Fragments of terra rock in the Apollo 12 soil samples and a structural model of the moon

Abstract

Samples of lunar soil collected by Apollo 12 in Oceanus Procellarum contain not only basalt fragments, obviously locally derived, but also a component (∼10%) of light-colored rock fragments that probably derive from the lunar terrae. Most of these are noritic in composition (i.e., consist of plagioclase and low-Ca pyroxenes in comparable amounts), not anorthositic as were the light-colored soil particles of Apollo 11. Texturally, they are recrystallized breccias. An evolutionary model is developed that is consistent with the properties of this and other lunar rock types, and with the geophysical properties of the Moon. (I) The surface layers of the Moon were melted to a considerable depth in early times, possibly by energy of accretion of the Moon. (II) Igneous differentiation occurred as the surface layer cooled; crystallizing plagioclase floated to the surface to form an anorthositic layer. Residual liquid beneath the anorthosite was noritic in composition. (III) Major planetesimal impacts stripped away the anorthosite cover locally, and liquid norite welled up to fill the holes (these are now the nonmascon maria, including Oceanus Procellarum). (IV) Additional major impacts occurred after complete solidification of the crustal system; (V) isostatic adjustment drove plugs of solid but plastic mantle material up into these basins (now the mascon maria). (VI) Much later, lava generated at depth in the Moon by radioactive decay erupted at the surface and pooled in the topographic lows (mare basins). The relationship of gravity and topography on the near side of the Moon can be used to construct models of crustal structure; substantial thicknesses (∼25 km) of norite beneath Oceanus Procellarum, and anorthosite beneath the central highlands, appear to be required to reconcile gravity with topography. Most of the Apollo 14 samples (crustal rocks excavated by the Imbrium impact) are noritic breccias similar to the Apollo 12 norites, except that they appear to have been melted for a brief time, probably by the Imbrium event itself.