Nature 453, 1220-1223 (26 June 2008) | doi:10.1038/nature07025; Received 5 December 2007; Accepted 14 April 2008

Implications of an impact origin for the martian hemispheric dichotomy

F. Nimmo1, S. D. Hart1, D. G. Korycansky1 & C. B. Agnor2

  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).

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 approx100 Myr after martian accretion13, comparable to that of the Moon-forming impact on Earth14.


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