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

Research areas:
Phys. Earth Planet. Int.
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 as an explanation for
the formation of the Northern Lowlands on Mars. Such
an impact probably led to strong and deep mantle
heating which may have had implications on the
magnetic evolution of the planet. We model the
effects of such an impact on the martian magnetic
field by imposing an impact induced thermal
heterogeneity, and the subsequent heat flux
heterogeneity, on the martian core-mantle boundary
(CMB). The CMB heat flux lateral variations as well
as the reduction in the mean CMB heat flux are
determined by the size and geographic location of
the impactor. 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
hemispheric magnetic dichotomy is mostly controlled
by the horizontal Rayleigh number Rah which
represents the vigor of the convection driven by the
lateral variations of the CMB heat flux. We show
that, for a given Rah, an impact induced CMB heat
flux heterogeneity is more efficient than a
synthetic degree-1 CMB heat flux heterogeneity in
generating strong hemispheric magnetic
dichotomies. Large Rah values are needed to get a
dichotomy as strong as the observed one, favoring a
reversing paleodynamo for Mars. 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.