A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean1,2,3,4,5,6,7. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency’s Mars Express spacecraft is providing new constraints on the martian subsurface8. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over ∼14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins9,10,11. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.
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We thank J. W. Head and D. D. Blankenship for constructive criticism and advice that greatly improved the paper. We also thank H. V. Frey for providing the locations and diameters of the QCDs. We are grateful to the other members of the MARSIS Science team and to the instrument operation staff. MARSIS is managed by the Agenzia Spaziale Italiana (ASI) and the National Aeronautics and Space Administration (NASA). The Mars Express mission is managed and operated by the European Space Agency. The research activities of the MARSIS PI and Italian investigators were supported by the Mars Express/ASI programme; the work of the US investigators was supported by the Mars Express/NASA project.
This file contains Supplementary Notes with supporting Supplementary Figures discussing MARSIS antenna and instrument operations, buried basins in Chyrse Planitia, Acidilia, Amazonia, Elyisum and Utopia Planitiea, estimate of MARSIS coverage of the northern lowlands and the Supplementary Figure S1.
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Earth, Planets and Space (2008)