Giant impacts, heterogeneous mantle heating and a past hemispheric dynamo on Mars
Introduction
Giant impacts have strongly influenced the internal structure and dynamics of the terrestrial planets during the primordial stages of their evolutions (Hartmann and Davis, 1975, Benz et al., 1988, Asphaug et al., 2006, Andrews-Hanna et al., 2008, Marinova et al., 2008, Nimmo et al., 2008, Jutzi and Asphaug, 2011). These events are plausible explanations for remarkable features of the solar system such as the small volume of Mercury’s mantle relative to its core (Benz et al., 1988, Gladman and Coffey, 2009), the Earth–Moon system (Canup, 2004) and the topographic martian and lunar hemispheric dichotomies (Marinova et al., 2008, Nimmo et al., 2008, Jutzi and Asphaug, 2011). Giant impacts have also been invoked to explain the initiation or cessation of the dynamos of the terrestrial planets and moons (Roberts et al., 2009, Arkani-Hamed and Olson, 2010, Reese and Solomatov, 2010, Monteux et al., 2013, Monteux and Arkani-Hamed, 2014). In these models, the impactors’ radii typically range between 100 and 1000 km. These impacts deliver a large amount of heat to the deep mantle, which is likely to strongly affect the efficiency of core cooling and in turn the dynamo activity. 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).
On Earth, the influence of lower mantle thermal heterogeneity on core magnetohydrodynamics has been extensively studied using numerical dynamos with imposed non-uniform outer boundary conditions. It has been shown that heterogeneous CMB heat flux causes a deviation from axisymmetry in the core flow (Aubert et al., 2007), in the time-average paleomagnetic field (Olson and Christensen, 2002) and in locations of intense magnetic flux patches on millennial time-scales (Bloxham, 2002, Amit et al., 2010). It may also explain the emergence of intense magnetic flux patches in the equatorial region (Amit and Choblet, 2012) and may even yield field locking (Gubbins et al., 2007, Willis et al., 2007). Heterogeneous CMB heat flux may also recover the lateral variations in the inner-core boundary seismic properties (Aubert et al., 2008, Amit and Choblet, 2009). Finally, reversal frequency and the trajectory of the paleomagnetic dipole axis during reversals may also be governed by the heterogeneous lower mantle (Glatzmaier et al., 1999, Kutzner and Christensen, 2004, Olson et al., 2010, Olson et al., 2013, Olson and Amit, 2014).
Heterogeneous mantle control has also been proposed to explain some features of planetary magnetic fields. Cao et al. (2014) found that high equatorial CMB heat flux breaks the core flow symmetry and produces north–south asymmetric magnetic fields which may explain the observed field of Mercury (Anderson et al., 2012). Stanley (2010) argued that temperature differences in the surrounding envelope of the convective zone of Saturn axisymmetrize its magnetic field. It has also been proposed that CMB heterogeneity may have controlled the shape of the current Martian magnetic field (Stanley et al., 2008). Mars is characterized by a striking magnetic field dichotomy, which is correlated with the topographic dichotomy. The Northern Lowlands are mostly devoid of significant magnetic fields. In contrast the southern highlands exhibit large and in some places intense magnetic field anomalies, up to 1500 nT at 90 km altitude as measured by Mars Global Surveyor (Acuña et al., 1998). This is two orders of magnitude larger than the crustal magnetic field on Earth. In terms of magnetized material, this suggests a thick (40 km) and intensely magnetized (up to 12 A/m) lithosphere to produce the observed magnetic field (Langlais et al., 2004), or any combination of a thinner lithosphere and a more intense magnetization (e.g., Parker, 2003).
The martian magnetic dichotomy can be explained using two end-members scenarios. In the external scenario, the dynamo was equally strong in both hemispheres, and the resulting magnetization was equally strong in both hemispheres. Then the magnetization of the northern hemisphere was removed or erased after the dynamo cessation, e.g., by a giant impact (Nimmo et al., 2008) or volcanic activity (Lillis et al., 2008). Alternatively a significant magnetization was never recorded in the northern hemisphere because surface conditions, lithological or alteration processes were different from those in the southern hemisphere (Rochette, 2006, Quesnel et al., 2009, Chassefière et al., 2013). In the internal scenario, the magnetization is strong only in the southern hemisphere because the dynamo was hemispheric to begin with (Langlais and Amit, 2008, Stanley et al., 2008, Amit et al., 2011).
Such an hemispheric dynamo could have been driven by CMB heat flux heterogeneity possibly caused by a very large-scale mantle convection pattern (Harder and Christensen, 1996, Zhong and Zuber, 2001, Elkins-Tanton et al., 2003, Elkins-Tanton et al., 2005, Ke and Solomatov, 2006, Roberts and Zhong, 2006) or by a giant impact (Roberts et al., 2009). In this study we propose a model for the magnetic field dichotomy in which the dynamo hemisphericity (internal origin) is related to a large impact (external origin). For that purpose, we model heterogeneous CMB heat flux resulting from giant impact heating and investigate its influence on the core dynamo by imposing it as a static, laterally-varying outer boundary condition on numerical dynamo models. In this approach, the CMB heat flux pattern and amplitude, as well as the reduction in the mean heat flux with respect to a reference pre-impact value, are determined by the impactor size, using a synthetic description of the impact heating zone. In Section 2 we describe our method. The results are presented in Section 3. Discussion, post-impact time evolution and applicability of our results to Mars are given in Section 4. Conclusions and possible planetary applications are highlighted in Section 5.
Section snippets
Impact heating at the Core Mantle Boundary
Large impacts brought to Mars a formidable amount of energy that is a function of the impactor mass and velocity, the latter strongly depending on the impacted planet radius R. After a large collision on a Mars-size body, a significant fraction of this energy is deeply buried as heat within the mantle and leads to a local temperature increase below the impact site. The size and the shape of the post-impact thermal anomaly depend on several parameters such as the size of the impactor, the
Influence of impact heating on core dynamo
Various CMB heat flux models corresponding to varying impactor sizes were examined. We considered two extreme geographic locations of the impactor, with a center falling on the geographical pole or on the equator. The impactor introduces warm material to the lower mantle, thus locally decreases the CMB heat flux. Apart from pattern dependence, the reduction in mean CMB heat flux (expressed by the Ra number compared to its reference homogeneous case) as well as the amplitude of the heat flux
Implication for crustal magnetic dichotomy
Our results show that large (and therefore large Ra) are needed in order to get a dichotomy as strong as the observed. As large Ra are likely to be associated with reversing dynamos (e.g., Kutzner and Christensen, 2002), the possibility to record a dichotomy in the crust depends on the duration of a chron relative to the duration of the crystallization of the IIMM induced by the impact. If a slow crystallization rate was coupled with short magnetic chrons (Dietrich and Wicht, 2013), then
Conclusion
Can the observed martian hemispheric magnetic dichotomy be the consequence of a giant impact that has led to an internal hemispheric magnetic field? Our results show that a ∼1000 km radius impactor can generate a sufficiently large scale thermal anomaly at the CMB so that a hemispheric dichotomy comparable to the observed is generated. An impact induced CMB heat flux heterogeneity is more efficient than a synthetic degree-1 CMB heat flux heterogeneity in generating strong hemispheric magnetic
Acknowledgments
J. Monteux was funded by Agence Nationale de la Recherche (Accretis Decision No. ANR-10-PDOC-001-01). We thank Johannes Wicht for providing the MAGIC code. We thank two anonymous reviewers for their careful and insightful reviews that greatly improve the manuscript. The authors also thank N. Mangold and J.P. Lorand for useful discussions.
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