Written in collaboration with Filippo Cicchetti 

Like Earth, Mercury has a magnetic field generated by its metallic core – making it the only other planet in the inner Solar System with a present-day global magnetic field. NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft orbited Mercury for more than four years, collecting unprecedented measurements before its mission ended with a planned impact on the planet’s surface in 2015. 

MESSENGER also revealed that parts of Mercury’s crust are magnetized, producing smaller magnetic fields in addition to the global field generated by the core. Now, Dr. Catherine Johnson, Professor of planetary geophysics in EOAS, and her collaborators, including EOAS graduate student Filippo Cicchetti, have developed a new model of Mercury’s crustal magnetization and explored its possible origins. The paper is Dr. Johnson’s inaugural article in PNAS since her induction into the National Academy of Sciences in 2023, one of the highest honours a scientist can receive. 

Their paper, published this week in the Proceedings of the National Academy of Sciences (PNAS), asks whether Mercury’s crust became magnetized by a stronger magnetic field in the planet’s past, or whether some of the magnetization we see today could instead be induced by its much weaker present-day field. Their results suggest that, depending on the iron content and magnetic properties of Mercury’s crust, the present-day field may explain a substantial portion of the magnetic signals observed by MESSENGER. 

The possibility that Mercury’s present-day field explains much of the observed crustal magnetization raises new questions that BepiColombo, the newest mission to Mercury, will soon be able to explore in much greater detail. How much of Mercury’s crustal magnetization truly records an ancient, stronger core magnetic field? How do different geological units, their iron content, and their magnetic properties contribute to the signals we observe? And what role have craters and the impacts that formed them played in shaping Mercury’s magnetic crust? BepiColombo’s observations will offer a new opportunity to test these ideas and build a clearer picture of Mercury’s magnetic history.