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Originally published in Science Express on 15 May 2008
Science 30 May 2008:
Vol. 320. no. 5880, pp. 1182 - 1185
DOI: 10.1126/science.1157546
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Research Articles

Mars North Polar Deposits: Stratigraphy, Age, and Geodynamical Response

Roger J. Phillips,1,2* Maria T. Zuber,3 Suzanne E. Smrekar,4 Michael T. Mellon,5 James W. Head,6 Kenneth L. Tanaka,7 Nathaniel E. Putzig,1 Sarah M. Milkovich,4 Bruce A. Campbell,8 Jeffrey J. Plaut,4 Ali Safaeinili,4 Roberto Seu,9 Daniela Biccari,9 Lynn M. Carter,8 Giovanni Picardi,9 Roberto Orosei,10 P. Surdas Mohit,11,12 Essam Heggy,13,14 Richard W. Zurek,4 Anthony F. Egan,1 Emanuele Giacomoni,9 Federica Russo,9 Marco Cutigni,9 Elena Pettinelli,15 John W. Holt,16 Carl J. Leuschen,17 Lucia Marinangeli18

The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter has imaged the internal stratigraphy of the north polar layered deposits of Mars. Radar reflections within the deposits reveal a laterally continuous deposition of layers, which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice. The packet/interpacket structure can be explained by approximately million-year periodicities in Mars' obliquity or orbital eccentricity. The observed ~100-meter maximum deflection of the underlying substrate in response to the ice load implies that the present-day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers. Alternatively, the response to the load may be in a transient state controlled by mantle viscosity. Both scenarios probably require that Mars has a subchondritic abundance of heat-producing elements.

1 Southwest Research Institute, Boulder, CO 80302, USA.
2 Washington University, St. Louis, MO 63130, USA.
3 Massachusetts Institute of Technology, Cambridge, MA 02139–4307, USA.
4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
5 University of Colorado, Boulder, CO 80309–0392, USA.
6 Brown University, Providence, RI 02912, USA.
7 U.S. Geological Survey, Flagstaff, AZ 86001, USA.
8 Smithsonian Institution, Washington, DC 20013–7012, USA.
9 Dipartimento InfoCom, Università di Roma "La Sapienza," I-00184 Rome, Italy.
10 Istituto Nazionale di Astrofisica, I-00133 Rome, Italy.
11 Scripps Institution of Oceanography, La Jolla, CA 92093, USA.
12 University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4.
13 Lunar and Planetary Institute, Houston, TX 77058, USA.
14 Institute de Physique du Globe de Paris, Paris, France.
15 Università Roma Tre, 00146 Rome, Italy.
16 Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA.
17 University of Kansas, Lawrence, KS 66045–7612, USA.
18 Universita' d'Annunzio 65127 Pescara, Italy.

* To whom correspondence should be addressed. E-mail: roger{at}boulder.swri.edu

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