In a paper published in Nature Communications, researchers modeled the cooling and magnetization of Martian impact basins to explore the effects of a reversing dynamo on magnetic fields. They found that magnetic reversals significantly weakened field strengths, causing large basins to appear demagnetized.
Study: Weak magnetism of Martian impact basins may reflect cooling in a reversing dynamo. Image Credit: MrMask100/Shutterstock.com
Their findings suggested that Mars's demagnetized basins did not necessarily indicate early dynamo cessation but could be explained by a long-lived, reversing dynamo. This scenario aligned with existing constraints on Mars's magnetic history.
Related Work
Past work investigated the early Martian dynamo’s role in climate evolution and interior processes, focusing on its debated cessation age. While weak magnetic fields above large impact basins have suggested an early shutdown, evidence from magnetized young volcanic and the meteorite Allan Hills 84001 (ALH 84001) indicated dynamo activity until at least 3.9–3.7 Ga.
Studies proposed that magnetic field reversals, forming oppositely magnetized sub-volumes within cooling basins, might explain these observations. Recent findings renewed interest in quantifying these effects with models that consider complex cooling processes and realistic magnetic properties.
Estimating Mars' Magnetic Field Strength
The study devised an analytical method to approximate the depth of uniformly magnetized layers on Mars by balancing polarity chron duration with the thermal cooling timescale. They derived formulas for depth estimation and correlated these with variables like Curie depth and reversal frequency to establish limits for magnetized material volumes.
Using estimates for remagnetization volumes, the researchers calculated expected magnetization and magnetic field strengths at different altitudes, ultimately constructing predictions for peak field strengths.
To analyze the impact effects, they set thermal conditions for Martian basins of various sizes, incorporating crater formation parameters like pressure and temperature perturbations. These calculations used empirical relationships and scaling laws to estimate transient crater dimensions, impact velocities, and post-impact temperature profiles.
Additional corrections were applied to consider stratigraphic uplift, utilizing equations derived from terrestrial impact studies. They also employed numerical simulations, especially for the larger basins, to provide precise temperature and pressure distributions.
Subsequently, cooling models were built using finite element analysis to simulate post-impact thermal evolution. These simulations involved conductivity, density, and the Martian crust’s thermal structure. Magnetization estimates were based on meteorite thermal unblocking spectra, projecting outcomes for various reversal histories.
Late impacts and geological processes were integrated, showing how stochastic events could influence magnetization. Simulations incorporated random reversal sequences, ultimately offering a comprehensive view of potential magnetic field outcomes.
Magnetic Field Evolution in Basins
The study modeled the magnetic field signals of basins in a reversing dynamo field, showing that higher reversal rates (over 1.5 Myr−1) reduced field strength. Numerical simulations generated field maps, revealing weaker fields at low reversal frequencies, particularly at 200 km altitude. Smaller basins had dipolar fields, while larger basins had more complex structures, with field strength peaks near the rim.
Increasing the mean reversal rate above 1.5 Myr−1 resulted in diminishing effects on peak field strength. This slower decline at higher reversal rates matched the predictions of an analytical model, suggesting a persistent magnetic contribution from shallow material.
Even at the highest reversal rates, several kilometers of material typically cooled before the first reversal, supporting this model. The magnetization of the layer cooled in the first polarity chron significantly impacted the final magnetic field strength, especially at high reversal rates.
Peak fields decreased with higher reversal frequencies, but the timing of reversals, especially early after basin formation, was key in determining field strength. For reversal rates above 1.5 Myr−1, most basins showed fields weaker than those above uniformly magnetized basins, indicating reversals significantly reduced Martian basin magnetic fields.
The study estimated that basins with peak fields below five nT at 200 km altitude would likely be misclassified as demagnetized, depending on basin size, reversal rate, and magnetic properties.
Variations in saturation remanence intensities led to uncertainties in determining whether basins appeared magnetized or demagnetized. Larger basins (>800 km) could appear magnetized across a range of reversal rates, while smaller basins would appear demagnetized at reversal frequencies ≥0.1 Myr−1.
In scenarios with lower saturation remanence, basins were more likely to appear demagnetized, especially at higher reversal rates. Late impacts and remagnetization could reduce magnetic field strength, particularly in large basins.
The model also showed that factors like unblocking spectra, melt migration, and post-impact processes influenced magnetic characteristics, with reversal rates being the dominant factor.
Conclusion
To sum up, the study modeled the cooling and magnetization of Martian impact basins, showing that a long-lived, reversing dynamo could significantly reduce basin field strengths, particularly for larger basins. Reversal frequencies above 1.5 Myr−1 led to weaker fields, especially in basins more significant than 800 km.
The results suggested that Martian basins formed in an active dynamo could appear demagnetized, challenging the idea of an early dynamo cessation. Ultimately, the findings supported a reversing Martian dynamo persisting until at least 3.7 Ga, with implications for core convection and atmospheric escape.
Journal Reference
Steele, S. C., et al. (2024). Weak magnetism of Martian impact basins may reflect cooling in a reversing dynamo. Nature Communications, 15:1, 1-16. doi: 10.1038/s41467-024-51092-4, https://www.nature.com/articles/s41467-024-51092-4
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