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Magnetic Properties of Martian Impact Basins Including the Effect of Mantle Excavation.

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Title: Magnetic Properties of Martian Impact Basins Including the Effect of Mantle Excavation.
Authors: Steele, S. C.1 (AUTHOR) sarah_steele@fas.harvard.edu, Fu, R. R.1 (AUTHOR), Day, J. M. D.2 (AUTHOR)
Source: Journal of Geophysical Research. Planets. Oct2025, Vol. 130 Issue 10, p1-18. 18p.
Subject Terms: *Geomagnetism, *Geomagnetic reversals, Magnetic properties, Magnetism, Astronomical observations, Magnetic materials, Martian craters
Abstract: The weak magnetism of many impact basins on Mars has historically been interpreted as evidence of the martian dynamo's cessation before ∼4.0 Ga. However, expected magnetic properties of these impact basins are poorly known because basin formation exhumes and recrystallizes mantle material, which has not been sampled by martian meteorites. Potential depletion of magnetic minerals in these mantle‐derived rocks, as observed when comparing terrestrial mantle xenoliths to typical crustal material, may lead to weak magnetization even in the presence of a strong, stable dynamo. A reversing or intermittently active dynamo may also permit higher magnetic mineral concentrations while remaining consistent with the weak detected magnetic field signal. To explore the relationship between ancient martian dynamo behavior and orbitally observed magnetic fields, we quantify the expected magnetic properties of large impact basins by modeling the crystallization of candidate impact melt compositions. Our results suggest maximum impact basin magnetization intensities in the 0.5–4 A/m range assuming a non‐reversing dynamo. Although these values are low compared to the 1–20 A/m range expected for unperturbed martian crust, they would still yield stronger magnetic fields than those observed above the large martian impact basins Hellas, Utopia, Isidis, and Argyre. This ultimately suggests that a reversing or intermittent dynamo may be necessary to explain the weak magnetism of these martian impact basins. A martian dynamo which persisted until 3.9 to 3.7 Ga and reversed at rates >5 Myr−1 remains the most straightforward explanation for all available orbital magnetic observations and paleomagnetic measurements. Plain Language Summary: Many impact basins on Mars have weak magnetic signals when measured at spacecraft orbital altitudes. Because impacts heat large volumes of rock which acquire magnetizations as they cool, these weak fields have often been taken as evidence that Mars's core dynamo was inactive when the basins formed 4.1–4.0 billion years ago. However, basins could also produce weak magnetic fields if they contain systematically lower concentrations of key magnetic minerals, such as magnetite and pyrrhotite, than typical martian crust. A dynamo that experienced polarity reversals, like the Earth's, could also result in weak basin fields for greater concentrations of magnetic minerals. We tested these proposed mechanisms by modeling the crystallization of martian impact melts and quantifying their resulting magnetic properties. We found that martian impact basins would be depleted in magnetic minerals compared to the crust but would still appear more magnetic than we presently observe if they formed in a strong and non‐reversing dynamo. Our results suggest that Mars' magnetic field probably persisted until 3.9 to 3.7 billion years ago and experienced polarity reversals. Key Points: Martian impact basins should be depleted in ferromagnetic minerals compared to the crustIf basins formed in a uniform 50 μT field, natural remanent magnetization intensities would range from 0.5 to 4 A/mReproducing the weak magnetic fields above Hellas, Utopia, Isidis, and Argyre likely requires a reversing or intermittent dynamo [ABSTRACT FROM AUTHOR]
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Abstract:The weak magnetism of many impact basins on Mars has historically been interpreted as evidence of the martian dynamo's cessation before ∼4.0 Ga. However, expected magnetic properties of these impact basins are poorly known because basin formation exhumes and recrystallizes mantle material, which has not been sampled by martian meteorites. Potential depletion of magnetic minerals in these mantle‐derived rocks, as observed when comparing terrestrial mantle xenoliths to typical crustal material, may lead to weak magnetization even in the presence of a strong, stable dynamo. A reversing or intermittently active dynamo may also permit higher magnetic mineral concentrations while remaining consistent with the weak detected magnetic field signal. To explore the relationship between ancient martian dynamo behavior and orbitally observed magnetic fields, we quantify the expected magnetic properties of large impact basins by modeling the crystallization of candidate impact melt compositions. Our results suggest maximum impact basin magnetization intensities in the 0.5–4 A/m range assuming a non‐reversing dynamo. Although these values are low compared to the 1–20 A/m range expected for unperturbed martian crust, they would still yield stronger magnetic fields than those observed above the large martian impact basins Hellas, Utopia, Isidis, and Argyre. This ultimately suggests that a reversing or intermittent dynamo may be necessary to explain the weak magnetism of these martian impact basins. A martian dynamo which persisted until 3.9 to 3.7 Ga and reversed at rates >5 Myr−1 remains the most straightforward explanation for all available orbital magnetic observations and paleomagnetic measurements. Plain Language Summary: Many impact basins on Mars have weak magnetic signals when measured at spacecraft orbital altitudes. Because impacts heat large volumes of rock which acquire magnetizations as they cool, these weak fields have often been taken as evidence that Mars's core dynamo was inactive when the basins formed 4.1–4.0 billion years ago. However, basins could also produce weak magnetic fields if they contain systematically lower concentrations of key magnetic minerals, such as magnetite and pyrrhotite, than typical martian crust. A dynamo that experienced polarity reversals, like the Earth's, could also result in weak basin fields for greater concentrations of magnetic minerals. We tested these proposed mechanisms by modeling the crystallization of martian impact melts and quantifying their resulting magnetic properties. We found that martian impact basins would be depleted in magnetic minerals compared to the crust but would still appear more magnetic than we presently observe if they formed in a strong and non‐reversing dynamo. Our results suggest that Mars' magnetic field probably persisted until 3.9 to 3.7 billion years ago and experienced polarity reversals. Key Points: Martian impact basins should be depleted in ferromagnetic minerals compared to the crustIf basins formed in a uniform 50 μT field, natural remanent magnetization intensities would range from 0.5 to 4 A/mReproducing the weak magnetic fields above Hellas, Utopia, Isidis, and Argyre likely requires a reversing or intermittent dynamo [ABSTRACT FROM AUTHOR]
ISSN:21699097
DOI:10.1029/2025JE009173