Mercury Crustal Magnetization Indicates a Stronger Ancient Dynamo
Isaac S. Narrett, Benjamin P. Weiss, Sarah C. Steele, John B. Biersteker
Published: 2025/10/2
Abstract
Mercury is the only other terrestrial planet in the solar system with an active dynamo magnetic field (~200 nT at the equatorial surface). Furthermore, Mercury's ~3.9-3.7 billion-year old (Ga) crust is strongly magnetized (~10 nT at ~30-km altitude), indicating the presence of a past dynamo. However, the source and strength of the field that magnetized this crust are unknown. To address this, we performed three-dimensional magnetohydrodynamic simulations of the ancient solar wind interaction with the planetary field and coupled them with thermal cooling and magnetization models. We show that the crustal magnetization was likely produced by a surface field of at least ~2,000 nT, likely >10,000 nT (i.e., ~10x or ~50-100x stronger than at present). Such strong fields likely exclude both the solar wind feedback and thermoelectric mechanisms for driving the dynamo at 3.7 Ga ago. Instead, our results are compatible with the past dynamo being powered by an enhanced energy flux that drove strong core convection.