by Jeff Medkeff
The large crater Ptolemaeus dominates the central nearside of the moon, presiding over the northern limits of the southern highlands. The region is bordered to the west by Mare Nubium and to the north by Mare Insularum, while the highlands extend well to the east and south.
Most of the craters in this area that were formed before the Imbrian basin clearly show the signs in the form of damage to their rims, and the piling up of Imbrian ejecta on the Imbrium-facing sides of their walls. The Imbrium impact scoured a radial pattern into the highlands and it is often difficult to tell where the pre-Imbrian highlands and the Imbrian ejecta meet. The highlands area is littered with craters and the remains of craters almost obliterated by Imbrian sculpture and, at the margin, by mare formation. The radial Imbrian ejecta is most visible in the lunar morning, when the colongitude is around 30°, and can most easily be seen by looking at the broad sweep of the highlands from Flammarion down to Arzachel using a low power.
Following the emplacement of the basins, mare flooding completed the job of obscuring and modifying the craters in the area to the north and west. Most, if not all, of the region is within the boundaries of the giant, pre-Nectarian Procellarum Basin, and it is no surprise that mare flooding has occurred in points of weakness in this basin. These weak areas include the major basins of the area, such as Nubium, Insularum, and Imbrium basins.
The floors of such large craters as Ptolemaeus, Alphonsus, Hipparchus, and Albategnius are flat and somewhat darker than the typical highlands surface in the more southern parts of the moon. These flat-floor deposits, along with similar deposits in Parrot, Halley, and other smaller craters, are called the Cayley Formation. At first glance these crater floors appear to be flooded with mare, a possibility that Goodacre adopted explicitly in his classic, The Moon. But the Apollo 16 samples show that the Cayley Formation is actually basin ejecta, probably deposited as a flow. Although some authorities suggest the Cayley Formation is from the Imbrian impact, Mutch disagrees and dates it later. Wilhelms simply states the Cayley Formation was emplaced by impact and may contain Imbrium basin material, while noting that its dating from Apollo 16 samples is highly uncertain. In an early 1973 paper, Chou, Wilhelms, and others suggest that the Cayley formation is the result of the Orientale impact, but this possibility was ruled out in a later paper. Yet another proposal is that the Cayley Formation results from the ejecta of secondary craters of some basin-impact primary. A discredited minority view is that the Cayley Formation is volcanic. In the end, it can be said only that the Cayley Formation is an impact deposit, perhaps a result of impact melt. Whatever its precise genesis, even casual crater counts show that the Cayley Formation is more heavily cratered than the nearby mare surfaces, and therefore must be older.
Sunrise: 2° (Spring)
Noon: 92° (Winter)
Sunset: 182° (Early Autumn)
Ptolemaeus is a pre-Nectarian crater, one of a few dozen craters of great age that formed shortly (geologically speaking) after the moon's formation. The nearby Nubium basin was formed at approximately the same time, although Imbrian ejecta makes it impossible to tell for certain which was formed first. Other pre-Nectarian craters in the area include Flammarion and Hipparchus. Ptolemaeus is therefore the largest and oldest crater in the Lunar Snowman, consisting of the progressively younger craters Alphonsus and Arzachel to the south.
More than eighty small craters are visible on the floor of this giant during good seeing, even at moderate lighting angles. The Consolidated atlas shows more than a hundred and twenty such small craters, right down to the resolution limits of the 61" Kuiper telescope. H. P. Wilkins, in The Moon, published a detail chart of Ptolemaeus that sought to depict these small craters and other details in the floor of Ptolemaeus. While interpreting his extremely busy and obscure map is difficult at best, about 3/4 of the craterlets he depicted are real. Goodacre's map, which is easier to use, shows 81 craters, all but a couple reliably. On a night of good seeing, this many are probably visible to a user of a large telescope, but the best resource for the modern observer is probably one of the the Consolidated prints or a medium-resolution Lunar Orbiter frame.
One interesting thing to do is to compare the amount of cratering on the floor of Ptolemaeus with that on the surface of the mare to the north. The most convenient place to make such a comparison is on Sinus Medii immediately to the north of Ptolemaeus. Be sure to select a part of the Sinus which is illuminated at about the same angle as Ptolemaeus' floor, and make sure it is really mare material you are looking at - some of the lighter areas of Sinus Medii are also part of the Cayley Formation, while the darker areas are definitely mare. If conditions are right and a large telescope is employed, the difference will be obvious - a good number of small craters will be seen in Ptolemaeus but only a few will be found in Sinus Medii.
This is a duplicate of the sort of observation that planetary scientists use to establish the relative age of different parts of the lunar surface - a technique called crater counts. The basis is that older surfaces have been exposed to more bombardment by meteoritic material than younger surfaces and will therefore show the scars more readily.
In addition to this speckling of smaller craters, two larger craters stand out clearly on Ptolemaeus' floor even in small instruments. Ammonius, the larger one, is found on the northeast side; while Ptolemaeus D lies to Ammonius' west. Neither crater is exceptional, although Ammonius is remarkably bright under high illumination. Under good conditions, its bowl-shaped appearance, typical of simple crater morphologies, can be made out. In the classic literature, curious reports of a central peak dog Ammonius, with Goodacre reporting that another observer has detected a central peak but making no statements for himself. Wilkins specifically discounts the same report as shown to be false. There is, of course, no sign of a central peak in this small crater in Lunar Orbiter imagery.
Just to the north of Ammonius are two larger craters that have been buried by the Cayley Formation. These craters partially overlap and the northerly one extends to Ptolemaeus' limb, their rims surviving as nothing more than gentle ring-shaped hills. These buried craters are best seen when the terminator is less than ten degrees away.
Several other buried craters are found in Ptolemaeus, most of the easily visible ones being in the southwest portion of the floor. Very low lighting is required to see them all. Under such conditions, it will be clear that the Cayley Formation is full of gentle hills, depressions, and ridges, and this undulating topography makes discerning buried craters from ordinary gentle hills, dales, and ridges rather difficult. Some of the relief visible on Ptolemaeus' floor at the very lowest lighting is only a matter of a few feet in height and requires the lowest of possible illuminations to see.
On the west side of Ptolemaeus is found the rille Rima Ptolemaeus. Although this is as easily visible as the deepest rille on the floor of Alphonsus, it is not clearly depicted by Rukl, nor is it labeled. This rille is actually a chain of overlapping craters, probably endogenic - possibly a series of maars. Its nature as a crater chain is apparent under very good seeing; very low lighting may help.
The rim of Ptolemaeus rises up to 3000 meters above its floor. The Imbrian scouring on the northwest rim is particularly prominent; while this rim drops abruptly to the crater floor, the southwest rim has caught some Imbrian ejecta at places and rises more gradually. To the south, the rim has been destroyed by the Alphonsus impact, causing Ptolemaeus' rim to apparently bow in towards the center of the crater. The fact that Alphonsus' rim is here intact and Ptolemaeus' is not is one of several clues that Ptolemaeus is the older crater. On the north side of Ptolemaeus, ejecta from the fresh crater Herschel has scoured the rim and deposited some radial material onto Ptolemaeus' floor. This ejecta extends at least as far as Ammonius.
Like many highlands areas, Ptolemaeus can be difficult to find and observe at high sun. It is worth the trouble, however. Ptolemaeus' floor is covered with a ropy bright albedo texture, with several large irregular dark spots, especially in the western end of the crater. These are more or less correlated with the very low relief features seen as the terminator crosses the crater floor. Most of the craters in Ptolemaeus' floor are bright; Ammonius even has a modest ray system about twice the size of the crater itself. Most of the other floor craters appear as bright spots at high sun, but a crater in the southest of the floor has a dark halo. Ptolemaeus' rim is not nearly as bright as on fresher craters, and tracing the rim completely around at full moon can be quite difficult.
Three long streaks of light cross Ptolemaeus' floor just before sunrise. These are caused by the several clefts in Ptolemaeus' wall that Goodacre noted. This Triple Sunrise Ray is one of the attractions of this crater.
Sunrise: 3° (Spring)
Noon: 93° (Winter)
Sunset: 183° (Early Autumn)
Like almost any other selection of nearby craters in this region, Flammarion, Spoorer, and Herschel provide an example of nearby craters of differing ages. They are located to the north and west of Ptolemaeus.
Flammarion is a pre-Nectarian crater as old as Ptolemaeus but, largely because of its proximity to the Imbrium Basin, is even more ruined. Its floor is covered with Cayley Formation material, and it has also suffered extensive modification in later time periods. The result is a very wrecked crater, with a nearly completely ruined north wall, in part the result of another, unnamed, crater that formed on Flammarion's rim in the distant past. The rim of this crater can be seen as two incomplete arcs that make Flammarion's rim appear to bulge outward to the north. In addition to the ruined north rim, there are several gaps in the wall to the west, and the crater rims are heavly eroded wherever they survive. To the southeast, the crater Herschel's ejecta blanket contributed to the erosion of Flammarion's walls and deposited some material on Flammarion's floor.
In addition to ejecta, several secondary craters, probably from Herschel, are found on Flammarion's floor and to the northwest. Several of these are telescopically observable under good seeing. The most easily seen pair are on the northwest part Flammarion's floor, not far east of the small crater Mosting A. The westernmost secondary is a small, arrow-head shaped gouge in Flammarion's floor, pointing back in the direction of Herschel. The more easterly of the two most conspicuous secondaries is a comma shaped gouge, also pointing back to Herschel, but to its northwest are two shallower gouges, lined up radially with Herschel. Under very low lighting, this secondary can be quite conspicuous. More secondaries, apparently from Herschel, are found between Flammarion and Mosting, to the northwest.
Under high lighting, Flammarion's floor is mottled with irregular albedo variations, like most of the rest of the Cayley Formation. Some observers have suggested this patchwork of albedo features is arranged in the shape of a face, with the mouth on the south part of Flammarion's floor, a nose almost centered, and two long, irregular eyes near the north rim. In addition to these broad, irregular albedo features, there are a few small craters with very bright ejecta blankets that might be seen under high illumination. These are almost certainly recent impacts, and the craters are not visible under low illumination.
On the north part of Flammarion's floor is found a graben, Rima Flammarion, which extends to the west. Under good seeing and lighting the fault can be seen to begin near Flammarion C, though its most conspicuous parts lie a few miles to the west of there. The graben is difficult to follow through a gap in Flammarion's wall, and although never as deep as it is within Flammarion, it can be picked up more easily north of Mosting A. This graben is one of several east-west trending graben in the area, and it lines up passably well with Rima Oppolzer to the east.
While Flammarion is so old and banged up it can be hard to identify under certain lighting conditions, Herschel is young, fresh, and provides some rare sharp relief and fine detail in the area. Hershel is a small crater, probably Eratosthenian in age, with a small central peak that extends north-south. Its walls slumped after it was formed, and the resulting terracing is obvious even in a small instrument. When the lighting is right, the piles of slumped-off rubble are visible on the crater floor, especially at the northeast and southeast sides. Herschel's ejecta blanket extends as far south as Ammonius, covering quite a bit of Ptolemaeus' floor. The ejecta can be traced northward over most of Sporer.
Sporer is really a crater within a crater. Sporer proper formed just to the east of center of a somewhat larger and older crater. The rim of the older crater can most easily be seen in the direction of Flammarion, arcing to Sporer's north. The ninety degrees of this older crater's rim is all that remains of it.
Sporer's overlapping-crater morphology, like that of Flammarion T and Flammarion U to the north, is typical of the area. Fresh craters are a rarity in the central highlands, and for the most part the area is saturated with craters - so that the formation of a crater anywhere in the area would wipe out at least one crater that existed before. In situations like these, doing crater counts to determine surface ages no longer works. Instead of the numbers of craters, attention must be paid to the sizes of the surviving craters. An area in which large craters have obliterated large craters is clearly old, while a surface saturated with smaller craters but lacking any large ones is clearly more recent.
Still to do this region:
Albategnius, Klein
Hipparchus
Small eastern craters Vogel to Hipparchus Z to Pickering
Davy To Lalande
Other features in this region:
part of Jeff Medkeff's Notes on Lunar Features
Jeff Medkeff's home page.
Jeff's astronomy pages.
Copyright © Jeff Medkeff, 2002, All Rights Reserved.