Free Astronomy Magazine July-August 2019

region was renamed Descartes For- mation. Petrologists have claimed that Apollo 16 should have landed on the Kant Plateau, as it resembled a primitive crustal block that, al- though cratered, did not seem masked by volcanism. But geolo- gists rejected this idea precisely for this reason: they were looking for evidence of volcanism. They pointed out that just beyond the western slope of the highland there was the Descartes Formation and that a landing in one of the valleys that seemed wedged in the Cayley Formation would have al- lowed a single mission to sample both types of terrain. The information provided by the direct observations of Apollo 16 upset the scientific logic surround- ing the choice of the landing site. The Cayley Formation was certainly not a volcanic plain. The nature of Descartes Formation was uncertain because it was not clear whether it had been sampled. If Stone Mountain was of volcanic origin, it would have been masked by South Ray ejecta. Although the Smoky Mountain area was not sampled, nothing was observed in North Ray that looked volcanic. Apollo 16 re- sults have suggested that there are probably few (if any) mountains of volcanic origin on the Moon. Nearly three and a half years after the first lunar steps were taken by Armstrong and Aldrin, the Apollo 17 mission began. The landing site was chosen al- ready knowing that it would be the final mission. As the selection had taken place prior to the Apollo 16 flight, it was influenced by the extent to which crucial events in the lunar history had already been understood or were likely to be ad- dressed by Apollo 16. The target of Apollo 17 has, there- fore, been hotly debated. Tycho, in the southern highlands, and Tsi- olkovsky, on the far side of the Moon, were of special interest but were impractical from an opera- tional point of view. The Humorum Basin, south of Oceanus Procel- larum, is partially flooded, while Gassendi Crater, 93 km in diameter, stretches between the edge and the north shore of the internal mare. A landing in this crater would have allowed us to sample its central summit, to date the crater and probably to shed light on the surrounding basin. The choice of the last landing site was nevertheless motivated by the need to refine the timeframe in which the lunar heat engine had been active. As the formation of Mare Imbrium was well-known, as was the awak- ening of the lavas that flooded most basins over the next 500 mil- lion years, the mission target was late volcanism. Marius Hills and Davy Rille did not deserve a ‘J’ mis- sion, so the choice narrowed down to Gassendi and Alphonsus Craters, the latter 100 km in diameter, which seemed to have volcanoes inside and a ‘dark cover’ on the east edge of Serenitatis. Although the Serenitatis Basin was not flooded by the lava until sometime later, it is thought that the process began before the start of the ascent in Imbrium. Obvi- ously, Serenitatis was not flooded at one time. There were dark ma- terials around the southeastern edge, and the general opinion was that it was significantly younger than the lighter shades in the mid- dle. A visit to this area would have allowed us to sample both the old and younger soils. Thanks to the experienced staff and the excellent performance of the equipment, all aspects of the final landing mission were con- ducted with competence, precision 20 ASTRONAUTICS