Maria T. Zuber, ^* David E. Smith, Sean C. Solomon, James B. Abshire, Robert S. Afzal, Oded Aharonson, Kathryn Fishbaugh, Peter G. Ford, Herbert V. Frey, James B. Garvin, James W. Head, Anton B. Ivanov, Catherine L. Johnson, Duane O. Muhleman, Gregory A. Neumann, Gordon H. Pettengill, Roger J. Phillips, Xiaoli Sun, H. Jay Zwally, W. Bruce Banerdt, Thomas C. Duxbury

Elevations from the Mars Orbiter Laser Altimeter (MOLA) have been used to construct a precise topographic map of the martian north polar region. The northern ice cap has a maximum elevation of 3 kilometers above its surroundings but lies within a 5-kilometer-deep hemispheric depression that is contiguous with the area into which most outflow channels emptied. Polar cap topography displays evidence of modification by ablation, flow, and wind and is consistent with a primarily H₂O composition. Correlation of topography with images suggests that the cap was more spatially extensive in the past. The cap volume of 1.2 × 10⁶ to 1.7 × 10⁶ cubic kilometers is about half that of the Greenland ice cap. Clouds observed over the polar cap are likely composed of CO₂ that condensed out of the atmosphere during northern hemisphere winter. Many clouds exhibit dynamical structure likely caused by the interaction of propagating wave fronts with surface topography.

M. T. Zuber, O. Aharonson, G. A. Neumann, and G. H. Pettengill are in the Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. D. E. Smith, J. B. Abshire, R. S. Afzal, J. B. Garvin, H. V. Frey, G. A. Neumann, X. Sun, M. T. Zuber, and H. J. Zwally are at the Earth Sciences Directorate, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA. P. G. Ford is at the Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. K. Fishbaugh and J. W. Head are in the Department of Geological Sciences, Brown University, Providence, RI 02912, USA. A. B. Ivanov and D. O. Muhleman are at the California Institute of Technology, Pasadena, CA 91125, USA. R. J. Phillips is in the Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA. C. L. Johnson and S. C. Solomon are in the Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington D.C. 20015, USA. W. B. Banerdt and T. C. Duxbury are at the Jet Propulsion Laboratory, Pasadena, CA 91109.
^*   To whom correspondence should be addressed: zuber{at}tharsis.gsfc.nasa.gov