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Impact Plasma Amplification of the Ancient Mercury Magnetic Field.

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Title: Impact Plasma Amplification of the Ancient Mercury Magnetic Field.
Authors: Narrett, Isaac S.1 (AUTHOR) narrett@mit.edu, Oran, Rona1 (AUTHOR), Chen, Yuxi2 (AUTHOR), Miljković, Katarina3 (AUTHOR), Tóth, Gábor2 (AUTHOR), Johnson, Catherine L.4 (AUTHOR), Weiss, Benjamin P.1 (AUTHOR)
Source: Journal of Geophysical Research. Planets. May2026, Vol. 131 Issue 5, p1-24. 24p.
Subject Terms: *Geomagnetism, Impact craters, Magnetic fields, Paleomagnetism, Magnetohydrodynamics, Planetary observations, Remanence
Abstract: Spacecraft measurements of Mercury indicate that it has a core dynamo with a surface field of 200–800 nT. These data also indicate that the northern hemisphere crust contains remanent magnetization likely produced by an ancient magnetic field. The inferred magnetization intensity is consistent with a wide range of paleofield strengths (0.2–50 μT), possibly indicating that Mercury once had a dynamo field much stronger than today. Recent modeling of ancient impacts on the Moon has demonstrated that plasma generated during basin‐formation can transiently amplify a planetary dynamo field near the surface. Simultaneously, impact‐induced pressure waves can then record these fields as a crustal shock remanent magnetization (SRM). Here, we present impact hydrocode and magnetohydrodynamic simulations of a Caloris‐sized basin (∼1,550 km diameter) formation event. Our results demonstrate that the ancient magnetospheric field (∼0.5–0.9 μT) created by the interaction of the ancient interplanetary magnetic field and Mercury's dynamo field can be amplified by the plasma up to ∼13 μT and, via impact pressure waves, recorded as SRM at the basin antipode. Such magnetization could produce ∼5 nT crustal fields at 20‐km altitude antipodal to Caloris detectable by future spacecraft like BepiColombo. Furthermore, impacts in the southern hemisphere that formed ∼1,000 km diameter basins (e.g., Andal‐Coleridge, Matisse‐Repin, Eitkou‐Milton, and Sadi‐Scopus) could impart crustal magnetization in the northern hemisphere, contributing to the overall remanent field measured by MESSENGER. Overall, the impact plasma amplification process can contribute to crustal magnetization on airless bodies and should be considered when reconstructing dynamo history from crustal anomaly measurements. Plain Language Summary: Mercury's modern global magnetic field is much weaker than Earth's, defying our understanding of planetary magnetic field generation. Orbiting spacecraft data show that Mercury's crust carries "fossil" or remanent magnetization. Understanding how this ancient magnetization was set can provide insights into Mercury's interior and evolution. As recently shown to work on the Moon, one possible explanation involves basin‐forming impacts. When a large impactor hits a planet, it can generate hot, electrically conducting vapor ("plasma") that can interact with and strengthen surface magnetic fields. Simultaneously, the impactor produces shock waves that can record this amplified magnetic field (setting this remanent magnetization). To study this, we combined modeling of large basin impacts with plasma simulations. Our results show that this process can amplify Mercury's weak field by a factor of ∼10–20. These amplified fields could have been recorded in rocks on the opposite side of the planet (antipode), which could generate magnetic anomalies detectable today. Future spacecraft like BepiColombo can test this idea by measuring magnetic anomalies antipodal to large impact basins. Key Points: Basin‐forming impacts generating plasma can transiently amplify Mercury's magnetic fieldThis plasma amplified field can be recorded antipodal to the impactThis process can explain some components of Mercury's remanent magnetism [ABSTRACT FROM AUTHOR]
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Abstract:Spacecraft measurements of Mercury indicate that it has a core dynamo with a surface field of 200–800 nT. These data also indicate that the northern hemisphere crust contains remanent magnetization likely produced by an ancient magnetic field. The inferred magnetization intensity is consistent with a wide range of paleofield strengths (0.2–50 μT), possibly indicating that Mercury once had a dynamo field much stronger than today. Recent modeling of ancient impacts on the Moon has demonstrated that plasma generated during basin‐formation can transiently amplify a planetary dynamo field near the surface. Simultaneously, impact‐induced pressure waves can then record these fields as a crustal shock remanent magnetization (SRM). Here, we present impact hydrocode and magnetohydrodynamic simulations of a Caloris‐sized basin (∼1,550 km diameter) formation event. Our results demonstrate that the ancient magnetospheric field (∼0.5–0.9 μT) created by the interaction of the ancient interplanetary magnetic field and Mercury's dynamo field can be amplified by the plasma up to ∼13 μT and, via impact pressure waves, recorded as SRM at the basin antipode. Such magnetization could produce ∼5 nT crustal fields at 20‐km altitude antipodal to Caloris detectable by future spacecraft like BepiColombo. Furthermore, impacts in the southern hemisphere that formed ∼1,000 km diameter basins (e.g., Andal‐Coleridge, Matisse‐Repin, Eitkou‐Milton, and Sadi‐Scopus) could impart crustal magnetization in the northern hemisphere, contributing to the overall remanent field measured by MESSENGER. Overall, the impact plasma amplification process can contribute to crustal magnetization on airless bodies and should be considered when reconstructing dynamo history from crustal anomaly measurements. Plain Language Summary: Mercury's modern global magnetic field is much weaker than Earth's, defying our understanding of planetary magnetic field generation. Orbiting spacecraft data show that Mercury's crust carries "fossil" or remanent magnetization. Understanding how this ancient magnetization was set can provide insights into Mercury's interior and evolution. As recently shown to work on the Moon, one possible explanation involves basin‐forming impacts. When a large impactor hits a planet, it can generate hot, electrically conducting vapor ("plasma") that can interact with and strengthen surface magnetic fields. Simultaneously, the impactor produces shock waves that can record this amplified magnetic field (setting this remanent magnetization). To study this, we combined modeling of large basin impacts with plasma simulations. Our results show that this process can amplify Mercury's weak field by a factor of ∼10–20. These amplified fields could have been recorded in rocks on the opposite side of the planet (antipode), which could generate magnetic anomalies detectable today. Future spacecraft like BepiColombo can test this idea by measuring magnetic anomalies antipodal to large impact basins. Key Points: Basin‐forming impacts generating plasma can transiently amplify Mercury's magnetic fieldThis plasma amplified field can be recorded antipodal to the impactThis process can explain some components of Mercury's remanent magnetism [ABSTRACT FROM AUTHOR]
ISSN:21699097
DOI:10.1029/2025JE009474