AS16-2518(M)

Figure 26

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Apollo 16 crossed the central highlands and then looked northward into southern Oceanus Procellarum. This region contains a mixture of terrae and of younger mare material that has flooded the terrae. The large terra mass in the foreground is Montes Riphaeus, which consist of the unflooded remnants of the rim of a large basin and the rims of two or three inundated craters. Islandlike continuations of the basin rim can be seen arcing eastward (right) from the north end of the mountains. The mare that occupies this basin in the lower right of the picture is called Mare Cognitum, the "Sea That Became Known" - but not until visited by the first successful U.S. lunar mission, Ranger 7, in July 1964. Two years and 9 months later, Surveyor 3 landed on the mare north of Montes Riphaeus along a ray of bright material emanating from the crater Copernicus. Surveyor was joined in November 1969 by Apollo 12. Then, in January 1971, after an earlier aborted try by Apollo 13, Apollo 14 landed on the terra bordering the east side of Mare Cognitum (near the horizon in this picture). The materials at the Apollo 14 landing site had been mapped as the Fra Mauro Formation and interpreted as ejecta from the Imbrium basin long before the landing. Although the site is nearly 1200 km south of the center of the Imbrium basin, the results of the mission substantiate this interpretation and verify the magnitude of the event that formed the basin. Islands and peninsulas of terra in the upper left part of the picture are also probably structures and deposits formed by the Imbrium impact.  - D.E.W.