Editing Apollo 16 (section)

===Landing site selection===
Apollo 16 was the second of Apollo's [[List of Apollo missions#Alphabetical mission types|J missions]], featuring the use of the [[Lunar Roving Vehicle]], increased scientific capability, and three-day lunar surface stays.<ref name=astronautix>{{cite web|last=Wade|first=Mark|title=Apollo 16|url=http://www.astronautix.com/flights/apollo16.htm|publisher=[[Encyclopedia Astronautica]]|url-status=dead|archive-url=https://web.archive.org/web/20111111125604/http://www.astronautix.com/flights/apollo16.htm|archive-date=November 11, 2011}}</ref> As Apollo 16 was the penultimate mission in the Apollo program and there was no major new hardware or procedures to test on the lunar surface, the last two missions (the other being [[Apollo 17]]) presented opportunities for astronauts to clear up some of the uncertainties in understanding the Moon's characteristics. Scientists sought information on the Moon's early history, which might be obtained from its ancient surface features, the [[Geology of the Moon#Highlands|lunar highlands]]. Previous Apollo expeditions, including Apollo 14 and [[Apollo 15]], had obtained samples of pre-[[Lunar mare|mare]] lunar material, likely thrown from the highlands by [[meteorite]] impacts. These were dated from before [[lava]] began to upwell from the [[Internal structure of the Moon|Moon's interior]] and flood the low areas and basins. Nevertheless, no Apollo mission had actually visited the lunar highlands.<ref name=overview>{{cite web|title=Descartes Surprise|date=September 14, 2006|url=http://www.hq.nasa.gov/alsj/a16/a16.summary.html|publisher=[[NASA]]|editor-first=Eric M.|editor-last=Jones|editor2-first=Ken|editor2-last=Glover|work=Apollo 16 Lunar Surface Journal|access-date=September 23, 2021|archive-date=December 31, 2020|archive-url=https://web.archive.org/web/20201231213226/https://www.hq.nasa.gov/alsj/a16/a16.summary.html|url-status=live}}</ref>

Apollo 14 had visited and sampled a ridge of material ejected by the impact that created the [[Mare Imbrium|Mare Imbrium impact basin]]. Likewise, Apollo 15 had also sampled material in the region of Imbrium, visiting the basin's edge. Because the Apollo 14 and Apollo 15 landing sites were closely associated with the Imbrium basin, there was still the chance that different geologic processes were prevalent in areas of the lunar highlands far from Mare Imbrium.<ref name=overview/> Scientist Dan Milton, studying photographs of the highlands from [[Lunar Orbiter]] photographs, saw an area in the Descartes region of the Moon with unusually high albedo that he theorized might be due to [[volcanic rock]]; his theory quickly gained wide support.{{sfn|Wilhelms 1993|p=284}} Several members of the scientific community noted that the central lunar highlands resembled regions on Earth that were created by volcanism processes and hypothesized the same might be true on the Moon. They hoped scientific output from the Apollo 16 mission would provide an answer.<ref name=overview/> Some scientists advocated for a landing near the large crater, [[Tycho (lunar crater)|Tycho]], but its distance from the lunar equator and the fact that the Lunar Module would have to approach over very rough terrain ruled it out.{{sfn|Chaikin 1995|pp=454–455}}

[[File:Apollo 16 landing site AS16-M-0440.jpg|thumb|left|300px|Location of the Apollo 16 landing site]]

The Ad Hoc Apollo Site Evaluation Committee met in April and May 1971 to decide the Apollo 16 and 17 landing sites; it was chaired by [[Noel Hinners]] of [[Bellcomm]]. There was consensus the final landing sites should be in the lunar highlands, and among the sites considered for Apollo 16 were the [[Descartes Highlands]] region west of [[Mare Nectaris]] and the crater [[Alphonsus (crater)|Alphonsus]].{{sfn|Wilhelms 1993|p=286}} The considerable distance between the Descartes site and previous Apollo landing sites would also be beneficial for the network of [[seismometer]]s, deployed on each landing mission beginning with Apollo 12.<ref name=lpi>{{cite web|title=Landing Site Overview|url=http://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/landing_site/|work=Apollo 16 Mission|publisher=[[Lunar and Planetary Institute]]|access-date=September 23, 2021|archive-date=November 7, 2012|archive-url=https://web.archive.org/web/20121107170544/http://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/landing_site/|url-status=live}}</ref>

At Alphonsus, three scientific objectives were determined to be of primary interest and paramount importance: the possibility of old, pre-Imbrium impact material from within the crater's wall, the composition of the crater's interior and the possibility of past volcanic activity on the floor of the crater at several smaller "dark halo" craters. Geologists feared, however, that samples obtained from the crater might have been contaminated by the Imbrium impact, thus preventing Apollo 16 from obtaining samples of pre-Imbrium material. There also remained the distinct possibility that this objective would have already been satisfied by the Apollo 14 and Apollo 15 missions, as the Apollo 14 samples had not yet been completely analyzed and samples from Apollo 15 had not yet been obtained.<ref name=lpi/>

On June 3, 1971, the site selection committee decided to target the Apollo 16 mission for the Descartes site.{{sfn|Wilhelms 1993|p=288}} Following the decision, the Alphonsus site was considered the most likely candidate for Apollo 17, but was eventually rejected. With the assistance of orbital photography obtained on the Apollo 14 mission, the Descartes site was determined to be safe enough for a crewed landing. The specific landing site was between two young impact craters, [[North Ray (crater)|North Ray]] and [[South Ray (crater)|South Ray]] craters – {{convert|1000|and|680|m|ft|abbr=on|sp=us}} in diameter, respectively – which provided "natural drill holes" which penetrated through the lunar [[regolith]] at the site, thus leaving exposed [[bedrock]] that could be sampled by the crew.<ref name=lpi/>

After the selection, mission planners made the Descartes and Cayley formations, two geologic units of the lunar highlands, the primary sampling interest of the mission. It was these formations that the scientific community widely suspected were formed by lunar volcanism, but this hypothesis was proven incorrect by the composition of lunar samples from the mission.<ref name=lpi/>