Icarus 144, 313–328 (2000) doi:10.1006/icar.1999.6310, available online at http://www.idealibrary.com on Lithospheric Loading by the Northern Polar Cap on Mars Catherine L. Johnson and Sean C. Solomon Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 E-mail: cjohnson@dtm.ciw.edu James W. Head III Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Roger J. Phillips Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130 David E. Smith Geodynamics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 and Maria T. Zuber Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 24, 1999; revised December 9, 1999 New topography data for the northern polar region on Mars, re- turned by the the Mars Orbiter Laser Altimeter (MOLA) during the aerobraking hiatus and science phasing orbits, allow characteriza- tion of the topography of the present northern polar cap and its environs. Models for loading of an elastic shell by an axisymmetric load approximating the present polar cap geometry indicate that the maximum deflection of the subice basement is in the range 1200 to 400 m foran elastic lithosphere of thickness 40 to 200 km over- lain by a cap of pure H 2 O ice. Corresponding model cap volumes increase from 1.5 to 1.8 × 10 6 km 3 , as elastic lithosphere thickness decreases from 200 to 40 km. The presence of sediments in the polar cap increases the depth to basement and resulting cap volume for a given value of elastic lithosphere thickness. One-dimensional heat flow calculations indicate that the temperature at the base of the cap may approach the melting point of cap material if the litho- sphere underlying the cap is thin. The basal temperature is 170 K fora 200-km-thick lithosphere overlain by pure ice but is as great as 234 K for a 40-km-thick lithosphere overlain by a cap with a high sediment/ice ratio. Constraints on elastic lithosphere thick- ness are weak, but geologic mapping and MOLA data suggest that a flexurally derived circumpolar depression filled with sediments is consistent with elastic lithosphere thickness values in the range 60–120 km. Gravity and topography over the whole cap are poorly correlated, possibly due to viscous relaxation of long-wavelength topography, but gravity and topography overthe western portion of the main cap are consistent with an elastic lithosphere thickness of 120 km, fora crustal thickness of 50 km. Both MOLA data and geological information suggest a formerly largernorthern polarcap. The relationship of time scales for changes in the polar cap volume and extent to time scales for viscous relaxation of topography has important implications for investigations of even present polar cap topography. Viscoelastic calculations show that the Maxwell time for an Earth-like mantle viscosity for Mars (10 21 Pa-s) is 10 5 yr. The Maxwell time scales directly with the martian mantle viscosity so that values as high as 10 7 yr are possible. Time scales for changes in polar cap volume are poorly constrained, but major changes in cap volume over periods of 10 6 –10 8 yr are consistent with current understanding of polar cap processes. c  2000 Academic Press Key Words: Mars; ices; geophysics; Mars, climate; Mars, interior. 1. INTRODUCTION The northern polar cap of Mars is an important volatile reser- voir for that planet. The partitioning of volatiles among the polar caps, the atmosphere, and the regolith has changed through time, a consequence of climate changes driven by variations in plan- etary obliquity (Kieffer and Zent 1992). In particular, the polar caps have likely undergone episodic changes in mass and spatial extent, producing a temporally varying load on the underlying lithosphere. It is difficult to determine the detailed time evolu- tion of the polar caps because of complex feedbacks among cli- mate variations, resulting changes in polar cap volumes, and the influence of the polar caps on planetary dynamics (Rubincam 313 0019-1035/00 $35.00 Copyright c  2000 by Academic Press All rights of reproduction in any form reserved.