Demagnetization by basin‐forming impacts on early Mars: Contributions from shock, heat, and excavation

Abstract

[1] Large hypervelocity impacts occurred frequently on ancient Mars, leaving many large impact basins visible today. After the planetary dynamo ceased operating, such impacts demagnetized the crust by way of (1) excavation of magnetized material, (2) heating, and (3) shock pressure. We investigate these three demagnetizing processes, both separately and in combination, using hydrocode simulations of large impacts on early Mars at a range of impact energies and using a new parameterization of the shock pressure-demagnetization behavior of candidate Martian minerals. We find that in general, shock pressure demagnetization is more important than thermal demagnetization, except in the combined case of very large impacts (more than~10 26 J) and low Curie temperature minerals such as pyrrhotite. We find that total demagnetized area has a power law dependence on impact energy (with an exponent of 0.6-0.72) and that depending on the magnetic mineral, the demagnetized area resulting for a given impact energy can vary over approximately an order of magnitude. We develop an empirical model that can be used to calculate total demagnetized area for a given impact energy and magnetic mineral (whose pressure-demagnetization properties are known). Once a reliable basin scaling law for ancient Mars (i.e., relating impact energy to final basin topography) is derived, this mineral parameterization and empirical model will allow robust constraints to be placed upon the primary Martian magnetic carrier(s).Citation: Lillis, R. J., S. T. Stewart, and M. Manga (2013), Demagnetization by basin-forming impacts on early Mars: Contributions from shock, heat, and excavation,

A spherical harmonic model of the lithospheric magnetic field of Mars

“…Figure b). This would be in accordance with the early dynamo shutdown hypothesis, as Schiaparelli's age is estimated to be approximately 3.92 Ga [ Werner , ], although the crater might be significantly older [ Lillis et al , ]. The unnamed crater labeled 2 appears to be at least partially demagnetized, and the profiles at 185 km and 0 km altitude are very similar, which may be due to larger coherence wavelengths as compared to Schiaparelli.…”

“…The magnetic signature of smaller (<1000 km in diameter) craters is more difficult to assess, as information on their magnetization cannot directly be extracted from data gathered at the MGS mapping phase orbit altitude [ Lillis et al , ]. To circumvent this problem, Lillis et al [] have used a statistical approach to investigate the magnetic signature of craters down to 300 km in diameter, and we will show here that the presented model is also capable of resolving the magnetic signature of small craters when it is downward continued to surface altitude. In this case, the diameter of the smallest crater for which a magnetic signature can be observed is limited by the resolution of the model, corresponding to ∼190 km.…”

“…The normalized circumferential averages of magnetic field intensity above the former craters as a function of radial distance from the crater's center are shown in Figure b for altitudes of 400 km (blue solid line), 185 km (black solid line), and at the surface (red solid line). Schiaparelli crater (labelled 1 here) was previously investigated by Lillis et al [], who performed a statistical analysis on the expected and observed magnetic fields at 185 km altitude and they argue that Schiaparelli is likely magnetized. The circumferentially averaged magnetic profile in Figure b (Crater 1) indicates that the signal at 185 km altitude indeed suggests that Schiaparelli is not completely demagnetized, but the presented model at lower altitudes reveals a near‐zero field region to be directly associated with the crater (cf.…”

Giant impacts, heterogeneous mantle heating and a past hemispheric dynamo on Mars

“…Although there is a higher probability that a giant impact will fall on low-latitudes of the planetary surface (Le Feuvre and Wieczorek, 2011), true polar wander events can ultimately place the resulting thermal anomaly at high-latitudes of the Core Mantle Boundary (CMB). Moreover, large impacts could be responsible for significant resurfacing and reset the magnetization of the pre-impact material (Langlais and Thébault, 2011;Lillis et al, 2013).…”

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.…”