1. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
2. Astronomy Unit, Queen Mary, University of London, London E1 4NS, UK

Correspondence to: F. Nimmo¹ Correspondence and requests for materials should be addressed to F.N. (Email: fnimmo@es.ucsc.edu).

Abstract

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 convection^{4, 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 hydrocode^{8, 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 meteorites^{12, 13} and suggests a dichotomy formation time 100 Myr after martian accretion¹³, comparable to that of the Moon-forming impact on Earth¹⁴.

1. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
2. Astronomy Unit, Queen Mary, University of London, London E1 4NS, UK

Correspondence to: F. Nimmo¹ Correspondence and requests for materials should be addressed to F.N. (Email: fnimmo@es.ucsc.edu).

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