THE CRUST OF THE MOON: CURRENT UNDERSTANDING AND SOME REMAINING PROBLEMS

The samples returned by Apollo show that the Moon has a crust and mantle, having undergone global differentiation soon after its formation 4.6 Ga ago. Thirty years later, details of the composition and structure of that crust remain active areas of study. Two remote sensing missions flown in the 1990's, Clementine and Lunar Prospector (LP), have provided us with global maps of surface composition of selected elements and minerals. Using the large craters and basins of the Moon as natural drill holes, we can infer the composition and structure of the crust.

The lunar crust is heterogeneous. Lateral variations have been understood for some time; differences among Apollo sites (e.g., Apollos 14 and 15 = basaltic, while Apollo 16= feldspathic) document that the Moon's crust varies on scales of tens to hundreds of kilometers. The global data provided by Clementine and LP show major compositional provinces, including a highly feldspathic area located on the central far side, that represents the primordial anorthosite crust created by plagioclase floatation in the magma ocean. A mafic zone on the central southern far side occupies the floor of the largest impact basin on the Moon, the South Pole-Aitken basin, over 2600 km in diameter. The impact that formed this basin stripped off the upper crust in this region, exposing lower crust and/or upper mantle.

This relation, and an enrichment in mafic material with increasing basin size, suggests that the lunar crust is also vertically heterogeneous, with iron- and magnesium-rich material more abundant at depth. The mafic lower crust may be the source region of the enigmatic lunar material “low-K Fra Mauro” basalt, a rock with a cotectic composition, but only found as an impact melt breccia, never as a pristine igneous rock. A corollary of such a supposition is that this material provides us with a direct sample of deep layers of the crust, only accessed by large, basin-forming impacts. Thus, LKFM melt rocks form in basin events and their ages address one of the key mysteries of early lunar history, the “cataclysm” which occurred 3.9 Ga ago, long after the Moon formed. Lunar science seeks to understand the interplay of these internal and external events.