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Letters to Nature

Nature 404, 161-164 (9 March 2000) | doi:10.1038/35004528; Received 3 December 1999; Accepted 7 January 2000

North–south geological differences between the residual polar caps on Mars

P. C. Thomas1, M. C. Malin2, K. S. Edgett2, M. H. Carr3, W. K. Hartmann4, A. P. Ingersoll, P. B. James6, L. A. Soderblom9, J. Veverka1 & R. Sullivan1

  1. Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853, USA
  2. Malin Space Science Systems, P.O. Box 910148, San Diego, California 92191, USA
  3. US Geological Survey, Menlo Park, California 94025, USA
  4. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
  5. Planetary Science Institute, Tucson, Arizona 85719, USA
  6. Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA
  7. US Geological Survey, Flagstaff, Arizona 86001, USA

Correspondence to: P. C. Thomas1 Correspondence and requests for materials should be addressed to P.C.T. (e-mail: Email: thomas@cuspif.tn.cornell.edu).

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Polar processes can be sensitive indicators of global climate, and the geological features associated with polar ice caps can therefore indicate evolution of climate with time. The polar regions on Mars have distinctive morphologic and climatologic features: thick layered deposits, seasonal CO 2 frost caps extending to mid latitudes, and near-polar residual frost deposits that survive the summer1, 2. The relationship of the seasonal and residual frost caps to the layered deposits has been poorly constrained3, 4, mainly by the limited spatial resolution of the available data. In particular, it has not been known if the residual caps represent simple thin frost cover or substantial geologic features. Here we show that the residual cap on the south pole is a distinct geologic unit with striking collapse and erosional topography; this is very different from the residual cap on the north pole, which grades into the underlying layered materials. These findings indicate that the differences between the caps are substantial (rather than reflecting short-lived differences in frost cover), and so support the idea of long-term asymmetry in the polar climates of Mars.