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The Lunar Crust: Global Structure and Signature of Major Basins

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The Lunar Crust: Global Structure and Signature of Major Basins JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. E7, PAGES 16,841-16,843, JULY 25, 1996 The lunar crust: Global structure and signature of major basins GregoryA. Neumannand Maria T. Zuber1 Departmentof Earth and PlanetarySciences, Johns Hopkins University, Baltimore, Maryland David E. Smith and Frank G. Lemoine Laboratoryfor TerrestrialPhysics, NASA/Goddard Space Flight Center,Greenbelt, Maryland Abstract. New lunar gravityand topography data from the ClementineMission provide a global Bougueranomaly map correctedfor the gravitationalattraction of mare fill in mascon basins.Most of the gravity signalremaining after correctionsfor the attractionof topographyand mare fill can be attributedto variationsin depthto the lunar Moho and thereforecrustal thickness. The large range of global crustalthickness (-20-120 km) is indicativeof major spatialvariations in meltingof the lunar exteriorand/or significant impact-related redistribution. The 61-km averagecrustal thickness, constrained by a depth-to-Mohomeasured during the Apollo 12 and 14 missions,is preferentiallydistributed toward the farside,accounting for much of the offset in center-of-figurefrom the center-of-mass.While the averagefarside thickness is 12 km greaterthan the nearside,the distributionis nonuniform,with dramaticthinning beneath the farside, South Pole-Aitkenbasin. With the globalcrustal thickness map as a constraint,regional inversions of gravityand topographyresolve the crustalstructure of majormascon basins to half wavelengthsof 150 km. In order to yield crustalthickness maps with the maximum horizontalresolution permittedby the data, the downwardcontinuation of the Bouguergravity is stabilizedby a three- dimensional,minimum-slope and curvature algorithm. Both mare and non-marebasins are characterizedby a centralupwarped moho that is surroundedby ringsof thickenedcrust lying mainly within the basinrims. The inferredrelief at this densityinterface suggests a deep structuralcomponent to the surficialfeatures of multiring lunar impactbasins. For large (>300 km diameter)basins, moho relief appearsuncorrelated with diameter,but is negativelycorrelated with basinage. In severalcases, it appearsthat the multiring structureswere out of isostatic equilibriumprior to mare emplacement,suggesting that the lithospherewas strongenough to maintaintheir stateof stressto the present. Introduction radar, characterizedby vertical errors of up to 500 m, to model gravity derived from line-of-sight tracking over the Grimaldi Large, uncompensated density anomalies ("mascons") basin. Bratt et al. [1985a] used topographic data from a coincide with most nearside lunar mare basins [Muller and variety of sources, and gravity calculated from a multidisk Sjogren, 1968]. These anomaliesarise in part from the effects mass model [Wong et al., 1971, 1975] to invert for the depth of impact processeson crustal structure[e.g., Wise and Yates, of the lunar moho in 5x5 degreeblocks. They assumedthat 1970; Bowin et al., 1975; Phillips and Dvorak, 1981; Bratt et the premare basins had achieved nearly complete isostatic al., 1985a,b] and in part from subsequentvolcanic flooding by balance through viscous relaxation [Solomon et al., 1982] denser mare basalt [Conel et al., 1968; Phillips et al., 1972; prior to being filled with mare basalts. The 2- to'4.5-km Solomon and Head, 1980]. Previous analyses[Sjogren and thicknesses of mare basalts inferred by Bratt et al. were Smith, 1976; Bills and Ferrari, 1977b; Thurber and Solomon, insufficient to producethe measuredpositive gravity anomaly 1978; Bratt et al., 1985a] have attempted to constrain the over the mare basins. They concluded that the mare basins crustal structure in associationwith the major basins in spite were characterized by significant central uplift of mantle of limited topographic coverage and uncertainties in the following impact. gravity field. Such deficiencies have so far prevented a Geophysical data from the Clementine mission and uniform and comprehensiveanalysis of lunar basin structure. reanalysis of historical tracking have now shown that the Phillips and Dvorak [1981] used topographyfrom Earth-based lunar lithosphere had a variable mechanical structureand was, on average, more rigid during the period of basin formation than previously thought [Zuber et al., 1994]. These data tNow at Departmentof Earth, Atmosphericand Planetary permit a consistent analysis and comparison of crustal Sciences,Massachusetts Institute of Technology,Cambridge. structure of lunar basins, without isostatic assumptions. We apply a completeBouguer correctionto the lunar gravity field of F.G. Lemoine et al. (GLGM2: A 70th degreeand order lunar Copyright 1996 by the American GeophysicalUnion. gravity model from Clementine and historical data, submitted to Journal of Geophysical Research, 1995; hereinaftrer Paper number96JE01246. referred to as submitted manuscript), using the Clementine 0148-0227/96/96J E-01246509.00 topography (D. E. Smith et al., The topography of the Moon 16,841 16,842 NEUMANN ET AL.: CRUSTAL STRU• OF LUNAR BASINS from the Clementine LIDAR, submitted to Journal of Uncertainty in the range data results from the behavior of Geophysical Research, 1996; hereinafter referred to as the electronic subsystems under varying conditions, submitted manuscript). The resulting Bouguer anomaliesare particularly the roughness of the surface. The detection modeled by topography on the lunar Moho, assuming a system incorporated a programmable range window that constant-densitycrust and mantle. The difference between the selectedonly those returns near the estimatedrange to lunar inferred moho topographyand the surfacetopography we shall surface,but in some casesthere were many false returnswithin refer to as the crustal thickness. a range window. Returns from the highland regions were A preliminary Clementine global crustal thickness map particularly hard to discriminate from noise due to greater [Zuber et al., 1994], in spherical harmonic degreesup to 30 dispersionof incident pulsesby rough terrain. As a result, (half wavelength resolution of 180 km), also showed there are areas of poor data quality as well as gaps due to substantial crustal thinning beneath the major basins. The operational problems. Nevertheless, the accuracy of the Bouguer anomaly was mapped to a single mass sheet at moho topographyis better than 100 m at the level of resolutionof depth. Finite amplitude effects were not considered,nor were the sphericalharmonic model. loads due to the greater density of mare basalts. In this study Astronomical occultations [Watts, 1963; Morrison and we apply a further correctionfor up to 10 km of mare lavasthat Martin, 1971] provide some data near the limbs, but are not may have filled major basins subsequentto impact [DeHon, geodetically referenced. Few of the major basins were 1979]. Thicknesses of surface flows are inferred primarily adequatelysampled by the near-equatorialground tracks of the from photogeologic mapping [DeHon and Waskom, 1976; laser altimeters aboard three Apollo Orbiters. These ground Horz, 1978; Solomonand Head, 1980; Head, 1982]. Regional tracks lie within 26øN to 26øS latitude, and are very sparse inversions using a quasi-continuous, minimum-structure compared with the Clementine data set. The RMS residual inversion technique resolve subsurface structure of major between the Apollo dataset and GLTM2 is more than a basins at the highest resolution permitted by the gravity and kilometer (D. E. Smith et al., submitted manuscript, 1996), topography data. with most of the uncertainties in topographic knowledge on Crustal thickness and density variations play a major role the farside. Many differences in altimetry are due to in the structure of lunar basins [Wise and Yates, 1970; Scott, undersampling. The variance of highland topography is as 1974; Bowin et al., 1975; Bills and Ferrari, 1977b; Phillips much as 1.25 km (1 standard deviation) over a distance of 80 and Dvorak, 1981; Bratt et al., 1985a]. The largest impact km, the approximatetrack-to-track spacingof the Clementine basins on the Moon, as well as other terrestrial planets, are orbits, compared to approximately 100 m variance over the surroundedby multiple, concentricmountain rings [Hartmann mare. Lemoine et al. (submitted manuscript, 1995) inferred and Kuiper, 1962]. The origin of theserings has been ascribed errors of up to several kilometers in the orbits used to reduce to the surficial effects of the impact, structuralmodification of the Apollo data. Interpreting the Apollo topography[Kaula et the initial crater, or in strengthvariations of the lunar surface al., 1972, 1973, 1974; Davies et al., 1987] required caution. [e.g., Van Dorn, 1968; Baldwin, 1972; Dence, 1973; Melosh, The error in the presentdata set is nearly an order of magnitude 1982; Spudis, 1993]. These macroscopicfeatures have been less. studiedmainly by meansof surfaceimages. We now probethe The historical data resulted in a topographic field to lunar interior to elucidate the density structure of impact spherical harmonic degree and order 12 [Bills and Ferrari, basins. 1977a], revealing an offset of 2 km of the center-of-figure from the center-of-mass toward the lunar farside. A long- Altimetry wavelength analysis of crustal structure suggested a global dichotomy [Kaula et al., 1972] arising either from different The LIDAR instrument aboard the polar-orbiting Clementine degreesof melting on the nearside and farside [Kaula et al., spacecraft [Nozette et al., 1994] rangedto most of the lunar 1972], asymmetrical
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