Two landing sites had been seriously considered for Apollo 16, the crater Alphonsus, 300 miles (480 kilometers) south of the moon's center, and the region some 340 miles (550 kilometers) east-southeast of the moon's center, north of the ancient crater Descartes. In both places geologists thought they would find highland material differing in composition from the Fra Mauro samples and the basalts filling the maria. The wall of Alphonsus was, some argued, pre-Imbrian highlands material, while dark craters on its floor were thought to consist of relatively young volcanic material that might have originated at great depths. North of Descartes, two formations (Cayley and Descartes) were of major interest. Evidence indicated that both were volcanic but of different types and ages. The area is the highest topographic region in the highlands on the visible face of the moon, indicating that the Descartes volcanics represent remobilized highlands. Analysis of these materials was expected to clarify the basic processes that formed the highlands.59
Preliminary discussions among interested scientists considered both sites but did not entirely agree on either.60 Alphonsus remained a strong candidate, but in view of the fact that it would be preferable to have more results from Apollo 14 and 15 before landing there, Descartes was preferred for Apollo 16. With one more mission left, Alphonsus could still be visited. The Apollo Site Selection Board approved Descartes as the landing site for Apollo 16 at its meeting on June 3, 1971.61
Science plans for the Descartes mission were much the same as they had been for Hadley Rille: to inspect, survey, and sample materials and surface features in the landing area; emplace and activate the lunar surface experiments; and conduct photography and remote sensing of the moon from lunar orbit. The surface package included two new instruments: a magnetometer and, in place of Apollo 15's suprathermal ion detector, an active seismometer that would be excited by several explosive charges after the astronauts left. Another new experiment was an automatic far-ultraviolet camera and spectrograph to photograph several galaxies from the moon's surface. Its objective was to study the distribution of interplanetary and intergalactic hydrogen.62
Apollo 16 blasted off from Kennedy Space Center at 12:54 p.m. Eastern Standard Time on April 16, 1972. Command module Casper and lunar module Orion arrived in lunar orbit three days later. All systems functioned well until Orion separated from the command module; a malfunctioning component in the main propulsion system caused Houston to delay the lunar module's descent for nearly six hours while it was checked out. When Mission Control was satisfied, Orion fired its descent engine and landed easily on the plain at Descartes at 9:33 p.m. EST on April 20.63
In the next 71 hours mission commander John Young and lunar module pilot Charles Duke laid out the surface instruments and conducted three traverses in their lunar rover, covering in all some 27 kilometers (nearly 17 miles). While they were busy on the surface, Ken Mattingly in Casper was occupied with operating the instruments in the service module. The only serious mishap on the surface occurred when Young tripped over the cable to the heat-flow sensors, pulling it loose from the central station and incapacitating the experiment.64
Young and Duke finished their exploration, loaded the 96 kilograms (210 pounds) of samples they had collected into Orion, and rejoined Mattingly in lunar orbit on April 23. They released the moon-orbiting subsatellite, but because of recurring problems with the service propulsion system, the spacecraft was not in the optimum orbit for the satellite. As a result, the satellite crashed into the moon after only five weeks.65 During the four-day return flight they conducted additional experiments with electrophoresis, a technique that offered advantages for separating certain biological preparations that could not be efficiently done in a gravity field. A normal landing in the Pacific, north of Christmas Island, completed the mission On April 27.66
Even before the Apollo 16 rock samples had been returned to Houston it was apparent that premission interpretations of the Descartes site had not been accurate. The service module instruments showed abnormally low radioactivity and high aluminum-silicon ratios in the area - both characteristic of typical lunar highlands. Furthermore, the laser altimeter found that the extensive plateau on which the site lay was almost four miles higher than the surrounding terrain, an elevation that seemed impossible to achieve by buildup of volcanic material.67 Preliminary examination of the samples confirmed what scientists had suspected as soon as Young and Duke began to explore: the site was not volcanic. Most of them appeared to be impact breccias rather than basalts.68 This result, confirmed by later analyses, "forced a reevaluation of the process of photogeology and site selection," because the majority opinion before the mission had affirmed that the site was volcanic in origin.69
The more samples returned from the moon the less clear the picture of its origin and evolution became to scientists. At the third lunar science conference, which included presentations of information from Apollo 15 and the Soviets' Luna 16, a more comprehensive picture of the moon was evident, but fewer investigators would claim to understand its evolution. The best summary at the time seemed to be that the moon is now relatively cold and inactive but that it has gone through a complex sequence of melting and resolidifying by internal or external heating.70 Toward the end of 1972 one baffled scientist, Gerald Wasserburg, summarized the frustrations of the lunar scientists: "we've got answers but not the questions," he said. "I'm not sure we're asking the questions in exactly the right way."71
59. N. W. Hinners, "Apollo 16 Site Selection," in Apollo 16 Preliminary Science Report, prepared by NASA MSC, NASA SP-315 (Washington, 1972).
60. Petrone to Mgr., Apollo Spacecraft Program, MSC, "Selection of Landing Sites for Apollo 16 and 17," Mar. 11, 1971; Lee R. Scherer to multiple addressees, "Apollo 16 and 17 Site Selection Discussions," May 5, 1971, with encl., "Minutes of the Ad Hoc Site Selection Committee Meeting on Preliminary Apollo 16 and 17 Site Selection Discussions," Apr. 14, 1971; Harold C. Urey to Hinners, May 11, 1971; James R. Arnold to Scherer, May 13, 1971; Paul W. Gast to Dir., Apollo Lunar Exploration, "Site for Apollo 16," May 14, 1971; John A. O'Keefe to Hinners, May 14, 1971.
61. Petrone, TWX to multiple addressees, subject: Apollo 16 Landing Site, June 10, 1971; James A. McDivitt to multiple addressees, "Apollo 16 Landing Site," June 11, 1971.
62. MSC, "Apollo 16 Mission Report," MSC-07230, Aug. 1972, pp. 12-1, A-7 to A-22; Richard R. Baldwin, "Mission Description," in Apollo 16 Preliminary Science Report.
63. Apollo 16 Mission Report, pp. 3-1, 3-2.
64. Baldwin, "Mission Description." The cable incident was the first to knock out an experiment, but every mission had experienced similar problems with cables that would not lie flat on the lunar surface. Furthermore, the space suits did not permit astronauts to see their own feet. Nonetheless, it caused one columnist to splutter, " . . . what we have been watching [is not] science. Those two klutzes up there on the moon, bumping into each other, unable to repair what their clumsiness has damaged, didn't look like scientists or lab technicians even. They looked like . . . a couple of miscast wahoo military officers. Nicholas von Hoffman, Two Klutzes On the Moon," Washington Post, Apr. 24, 1972. Von Hoffman was rather hyperbolically protesting the continuation of Apollo and the amount of time devoted to coverage of the missions by the television networks.
65. Apollo 16 Mission Report, p. 1-3; O'Toole, "90-Lb. Scientific Satellite Crashes On Moon, Ends Study 47 Weeks Early," Washington Post, May 31, 1972.
66.Apollo 16 Mission Report, pp. 5-20 to 5-21, 3-9; Baldwin, "Mission Description."
67. O'Toole, "Scientists Think Apollo 16 Rocks Most Important Haul Yet," Washington Post, Apr. 30, 1972.
68. Idem, "Metallic Iron in Rocks Baffles Moon Scientists," ibid., May 17, 1972; "Moon Theory Upset By Study of Rocks Astronauts Gathered," Wall Street Journal, May 17, 1972.
69. Hinners, "Apollo 16 Site Selection."
70. Allen L. Hammond, "Lunar Research: No Agreement on Evolutionary Models," Science 175 (1972):868-70.
71. Wilford, "Apollo 17 Crew to Seek Questions for Answers Found by Previous Moon Flights," New York Times, Nov. 12, 1972.