A hybrid origin of Martian crustal dichotomy: Degree-1 convection following a giant impact

Robert I. Citron

Dept. Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA; ricitron@berkeley.edu

 

This webpage is a summary of: Citron, R. I., M. Manga, and E. Tan (2018), A hybrid origin of the Martian crustal dichotomy: Degree-1 convection antipodal to a giant impact, Earth planet. Sci. Lett., 491, 58-66.

The oldest surface feature on Mars is its crustal dichotomy, a ~ 5 km difference in surface elevation and ~ 26 km difference in crustal thickness between the northern and southern hemispheres. This dichotomy originated within 100s of Myr of when the planet formed (Figure 1). There is growing consensus that this planet-scale feature is the result of a giant impact (e.g., Marinova et al., 2008) and this can explain the elliptical shape of the dichotomy boundary (Andrews-Hanna et al., 2008). However, it has also been suggested that the dichotomy formed as a result of degree-1 mantle convection (e.g., Roberts & Zhong, 2006). A superplume could have created the dichotomy by thickening the southern crust, and associated melt production might explain the strength and pattern of remnant crustal magnetism in Mars’ southern hemisphere (Citron & Zhong, 2012). Migration of such a superplume could also explain the formation of Mars’ most dominant volcanic province, Tharsis, on the dichotomy boundary (Šrámek & Zhong, 2010) [Ed: See also Plumes on Mars?]. Although a giant impact on Mars at ~ 4.5 Ga is likely (Bottke & Andrews-Hanna, 2017), several features potentially associated with dichotomy formation cannot be explained by an impact alone.

Figure 1. Maps of martian topography and crustal thickness highlight the dichotomy between the northern lowlands and southern highlands (data from Smith et al. (2003) and Genova et al. (2016)). The red region in the western hemisphere is the Tharsis volcanic province. Click here or on Figure for enlargement.

The present work began as an attempt to investigate a hybrid model of dichotomy that includes the effects of both a giant impact and superplume formation, potentially connecting a giant impact to the formation of the Tharsis volcanic province. We hypothesized that a giant impact in one hemisphere induced superplume formation in the opposite hemisphere (Figure 2). In this scenario, a giant impact excavated a significant fraction of ancient crust in the present-day northern hemisphere. While a new northern crust would form relatively rapidly it would arise from a mantle already depleted in radiogenic-heat producing elements, by extraction of the original crust, compared with the older, more enriched southern crust (Thiriet et al., 2018, Ruedas & Breuer, 2017). Transient upwellings under the impact site would dissipate over longer timescales (10s to 100s of Myr) while the dominant upwelling(s) would migrate to underlie the thicker, insulating crust of the southern hemisphere. Melt generation from these upwelling(s) could then further thicken the crust in the hemisphere opposite the impact.

Figure 2. A giant impact causes excavation, heating, and a transient upwelling in the northern hemisphere (a), resulting in degree-1 convection forming under the thicker, enriched southern crust (b). This enhances the initial crustal variation produced by the impact, and forms melt residue (c).

While superplume formation on Mars has been investigated before (e.g., Roberts & Zhong, 2006), such studies used a large (x25) viscosity jump in the mid-mantle to initiate a degree-1 convective pattern. We tested if degree-1 convection can instead result from hemispherical differences in crustal thickness and concentrations of radiogenic-heat producing elements, as expected following a giant impact. We conducted 3D mantle convection simulations for various post-giant-impact states of early Mars. The southern crust was parameterized by an increase in the concentration of radiogenic-heat producing elements relative to the mantle and northern crust. We also tracked melt production using a tracer method.

We found that the post-impact distribution of crust and radiogenic heating elements acted as a primary driver of superplume formation (Citron et al., 2018). In our simulations a superplume formed under the southern crust, which was more enriched in radiogenic-heat producing elements, within < 100 Myr. This occurred even when the simulation began with a post-impact temperature perturbation in the northern hemisphere (Figure 3). The transient upwelling caused by impact heating dissipated over longer timescales as the insulating effect of the thicker, enriched southern crust controlled the flow pattern.

Figure 3. Example mantle convection simulation over time for early Mars after a giant impact. Yellow contours outline hotter mantle material at a residual temperature of 80 K. The southern crust (solid grey line) is enriched in radiogenic-heat producing elements relative to the northern crust (dashed grey line). The simulation begins with a temperature perturbation from a giant impact in the northern hemisphere, which quickly dissipates. Over time, a superplume develops under the more enriched crust in the southern hemisphere. Click here or on Figure for enlargement.

This effect is analogous to supercontinent cycles on Earth, where the insulating effect of continental crust has been hypothesized to promote sub-continental warm upwellings that encourage supercontinent break-up.

On a one-plate planet such as Mars, a superplume could produce additional crust in the hemisphere where it develops. The amount of additional melt produced by the superplume in our simulations is consistent with crustal thickness estimates, and may explain the stronger remnant crustal magnetic signatures observed in Mars’ southern hemisphere. While a giant impact would have likely erased any prior remnant magnetic signatures, the unique pattern of lineations of opposite polarity observed in the remnant magnetism on Mars might result from crust production spreading from a superplume center (Citron & Zhong, 2012).

Superplume formation on early Mars may also be linked to the formation of Tharsis on the dichotomy boundary. Plume migration from the south pole to Tharsis’ location is supported by observations of volcanic resurfacing, demagnetization, and increased crustal thickness along that path (Hynek et al., 2011; Cheung & King, 2014). If the large upwelling under the southern crust caused significant melting, it could have resulted in a sub-crustal layer of highly viscous melt residue. Sufficient melt residue above the superplume could induce plume migration or lithospheric rotation (Zhong, 2009;  Šrámek & Zhong, 2010), resulting in the formation of Tharsis on the dichotomy boundary. While such superplume migration requires further study, the timescale of superplume formation (~100 Myr) is sufficient to allow for additional melting and plume migration between dichotomy formation at ~ 4.5 Ga and the emplacement of Tharsis at ~ 3.7 Ga.

Our results suggest that there may be a link between the formation of Tharsis and a giant impact [Ed: See also Impact-induced Martian mantle plumes: Origin of Tharsis], and that superplume formation may be a natural result of a giant impact on early Mars. This hybrid model of Mars’ crustal dichotomy is still just one of many attempts to understand Mars’ early geodynamic evolution. Other models of dichotomy formation not yet discussed include one-ridge convection (Keller & Tackley, 2009) and a mega-impact followed by superplume formation in the same hemisphere as the impact (e.g., Golabek et al., 2011). New constraints on Mars’ crust, mantle and core should soon be provided by NASA’s InSight mission which recently landed on Mars. As the first geophysics mission to Mars, InSight should provide a new perspective on present-day Mars as well as its past geodynamic evolution.

References

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