Letter

Nature 444, 905-908 (14 December 2006) | doi:10.1038/nature05356; Received 9 June 2006; Accepted 12 October 2006

MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars

Thomas R. Watters1, Carl J. Leuschen2, Jeffrey J. Plaut3, Giovanni Picardi4, Ali Safaeinili3, Stephen M. Clifford5, William M. Farrell6, Anton B. Ivanov3, Roger J. Phillips7 and Ellen R. Stofan8

  1. Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington DC 20560, USA
  2. Center for Remote Sensing of Ice Sheets, The University of Kansas, Lawrence, Kansas 66045, USA
  3. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
  4. Infocom Department, "La Sapienza" University of Rome, 00184 Rome, Italy
  5. Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA
  6. NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
  7. Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA
  8. Proxemy Research, Laytonsville, Maryland 20882, USA

Correspondence to: Thomas R. Watters1 Correspondence and requests for materials should be addressed to T.R.W. (Email: watterst@si.edu).

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 approx14 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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