Geological Chronicle
Located in the lunar highlands on the eastern end of Mare Nubium and slightly overlapping Ptolemaeus to the north, Alphonsus is an ancient impact crater dating from the pre-Nectarian era. Its outer walls are slightly distorted into a somewhat hexagonal form, and the boundary with Ptolemaeus is broken and irregular. A low ridge system of deposited ejecta bisects the floor and includes Alphonsus Alpha, a pyramid-shaped central peak rising 1.5 km above the interior surface. This peak consists of highland anorthosite rather than volcanic rock, and its infrared spectrum shows pure crystalline plagioclase.
The crater floor is fractured by an elaborate system of rilles and contains four or five smaller craters surrounded by symmetric dark halos. While some researchers believe these dark-halo craters are volcanic in origin, others conclude they formed when impacts excavated darker mare material from beneath lighter regolith. Because these features were thought to be volcanic vents that might yield xenoliths of the lunar mantle, Apollo planners evaluated Alphonsus as a candidate landing site for Apollo 16 and Apollo 17 before selecting other locations.
Alphonsus is also among the sites noted for transient lunar phenomena, with observers reporting glowing red-hued clouds. On October 26, 1956, Dinsmore Alter photographed a blurring of floor rilles in violet light that did not appear in infrared exposures, and in 1958 Nikolai A. Kozyrev observed a mist-like cloud displaying spectral indications of carbon matter. Closer examination came when the Ranger 9 probe impacted northeast of the central peak, returning a close-up photograph that revealed sharp new craters alongside older depressions filled with fragmented material.