When Impacts Supercharged Mercury’s Ancient Magnetic Field

Mercury’s magnetic field, first measured by NASA’s MESSENGER spacecraft, is far weaker than Earth’s, only about 200 nanotesla (nT) at the surface, compared to Earth’s 30,000–60,000 nT. Yet, its crust contains strong traces of ancient magnetism, implying that Mercury once hosted a much stronger magnetic field. In this paper, Isaac Narrett and colleagues explore an intriguing explanation: that giant impacts on Mercury could have temporarily amplified its magnetic field through the generation of electrically charged plasma clouds.

The Puzzle of Mercury’s Magnetic History

MESSENGER data revealed crustal magnetization concentrated in Mercury’s northern hemisphere, suggesting that rocks there recorded an ancient magnetic field up to 50 microtesla (μT), hundreds of times stronger than the planet’s current field. Scientists have long debated whether this implies a once-powerful internal “dynamo” (a magnetic field generated by the planet’s liquid core) or an external process. Narrett and collaborators focus on the role of giant impacts, such as the formation of the Caloris Basin, a crater over 1,500 km wide that formed about 3.9–3.7 billion years ago.

Simulating a Giant Impact

The team modeled the Caloris impact using two types of computer simulations. First, a hydrocode model simulated how the impact would vaporize part of Mercury’s crust, creating a cloud of hot, conductive plasma that expands around the planet. Then, a magnetohydrodynamic (MHD) model simulated how this plasma interacts with Mercury’s magnetic field and the interplanetary magnetic field carried by the young Sun’s solar wind. Their models incorporated realistic early-solar conditions, when the solar wind and magnetic field were far stronger than today’s.

Amplifying the Magnetic Field

According to the simulations, the plasma cloud from the Caloris impact could compress and amplify Mercury’s preexisting magnetic field by as much as 10–20 times, boosting a background field of roughly 0.5–0.9 μT to peaks near 13 μT. The amplification would last only about 20 minutes, but during that time, the strong fields could be “recorded” in rocks at the point opposite the impact, the antipode. The process, known as shock remanent magnetization (SRM), occurs when seismic pressure waves align magnetic minerals in the direction of the ambient field, freezing a record of it as the rock cools.

Magnetizing the Antipode

Narrett’s team found that pressures of about 0.4 gigapascals (GPa) at the Caloris antipode coincided with the period of maximum field amplification. Under these conditions, Mercury’s crustal materials, particularly iron-rich minerals like kamacite and martensite, could acquire stable magnetic signatures. The authors predict that such regions could generate detectable magnetic fields of around 5 nT at spacecraft altitudes of 20 km. These antipodal magnetic anomalies could be verified by ESA’s BepiColombo mission, currently en route to Mercury.

Broader Implications

The study also suggests that similar southern-hemisphere impacts (like the Andal-Coleridge and Matisse-Repin basins) could have magnetized their northern antipodes, contributing to the crustal magnetism observed by MESSENGER. However, the team concludes that impact-driven plasma amplification alone cannot explain all of Mercury’s widespread crustal magnetization, especially the strongest signals in the north. It remains likely that Mercury’s ancient core dynamo was stronger in the past.

Looking Ahead

Narrett and colleagues emphasize that future low-altitude measurements by BepiColombo could test their predictions by mapping magnetic anomalies antipodal to large basins. Together with possible future sample-return missions, these observations could clarify how giant impacts, plasma physics, and planetary magnetism shaped Mercury’s evolution, and perhaps other airless worlds as well.

Source: Narrett