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Crustal Structure of Mars from Gravity and Topography G JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, E08002, doi:10.1029/2004JE002262, 2004 Crustal structure of Mars from gravity and topography G. A. Neumann,1,2 M. T. Zuber,1,2 M. A. Wieczorek,3 P. J. McGovern,4 F. G. Lemoine,2 and D. E. Smith2 Received 9 March 2004; revised 1 June 2004; accepted 11 June 2004; published 10 August 2004. [1] Mars Orbiter Laser Altimeter (MOLA) topography and gravity models from 5 years of Mars Global Surveyor (MGS) spacecraft tracking provide a window into the structure of the Martian crust and upper mantle. We apply a finite-amplitude terrain correction assuming uniform crustal density and additional corrections for the anomalous densities of the polar caps, the major volcanos, and the hydrostatic flattening of the core. A nonlinear inversion for Moho relief yields a crustal thickness model that obeys a plausible power law and resolves features as small as 300 km wavelength. On the basis of petrological and geophysical constraints, we invoke a mantle density contrast of 600 kg mÀ3; with this assumption, the Isidis and Hellas gravity anomalies constrain the global mean crustal thickness to be >45 km. The crust is characterized by a degree 1 structure that is several times larger than any higher degree harmonic component, representing the geophysical manifestation of the planet’s hemispheric dichotomy. It corresponds to a distinction between modal crustal thicknesses of 32 km and 58 km in the northern and southern hemispheres, respectively. The Tharsis rise and Hellas annulus represent the strongest components in the degree 2 crustal thickness structure. A uniform highland crustal thickness suggests a single mechanism for its formation, with subsequent modification by the Hellas impact, erosion, and the volcanic construction of Tharsis. The largest surviving lowland impact, Utopia, postdated formation of the crustal dichotomy. Its crustal structure is preserved, making it unlikely that the northern crust was subsequently thinned by internal processes. INDEX TERMS: 1227 Geodesy and Gravity: Planetary geodesy and gravity (5420, 5714, 6019); 5420 Planetology: Solid Surface Planets: Impact phenomena (includes cratering); 5410 Planetology: Solid Surface Planets: Composition; 5415 Planetology: Solid Surface Planets: Erosion and weathering; 5430 Planetology: Solid Surface Planets: Interiors (8147); KEYWORDS: crustal dichotomy, impact basins, Martian crust Citation: Neumann, G. A., M. T. Zuber, M. A. Wieczorek, P. J. McGovern, F. G. Lemoine, and D. E. Smith (2004), Crustal structure of Mars from gravity and topography, J. Geophys. Res., 109, E08002, doi:10.1029/2004JE002262. 1. Introduction order 80, but crustal structure was interpreted cautiously to degree 60, or 360 km wavelength, owing to the presence of [2] The first reliable model of the structure of the crust noise. Tracking normal equations have since been generated and upper mantle of Mars from remote observations was to degree 75 [Yuan et al., 2001], to degree 80 (supplemented produced by Zuber et al. [2000] using data from the Mars by altimetric crossovers) [Lemoine et al., 2001], 85 [Tyler et Orbiter Laser Altimeter (MOLA) and the Radio Science al., 2002], and higher [Tyler et al., 2003, 2004], using new investigation of the Mars Global Surveyor (MGS) space- constants for the rotation rate and the orientation of the craft. Zuber et al. [2000] assumed a uniform crustal density Martian spin pole provided by the IAU2000 rotation model and solved for the global variations in crustal thickness [Folkner et al., 1997; Seidelmann et al., 2002]. Gravity using a gravity field derived from preliminary MGS models now incorporate tracking data coverage from the tracking [Smith et al., 1999a]. In that study, spherical Primary and Extended MGS missions and the early phases harmonic potential coefficients were derived to degree and of the Mars Odyssey mission. In the present study we exploit these advances in gravity modeling to present a 1 refined crustal inversion, which we also interpret in the Department of Earth, Atmospheric and Planetary Sciences, Massachu- context of Mars’ thermal evolution. setts Institute of Technology, Cambridge, Massachusetts, USA. 2Laboratory for Terrestrial Physics, NASA Goddard Space Flight [3] Volcanic constructs and giant chasms generate signif- Center, Greenbelt, Maryland, USA. icant power in the gravity signal at wavelengths as short as 3De´partement de Ge´ophysique Spatiale et Plane´taire, Institut de 250 km, or degree 85 [Anderson and Grimm, 1998]. The Physique du Globe de Paris, Paris, France. 4 gravity signal at spacecraft altitudes, stacked over thousands Lunar and Planetary Institute, Houston, Texas, USA. of orbits, is not yet exhausted at degree 85 [Lemoine et al., Copyright 2004 by the American Geophysical Union. 2001]. Gravity anomalies at these wavelengths are similar 0148-0227/04/2004JE002262$09.00 in spatial scale to many craters and basins, and when E08002 1of18 E08002 NEUMANN ET AL.: CRUSTAL STRUCTURE OF MARS E08002 combined with topography, contain information about the [8] The model of Zuber et al. [2000] varied from a response of the crust to bolide impacts [Melosh, 1989]. minimum of 3 km to a maximum of 92 km thickness, with Higher resolution of crustal structure than obtained by the northern lowlands characterized by a relatively uniform Zuber et al. [2000] is now feasible. To the extent that 35-km-thick crust. The crustal thickness, when averaged recent high-degree potential models exhibit statistical over the globe, was 43.5 km owing to the flattening of power-law behavior and are coherent with topography, polar topography (not 50 km as stated). The depth of we apply topographic corrections to obtain a Bouguer crustal interfaces is not known from seismic measurements, anomaly. unlike the Earth and Moon, so absolute measurements were [4] Earlier studies [Phillips et al., 1973; Bills and Ferrari, constrained by two considerations. First, the amplitude of 1978; Frey et al., 1996; Kiefer et al., 1996] used spherical crust-mantle deflections beneath the Isidis basin did not harmonic gravity and topographic data sets of relatively low permit a significantly shallower crust-mantle density inter- degree. Long-wavelength errors up to 5 km in the topogra- face (Moho). Second, crustal thickness much greater than phy [Smith et al., 1999b] hindered the early interpretations. 50 km was deemed unlikely, as any dichotomy in such a Potential coefficients to degree and order 50 were derived thick crust would have viscously relaxed during Mars’ from reanalysis of Viking and Mariner spacecraft tracking early thermal history [Zuber et al.,2000;Nimmo and using a power-law constraint [Smith et al., 1993; Konopliv Stevenson, 2000]. and Sjogren, 1995], although the coefficient uncertainty [9] Zuber et al. [2000] neglected the lower density of the exceeded signal above degree 30. With tracking below polar ice caps [Johnson et al., 2000], the higher density of 500 km available only at 40°S–50°N, these fields had poor the Tharsis volcanoes [McGovern et al., 2002], and the resolution in the polar regions, and could only partially flattening of the core-mantle boundary [Folkner et al., 1997; resolve the gravity signature of first-order topographic Yoder et al., 2003]. We apply these additional corrections features such as the equatorial mountains and the highland- based on inferred compositions. The residual anomaly is lowland boundary. then filtered and inverted using the finite-amplitude method [5] A roughly hemispheric Martian dichotomy [Mutch et of Wieczorek and Phillips [1998] to resolve thickness al., 1976; Carr, 1981] between the smooth northern low- variations at wavelengths of 300 km. The inferred Moho lands and the rougher southern highlands has been defined shape has a power-law behavior matching that of the in terms of differences in elevation and surface cratering surface. age. Lacking accurate topography the relationship of the geological dichotomy to crustal structure was unclear. Some of the dichotomy boundary coincides with the margins of 2. Data the Hellas and Isidis impacts, and part of the lowlands could [10] In this section we describe the topography, gravity, be identified with the Utopia Basin [McGill, 1989], but and assumed density values used in the inversion, as well as direct evidence for an impact origin was lacking. Using caveats associated with these models and parameters that remote radar soundings and radio occultations, Smith and bear on the final results. Zuber [1996] characterized the topographic dichotomy by a pole-to-pole slope represented by a displacement of 3 km 2.1. Topography and Reference Surfaces between the center of figure and the center of mass. Early [11] Topography of Mars has been obtained by the Mars MOLA topography revealed a 2–4 km scarp along some Orbiter Laser Altimeter (MOLA) [Zuber et al., 1992; Smith parts of the boundary, similar to terrestrial passive margins et al., 2001b], which measured the planetary radius at [Frey et al., 1998]. 600 million locations. Unique among planets, the shape [6] Martian topography [Smith and Zuber, 1996; Smith et of Mars about its center of mass is known globally at meter- al., 1999b, 2001b] was found to be bimodal when measured level accuracy [Neumann et al., 2001]. Cartographers about the planetary center of mass, with peaks at 1.5 km describe positions on Mars with reference to an ellipsoid above and 4 km below the datum, but unimodal when with mean equatorial and polar radii of 3396.19 km and measured about the center of figure, suggesting a planet- 3376.2 km respectively [Seidelmann et al., 2002]. Geo- wide redistribution of mass. On the basis of the early physicists, however, often adopt a spherical datum of radius studies, several workers postulated a global geophysical R = 3396 km for modeling the gravitational attraction of dichotomy, or decrease in crustal thickness from south to surface relief. More properly termed the planetary shape, we north, to explain the geological dichotomy [e.g., Phillips et will refer to the deviations h of planetary radius from this al., 1973; Bills and Ferrari, 1978; Phillips, 1988].
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