Alphonsus (crater)
Lunar Orbiter 4 image
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| Coordinates | 13°24′S 2°48′W / 13.4°S 2.8°WCoordinates: 13°24′S 2°48′W / 13.4°S 2.8°W |
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| Diameter | 119 km |
| Depth | 2.7 km |
| Colongitude | 4° at sunrise |
| Eponym | Alfonso X |
Alphonsus is an ancient impact crater on the Moon that dates from the pre-Nectarian era. It is located on the lunar highlands on the eastern end of Mare Nubium, west of the Imbrian Highlands, and slightly overlaps the crater Ptolemaeus to the north. To the northwest is the smaller Alpetragius.
The surface of Alphonsus is broken and irregular along its boundary with Ptolemaeus. The outer walls are slightly distorted and possess a somewhat hexagonal form.
A low ridge system of deposited ejecta bisects the crater floor, and includes the steep central peak designated Alphonsus Alpha (α). This pyramid-shaped formation rises to a height of 1.5 km above the interior surface. It is not volcanic in origin, but rather is made of anorthosite like the lunar highlands.
The floor is fractured by an elaborate system of rilles and contains three smaller craters surrounded by a symmetric darker halo. These dark-halo craters are cinder cone-shaped and are believed by some to be volcanic in origin, although others think they were caused by impacts that excavated darker mare material from underneath the lighter lunar regolith.
The Ranger 9 probe impacted in Alphonsus, a short distance to the northeast of the central peak. Harold Urey said of a close-up photograph of Alphonsus:
Alphonsus was one of the primary alternative landing sites considered for both the Apollo 16 and the Apollo 17 missions.
Alphonsus is one of the sites noted for transient lunar phenomena, as glowing red-hued clouds have been reported emanating from the crater. On October 26, 1956, the lunar astronomer Dinsmore Alter noted some blurring of the rilles on the floor of Alphonsus in the photographs he took in violet light. The same blurring did not occur in the infrared photographs he took at the same time. However, few professional astronomers found this evidence of volcanic activity on the Moon very convincing.
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