The Geologic Investigation of the Taurus-littrow Valley: Apollo 17 Landing Site

Astronauts Cernan and Schmitt, of Apollo 17, landed in the Taurus-Littrow valley of the Moon on December 11,1972. Their major objectives were (1) to sample very ancient lunar material such as might be found in pre-Imbrian highlands as distant as possible from the Imbrium basin and (2) to sample pyroclastic materials that had been interpreted as significantly younger than the mare basalts returned from previous Apollo landing sites. The crew worked approximately 22 hours on the lunar surface; they traversed about 30 km, collected nearly 120 kg of samples, took more than 2,200 photographs, and recorded many direct geologic observations. The lunar surface data, sample results, and geologic interpretation from orbital photographs are the bases for this geologic synthesis. The Taurus-Littrow massifs are interpreted as the upper part of the thick, faulted ejects deposited on the rim of the transient cavity of the large southern Serenitatis basin, which was formed about 3.9 to 4.0 b.y. ago by the impact of a planetesimal. The target rocks, predominantly of the dunite-anorthosite-norite-troctolite suite or its metamorphosed equivalents, were fractured, sheared, crushed, and melted by the impact. The resulting mixture of crushed rock and melt was transported up and out of the transient cavity and deposited on and beyond its rim. Hot fragmental to partly molten ejecta and relatively cool cataclasite and relict target rocks were intermixed in a melange of lenses, pods, and veins. Crystallization of melts and thermal metamorphism of fine-,grained fragmental debris produced breccia composed of rock and mineral fragments in a fine-grained, coherent, crystalline matrix. Such breccia dominates the massif samples. High-angle faults that bound the massifs were activated during formation of the basin, so that structural relief of several kilometers was imposed on the ejecta almost as soon as it was deposited. Massive slumping that produced thick wedges of colluvium on the lower massif slopes probably occurred nearly contemporaneously with the faulting. Material of the Sculptured Hills, perhaps largely cataclasite excavated from the southern Serenitatis basin by the same impact, was then deposited on and around the massifs. Basalt, estimated to be about 1,400 m thick in the landing site, flooded the Taurus-Littrow graben before approximately 3.7 b.y. ago. The basalt (subfloor basalt) is part of a more extensive unit that was broadly warped and cut by extensional faults before the accumulation in Mare Serenitatis of younger, less deformed basalts that overlap it. A thin volcanic ash unit, probably about 3.5 b.y. old, mantled the subfloor basalt and the nearby highlands. It, too, was subsequently overlapped by the younger basalt of Mare Serenitatis. In the time since deposition of the volcanic ash, continued bombardment by primary and secondary projectiles has produced regolith, which is a mechanical mixture of debris derived mainly from the subfloor basalt, the volcanic ash, and the rocks of the nearby massifs and Sculptured Hills. The regolith and the underlying vol canic ash form an unconsolidated surficial deposit with an average thickness of about 14 m, sufficiently thick to permit abnormally rapid degradation of the smaller craters, especially those less than 200 m in diameter, so as to create a surface that appears less cratered than other mare surfaces. Admixed volcanic ash gives the surface a distinctive dark color, which, in combination with the less cratered appearance, led to its interpretation before the mission as a young dark mantling