Alphonsus (top), Arzachel (bottom), and Alpetragius (middle left). Sketch by Tracy Wilson.
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Alphonsus is a large, famous crater near the center of the lunar nearside, and is one of lunar observers’ perennial favorites. It is well known as the site of the impact of Ranger 9, and its attraction for telescopic observers originates from its complex appearance and the wide variety of geological formations found there. The crater is in the middle of the Lunar Snowman arrangement of three craters. Ptolemaeus comprises the Snowman’s base while Arzachel supplies the head. Mare Nubium to the west and the Imbrian Highlands to the east complete the context. There is a wealth of detail visible in Alphonsus even with small telescopes, and at all angles of illumination, and we will concentrate here on the crater itself rather than with its surroundings.
Alphonsus is a very old impact crater, dating from the immediate post-Nectarian age, as does Alpetragius to the southwest. Nearby Ptolemaeus is an even older pre-Nectarian crater, while Arzachel is younger; the lunar snowman therefore preserves in order three different crater ages in a fairly small area. Later, during the Lower Imbrian period, the huge impact which formed the Imbrium Basin occurred, depositing Imbrium debris over the area and giving rise to the Alpes Formation to the north of Ptolemaeus. Alphonsus escaped being overwhelmed by the ejecta and uplift, but the crater walls were eroded and almost certainly settled as a result of the Imbrian impact, and the stresses of that impact also probably further weakened the crust in the floor of the crater. Around the same time as the Imbrian impact, Arzachel was created by a smaller impact, which coved the southern part of Alphonsus' floor with its ejecta. Even later, during the Upper Imbrian and Eratosthenian age, while the Nubium Basin was filling with basalt to become Mare Nubium, a number of grabens formed and several small volcanic eruptions took place on Alphonsus’ floor. The formation of Alphonsus' central peak brought deep bedrock to the surface, with important implications for our understanding of the formation of the moon. Under low lighting, the most noticeable feature of Alphonsus is the ridge running from the south central part of the crater toward the north-northwest, passing just west of Alphonsus a, the central peak of Alphonsus. The ridge is especially striking near sunset, but it can be seen a few days away from the terminator in even very small telescopes. At the north end of the ridge, the small crater Alphonsus G is sometimes a challenge in a small instrument. It has always seemed more crater-like under morning lighting to me, while during the evening it is harder to distinguish from the small graben immediately to its southeast. The ridge is probably the result of uplift in the floor of Alphonsus, the cause of which remains disputed.
The entire southern half of Alphonsus’ floor is covered with fan-shaped ejecta from Arzachel, the crater to the south. This fan-shaped feature is easily observed with a small telescope under low lighting, but unlike the ejecta blankets around free-standing craters that impacted into mare, only a small portion of Arzachel’s ejecta blanket is visible here. Arzachel’s ejecta is best preserved within Alphonsus, since the highland units around Arzachel interfere with discerning the ejecta clearly, except perhaps to the south. Sometime after the emplacement of that ejecta, a complex network of rilles formed on the floor of Alphonsus. These rilles, collectively referred to as the Rimae Alphonsus, are best seen when Alphonsus is fully illuminated and the terminator six or eight degrees from the rim. The most conspicuous of these rilles, the Rima Alphonsus I, appears in its north segment to be a crater chain. This appearance should be just visible in the best seeing.
Although many maps plot only the two relatively large rilles in the eastern part of the crater, several smaller rilles cover the eastern and southern part of the crater floor. Although these smaller rilles are challenging, they are sometimes seen telescopically. Most of the rilles trend N-S, but this may be an artifact of preferential solar illumination – rilles running east-west will never be well illuminated as relief features. All of these rilles are thought to be graben, the result of faulting and settling of terrain in the weakened crust of the crater floor, but it is clear that these graben have been modified by subsequent volcanic activity.
Associated with the rilles is a remarkable collection of three obvious black spots. This collection of black spots is easily the most conspicuous feature of Alphonsus at a high sun and one of the most conspicuous things to be seen anywhere on the full moon. They are more difficult to discern at low sun, as they are principally albedo variations. Close inspection, especially under high lighting, will reveal more than three patches. The large patch to the northeast sometimes resolves into two distinct patches of dark material, the south-easterly of which is centered around a conspicuous, though very small, crater. A short distance to the south, another patch of dark material can be seen, again associated with a small crater sometimes visible telescopically. In the southeast of the crater floor is a large dark patch associated with Alphonsus R.
The large dark patch on the western floor is also associated with a small crater that is visible telescopically, but take care not to confuse this crater with some slumped material from the crater wall that lies near here. In between this large dark patch and the central peak is a much smaller dark patch which is generally easily seen despite its small size relative to the others.
Additional minor patches of dark material are found near Alphonsus A, Alphonsus J, a short distance to the southeast of the large western patch, as well as several spots between the large western patch and Alphonsus G. Many of these are not hard to see, though they appear indistinct telescopically. In all, some 12 such patches, each associated with small craters, have been identified from Lunar Orbiter and Clementine imagery.
The dark patches are the result of volcanic processes, and the associated craters are a relatively rare example of endogenic craters on the lunar surface formed not by impact, but by internal lunar processes. Spectroscopy, along with an important sample of similar material collected by Apollo 17, has revealed the dark patches to be composed of dark volcanic glasses and clasts. The eruptions have not created any structural features to speak of, however, and this has led to the understanding that the eruptions were explosive. In such a situation, gasses build up below a rock ‘plug’ at the surface of a magma conduit, until the pressure becomes too much to resist. At that point, the plug catastrophically gives way and the pressure escapes explosively, spraying gas and a small amount of magma all over the place. In these kinds of eruptions, the new material typically stays within a few miles of the vent, consistent with what is seen at Alphonsus. Something between half and 90% of the material in these dark patches is the result of sprayed out magma cooling into volcanic glasses and other pyroclasts, and these formed the highly visible pyroclastic blankets that can be seen telescopically. The rest of the material came from the excavation of the small craters associated with each blanket. On this theory, the associated craters are fumaroles or maars. Because of the nearly identical composition of these blankets with similar features in Mare Nubium, it is thought that these eruptions took place during the early stages of the lava filling of the Nubium basin, a short distance to the west.
Another notable feature of Alphonsus is its bright central peak, designated Alphonsus a. Ranger 9 hard-landed in Alphonsus a short distance to the northeast of this peak. Both this peak and the northeast walls of the crater have substantial anorthosite deposits, and have been important to theories that the moon once had a magma ocean. If there was substantial liquid magma about at any time, anorthosite should be quite common. Alphonsus’ central peak uplifted from a depth of about 10 km below the lunar surface; interestingly, Arzachel’s peak came from slightly less than this depth but contains no anorthosite. Apparently there is a subsurface layer of anorthosite that extends just a bit deeper than 10km, or else the layer does not extend far enough south to show up in Arzachel’s peak. Alphonsus’ floor and Arzachel’s peak have the same general composition, which may give slightly more weight to the theory that the anorthosite deposit is deeper than Arzachel’s peak uplift. So far, anorthosite has been found mostly on or near basin rings, another place where deposits of material from deep excavations may be found.
Goodacre notes that Alphonsus’ walls are characterized by radial valleys or depressions, rather than the more usual slump-terracing found in many crater walls. This is well visible in Lunar Orbiter imagery and appears to be the result of ground surge scouring the crust at the time of the Alphonsine impact, modified by the effects of the Imbrian impact later on.
Alphonsus is also one of those remarkable places where observers have consistently reported hazes, obscurations, and other transient phenomena. Although the reliability of visual observations of these phenomena have been called into question, N. A. Kozyrev succeeded in taking a spectrogram of an emission in Alphonsus on November 3, 1958. The resulting spectra touched off a controversy on the question of whether or not C2 molecules were present, but it appears that questions surrounding the spectrogram and its interpretation were never settled. Although I have been an avid Alphonsus watcher for some twenty years, I have never noticed anything variable about its appearance. I suspect that much of the variability reported in its appearance is the result of the wide variety of structural and albedo detail to be seen here – and that observers sometimes find the effects of local seeing most noticeable in such areas.
Lunar Orbiter photograph of Alphonsus.
Images:
Sketch of Alphonsus, Arzachel, and Alpetragius by Tracy Wilson.
External Images:
Consolidated: Low evening, Moderate evening, Low morning, Moderate morning, Full Moon
Lunar Orbiter: Alphonsus
References:
Alter, Dinsmore; The Kozyrev Observations of Alphonsus; Publications of the Astronomical Society of the Pacific, Vol. 71, No. 418, p.46 1959
Buratti, et al; Lunar Transient Phenomena: What Do the Clementine Images Reveal?; American Astronomical Society, DPS meeting #28, #17.02 1996
LeLong, et al; Lateral heterogeneity of the lunar crust in Alphonsus region; Conference Paper, 28th Annual Lunar and Planetary Science Conference, p. 801, 1997
Phillips & Arpigny; Comments on the Identification of the Emission Feature Observed by Kozyrev in the Crater Alphonsus; Astrophysical Journal, vol. 149, p.275 1967
Poppendiek & Bond; Recent Observations of Possible Volcanic Activity within the Lunar Crater Alphonsus; Publications of the Astronomical Society of the Pacific, Vol. 71, No. 420, p.233 1959
Weitz, et al; Geology of the lunar regional dark mantle deposits as seen by Clementine UVVIS data; Conference Paper, 28th Annual Lunar and Planetary Science Conference, p. 533, 1997
Zisk, et al; Alphonsus crater - Floor fracture and dark-mantle deposit distribution from new 3.0-cm radar images; Geophysical Research Letters (ISSN 0094-8276), vol. 18, Nov. 1991, p. 2137-2140 1991
part of Jeff Medkeff's Notes on Lunar Features
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