectively be matched by dynamo models: how does the thermal state and precise core composition (e.g., the relative amounts of Si, S, and C) control the solidification history of the inner core, and the radial extent of dynamo region(s) in the outer core? To what extent do core-mantle interactions, physical or chemical, affect core dynamics now and in the past? I will discuss how upcoming BepiColombo data can address some of these open questions

Catherine Johnson is a Professor at the University of British Columbia and a Senior Scientist at the Planetary Science Institute. Her research focuses on understanding the structure and interior evolution of terrestrial planets, moons and asteroids. She is a Guest Investigator on the Bepi-Colombo mission and was previously a Participating Scientist on the MESSENGER (Mercury) mission, and a Co-Investigator on the InSight (Mars) and OSIRIS-REx (asteroid Bennu) missions. She is a member of the U. S. National Academy of Sciences, the American Academy of Arts and Sciences, and a fellow of the Royal Society of Canada and the AGU. Beyond geophysics she enjoys being outside, cooking and eating good food with excellent company, her local community sewing studio, and teaching yoga.

Speaker suggested reading: Johnson, C. L., Plattner, A. M., Cicchetti, F., & Miljković, K. (2026). The distribution and origin of Mercury's crustal magnetization. Proceedings of the National Academy of Sciences, 123(35), e2535723123. https://doi.org/10.1073/pnas.2535723123