Mercury may not need an ancient dynamo to explain its magnetic crust

A recent study utilizing data from NASA's MESSENGER mission suggests most of Mercury's crustal magnetic fields can be explained by iron-rich rocks in the crust responding to the planet's weak core magnetic field, rather than originating from an ancient field as previously thought. Mercury is the only planet other than Earth in the solar system to have both a global magnetic field from its core and crustal magnetization from rocks magnetized by past or present-day core fields.

MESSENGER observed Mercury from 2011 to 2015, providing data to study the magnetic field's strength and distribution. A decision was made near the end of the mission to lower its orbit, enabling the detection of crustal magnetic fields. Lon Hood from the Lunar and Planetary Laboratory at the University of Arizona, who worked on Mercury's crustal magnetism using MESSENGER data, explains that crustal magnetism was thought to be partly due to magnetization formed when the crustal sources developed in the planetary magnetic field.

However, determining the contributions of induced and remanent magnetization in the crustal magnetic field has been a challenge. The new study, led by Catherine Johnson from the University of British Columbia, used MESSENGER observations to develop a model of Mercury's magnetization and identify its sources. The researchers calculated the expected induced magnetization strength using low-altitude observations and assumptions on crustal thickness, magnetic mineralogy, iron content, and the core field.

They found induced magnetization is relatively weak, accounting for over 85% of the area north of 38°N, suggesting magnetized rocks are concentrated within the upper crust.