DOI: 10.1126/science.287.5459.1788, 1788 (2000);287 Science, et al.Maria T. ZuberGlobal Surveyor Topography and GravityInternal Structure and Early Thermal Evolution of Mars from Mars This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles ): January 31, 2011 www.sciencemag.org (this infomation is current as ofThe following resources related to this article are available online at http://www.sciencemag.org/content/287/5459/1788.full.htmlversion of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/content/287/5459/1788.full.html#relatedfound at:can berelated to this article A list of selected additional articles on the Science Web sites http://www.sciencemag.org/content/287/5459/1788.full.html#ref-list-1, 9 of which can be accessed free:cites 34 articlesThis article 248 article(s) on the ISI Web of Sciencecited by This article has been http://www.sciencemag.org/content/287/5459/1788.full.html#related-urls11 articles hosted by HighWire Press; see:cited by This article has been http://www.sciencemag.org/cgi/collection/planet_sciPlanetary Sciencesubject collections:This article appears in the following registered trademark of AAAS. is aScience2000 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on January 31, 2011www.sciencemag.orgDownloaded fromcomponents of Ar were clearly associated withanomalous Ca/K (determined from37Ar/39Ar), whichwe construe to reflect Ca-rich clasts that were in-completely degassed during impact melting; we ex-cluded such analyses from isochron regressions. Insome other cases, more than two components of Arare indicated by nonlinear scatter on isochron dia-grams. In such cases the age error is multiplied by thesquare root of the mean squared weighted deviates,such that isochron ages with excess scatter are de-weighted and have little impact on our conclusions.Terrestrial atmospheric contamination is efficientlyremoved in sample cleaning and preanalysis bakeout,as indicated by consistently low40Ar/36Ar in even thelowest temperature extraction steps and by the lin-earity of the isochron data.22. Summary data for all isochron ages are available atScience Online (www.sciencemag.org/feature/data/1044416.shl).23. P. Eberhardt, Proc. Lunar Planet. Sci. Conf. 3, 1821(1972).24. T. R. McGetchin, M. Settle, J. W. Head, Earth Planet.Sci. Lett. 20, 226 (1973).25. G. H. Heiken, T. Vaniman, B. French, Eds., LunarSourcebook (Cambridge Univ. Press, New York,1991).26. E. C. T. Chao, USGS J. Res. 1, 1 (1973).27. R. V. Morris, R. Score, C. Dardano, G. Heiken, Tech.Rep. JSC 19069 (Lyndon B. Johnson Space Center,1983).28. F. Ho¨rz, R. Grieve, G. H. Heiken, P. Spudis, A. Binder,in Lunar Sourcebook, G. H. Heiken, T. Vaniman,B. French, Eds. (Cambridge Univ. Press, New York,1991), pp. 285–356.29. J. P. Grotzinger, S. A. Bowring, B. Z. Saylor, A. J.Kaufman, Science 270, 598 (1995).30. J. P. Grotzinger, Am. Assoc. Petrol. Geol. 18, 1954(1997).31. P. R. Renne et al., Chem. Geol. (Isot. Geosci. Sect.)145, 117 (1998).32. Supported by the Ann and Gordon Getty Foundation.We thank W. Alvarez, A. Deino, J. Donovan, D. Karner,K. Ludwig, and T. Teague for discussions and variousother invaluable contributions to this research, K.Nishiizumi for curating the samples, and NASA forproviding them.11 August 1999; accepted 18 January 2000Internal Structure and EarlyThermal Evolution of Mars fromMars Global SurveyorTopography and GravityMaria T. Zuber,1,4* Sean C. Solomon,2Roger J. Phillips,3David E. Smith,4G. Leonard Tyler,5Oded Aharonson,1Georges Balmino,6W. Bruce Banerdt,7James W. Head,8Catherine L. Johnson,2Frank G. Lemoine,4Patrick J. McGovern,2† Gregory A. Neumann,1,4David D. Rowlands,4Shijie Zhong1Topography and gravity measured by the Mars Global Surveyor have enableddetermination of the global crust and upper mantle structure of Mars. Theplanet displays two distinct crustal zones that do not correlate globally withthe geologic dichotomy: a region of crust that thins progressively from southto north and encompasses much of the southern highlands and Tharsis provinceand a region of approximately uniform crustal thickness that includes thenorthern lowlands and Arabia Terra. The strength of the lithosphere beneaththe ancient southern highlands suggests that the northern hemisphere was alocus of high heat flow early in martian history. The thickness of the elasticlithosphere increases with time of loading in the northern plains and Tharsis.The northern lowlands contain structures interpreted as large buried channelsthat are consistent with northward transport of water and sediment to thelowlands before the end of northern hemisphere resurfacing.The interior structure of Mars contains infor-mation on bulk composition, differentiationand melting history, and mantle dynamics,all of which bear on the planet’s thermalevolution. Global models of martian topogra-phy from the Mars Orbiter Laser Altimeter(MOLA) (1) and gravity from Doppler track-ing (2) obtained during the mapping missionof the Mars Global Surveyor (MGS) space-craft (3) have now achieved the coverage andquality required for the first reliable determi-nation of Mars’s crustal and upper mantlestructure.We analyzed updated models of topogra-phy (4 ) (Fig. 1A) and free-air gravity (5)(Fig. 1B) expanded to spherical harmonicdegree and order 60, corresponding to a spa-tial resolution of !178 km. Global correla-tions of the fields are shown in Web figure 1(6). To investigate the martian internal massdistribution, we calculated Bouguer gravityanomalies, for which the gravitational attrac-tion of surface topography is subtracted fromthe free-air gravity field to reveal the subsur-face mass distribution. Bouguer gravityanomalies have been calculated for Mars, butthe uncertainties limited geophysical inter-pretation (7).The simplest interpretation of the Bouguergravity is that subsurface mass variations area consequence of