We thank J. Preimesberger for assistance with preliminary experiments, R. DiBiase, W. Dietrich, N. Izumi, G. Parker, J. Prancevic, J. Turowski and M. Yokokawa for discussion, and A. Wickert for a review. We acknowledge funding from the National Science Foundation (grant EAR-1147381 to M.P.L. and a Graduate Research Fellowship to J.S.S.), NASA (grant 12PGG120107 to M.P.L.), and the Alexander von Humboldt Foundation (postdoctoral fellowship to J.S.S.). This work includes data services provided by the OpenTopography Facility with support from the National Science Foundation under NSF Award Numbers 1557484, 1557319 and 1557330, and EAR-1043051.

Reviewer information

Nature thanks Andrew Wickert and the other anonymous reviewer(s) for their contribution to the peer review of this work.