The Origin and Nature of Lunar Mascons


  • J. J. Gilvarry


The origin of the mascons is explained in terms of a primordial atmosphere and hydrosphere of the moon lasting of the order of 1 b.y. from the origin 4.5 b.y. ago. On this basis, the mascons arose in four steps: (1) excavation of a large crater by meteoritic impact at a time close to the moon's origin, (2) subsequent isostatic adjustment of its rim and true floor, (3) rigidification of the crater and extensive depositon of sediments in its interior while an atmosphere and hydrosphere existed, and (4) eventual dessication of the sediments in the floor when the atmosphere and hydrosphere vanished. Rigidification of the crater (within a time about 1 m.y. after the moon's origin) prior to the main deposition of the sediments implies that the mascon load is supported by the strength of the underlying rock to produce a positive gravitational anomaly (independently of sediment density). The same history of the hydrosphere explains the presence of negative mascons in the irregular maria as a consequence of low sedimentation in these areas. The thicknesses of the mascons (at the apex, for a conical shape) in the prominent circular maria are obtained by scaling from the computer results of Conel and Holstrom for a sediment density (2.4 g/cm3) corresponding to shale. By means of Haskell's equation, it is shown that the lunar craters exclusive of those forming mascon basins cannot be compensated isostatically. It follows that the curves of Baldwin and of Gilvarry for the diameter-depth and diameter-rim relations for lunar craters can be extrapolated to infer the uncompensated depths and uncompensated rim heights of the primordial mascon craters. Available measurements of these parameters for the present time permit evaluation of isostatic adjustments in dimensions of mascon craters. On the basis of these data, a plot of the change in rim volume corresponding to isostatic subsidence of the crater rim against the volume corresponding to isostatic elevation of the cratic floor yields equality over two powers of 10 in the variables, within the accuracy of the data. This quantitative check tends to confirm strongly the isostasy of the true floors of the mascon craters and the support by the mantle rock of the mascon loads, as postulated in the theory.