Consequences of Giant Impacts on the Martian dynamo

Wednesday, 16 December 2015: 14:10
2007 (Moscone West)
Julien Monteux, Laboratoire Magmas et Volcans, Clermont-Ferrand, France, Hagay Amit, CNRS, Paris Cedex 16, France, Jafar Arkani-Hamed, University of Toronto, Physics, Toronto, ON, Canada, Gael Choblet, LPGN Laboratoire de Planétologie et Géodynamique de Nantes, Nantes Cedex 03, France, Benoit Langlais, Lab Planetologie Geodynamique, Nantes, France, Gabriel Tobie, University of Nantes, Nantes, France, Catherine L Johnson, University of British Columbia, Department of Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada and Mark Jellinek, University of British Columbia, Vancouver, BC, Canada
The Martian surface exhibits a strong dichotomy in elevation, crustal thickness and magnetization between the southern and northern hemispheres. A giant impact has been proposed to explain the formation of the Northern Lowlands on Mars. Such an impact probably led to strong and deep mantle heating and merging between the two cores. These processes will have implications on the thermal state and on the magnetic evolution of the planet. We model the effects of such an impact on the Martian magnetic field (1) by characterizing the thermochemical consequences of the sinking of the impactor’s core as a single diapir, (2) by imposing a heat flux heterogeneity on the Martian core-mantle boundary (CMB). Our results show that large viscosity contrasts between the impactor’s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr. Direct impact heating of Martian core favor thermal stratification of the core and core dynamo cessation. The merging of the impactor’s core with the Martian core only delays the re-initiation of the dynamo for a very short time. While the core thermal stratification is likely to be evacuated rapidly, the impact induced thermal anomaly within the mantle is likely to remain stable for a longer timescale above the CMB. This thermal anomaly generates a large scale cooling heterogeneity at the CMB and a magnetic field dichotomy. A polar impactor leads to a north–south hemispheric magnetic dichotomy that is stronger than an east–west dichotomy created by an equatorial impactor. The amplitude of the magnetic dichotomy is mostly controlled by the horizontal Rayleigh number that represents the vigor of the convection driven by the lateral variations of the CMB heat flux. Our results imply that an impactor radius of 1000 km could have recorded the magnetic dichotomy observed in the Martian crustal field only if very rapid post-impact magma cooling took place.