Mineralogical enhancement of crustal magnetization on Mars

Abstract:

Orbital magnetometers and the InSight lander discovered strong crustal magnetic fields (~10 μT) on Mars despite the lack of a detectable core dynamo. These strong crustal magnetic fields remains unexplained given that previous models of Mars’ ancient core dynamo predict a magnetic field of approximately the strength of Earth’s current field and would not produce strong remanent magnetization if Earth-like lithologies are assumed. However, the crust of Mars is more iron-rich than Earth’s, and thus could include significantly more magnetic phases such as magnetite, hematite, pyrrhotite, and titanomagnetite.

Here, we explore the relationship between the strength of Mars’ crustal magnetization and its composition in multivariate space. Using the recent Langlais model for Mars crustal magnetization and maps of surface composition from GRS, TES, CRISM and OMEGA. We find that in the Terra Sirenum region, the variance contribution (change in magnetization with predictor variables) is ~11%. The key predictor variables observed through our multivariate analysis, are absorptions at 0.53 and 1 μm (from OMEGA and CRISM, respectively; both Fe indicators). Surface materials detected by these spectrometers, typically regolith or weathered bedrock, have likely been subjected to demagnetizing processes since the decline of Mars’ dynamo. To explain the correlation between surface composition and bulk/buried magnetization, we assume that there are compositional similarities between the uppermost material and layers with remanent magnetization. This implies that the ~11% variance contribution is likely an underestimate of the compositional enhancement in Terra Sirenum as some portion of the region has almost certainly undergone demagnetization processes and regolith transport/mixing that affect the correlation we observe.

Previously, the magnitude of Mars’ crustal magnetization conflicted with models of potential ancient core dynamos. Our findings significantly reduce the discrepancy between models of ancient field strength and modern crustal magnetization suggesting that this at least partially due to compositional enhancements rather than solely a strong paleofield. In addition to providing an insight into the region’s thermal history- that heating/other demagnetizing processes are largely absent.