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Implications of an impact origin for the martian hemispheric dichotomy


The observation that one hemisphere of Mars is lower and has a thinner crust than the other (the ‘martian hemispheric dichotomy’)1,2,3 has been a puzzle for 30 years. The dichotomy may have arisen as a result of internal mechanisms such as convection4,5. Alternatively, it may have been caused by one6 or several7 giant impacts, but quantitative tests of the impact hypothesis have not been published. Here we use a high-resolution, two-dimensional, axially symmetric hydrocode8,9 to model vertical impacts over a range of parameters appropriate to early Mars. We propose that the impact model, in addition to excavating a crustal cavity of the correct size, explains two other observations. First, crustal disruption10 at the impact antipode is probably responsible for the observed antipodal decline in magnetic field strength11. Second, the impact-generated melt forming the northern lowlands crust is predicted to derive from a deep, depleted mantle source. This prediction is consistent with characteristics of martian shergottite meteorites12,13 and suggests a dichotomy formation time 100 Myr after martian accretion13, comparable to that of the Moon-forming impact on Earth14.

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Figure 1: Averaged crustal properties as a function of angular distance from impact centre (from ref.6) at 170° E, 50° N.
Figure 2: Results from typical axially symmetric impact simulation using Zeus hydrocode8,9.
Figure 3: Impact crustal excavation radius and melt production.


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This work was funded by NASA’s MFR programme.

Author Contributions F.N. and S.D.H. carried out the two-dimensional runs and analysed the results, D.G.K. modified the two-dimensional code for the present application and C.B.A. carried out the SPH runs. F.N. conceived the project and wrote the first draft of the paper.

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Correspondence to F. Nimmo.

Supplementary information

Supplementary Information

The supplementary information contains three sections: 1) model results; 2) benchmarking and comparisons with smoothed-particle hydrodynamics (SPH) approach; 3) discussion of ejecta curtain velocity and behaviour. The file also contains Supplementary Figures S1-S7 with Legends. (PDF 650 kb)

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Nimmo, F., Hart, S., Korycansky, D. et al. Implications of an impact origin for the martian hemispheric dichotomy. Nature 453, 1220–1223 (2008).

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