Viscous and impact demagnetization of Martian crust

Abstract

Magnetization of Martian crust has been modified by impact‐induced shock waves and viscous decay since the cessation of the core dynamo of Mars at around 4 Gyr ago. Thermal evolution models of Mars suggest that the potentially magnetic layer was about 85 km thick during the active period of the core dynamo, assuming magnetite as the major magnetic carrier. The lower boundary of the magnetic layer has gradually decreased, by a total of about 30 km, through viscous decay of magnetization. The large impacts that created the giant basins Hellas, Argyre, and Isidis have almost completely demagnetized the crust beneath the basins. The shock wave pressure produced by impacts that created craters of diameters 300–1000 km is expected to significantly demagnetize the crust beneath the craters. However, except for a few craters, there is no signature of appreciable demagnetization. This implies that either the magnetic carriers have high coercivity and have resisted demagnetization, or magnetic source bodies are deep seated, or they have acquired magnetization after the intensive impact cratering period. An alternate possibility is that the scaling laws proposed for small craters do not apply to the large craters considered in this paper.

Constraining the Date of the Martian Dynamo Shutdown by Means of Crater Magnetization Signatures

“…Due to the high temperature and pressure that arise when an impact takes place, most of the crust of the impact basin gets demagnetized, both laterally and in depth, through excavation, shock, and heating (see, e.g., Arkani‐Hamed, ; Lillis, Stewart, et al, ; Louzada et al, ; Shahnas & Arkani‐Hamed, ). In the presence of an ambient magnetic field, it gets remagnetized by acquiring thermoremanent magnetization as the affected crust cools below its magnetic blocking temperature.…”

Magnetic zones of Mars: Deformation‐controlled origin of magnetic anomalies

“…If such minerals are present within the Mars crust, they would inherit a larger magnetic coercivity. There is some evidence that the more intense magnetic anomalies consist of magnetic material of higher coercivity based on distribution of magnetic anomalies near impact craters (Kletetschka et al 2004b) and magnetic anomaly modeling (Shahnas and Arkani-Hamed 2007). Therefore we suggest that magnetic anomalies on Mars may have acquired anomalously intense magnetization by virtue of separation of magnetic minerals from the silicate in the forms of continuous bands (gneiss texture).…”

Mars: a small terrestrial planet

“…23) proves that a dynamo was active in the liquid core, once in the past. The northern lowlands, as well as several the giant basins, are devoid of significant anomalies, suggesting an early magnetic field cessation on in the Noachian before the last giant impacts that would have reset the crust (e.g., Shahnas and Arkani-Hamed 2007). However, the exact timing of the cessation remains debated because of the presence of anomalies related to post-Noachian volcanism, such as the volcano Apollinaris Patera (Langlais and Purucker 2007;Milbury and Schubert 2010).…”