THE NATURE OF THE TYPICAL LUNAR MOUNTAIN WALLED PLAINS. Dinsmore Alter Griffith Observatory, Los Angeles, California

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1 THE NATURE OF THE TYPICAL LUNAR MOUNTAIN WALLED PLAINS Dinsmore Alter Griffith Observatory, Los Angeles, California This is the third of a series of papers concerning the evolution of the present surface of the moon. 1 In a study of this nature, it is necessary to refer to many features by the proper names used by selenographers. Unfortunately, however, these names often are mystifying to a reader. In the second paper of the series, published in the February 1956 issue of these Publications, two pages were devoted to a photographic map of the moon from which x and y coordinates can be read easily. Table I lists the names and coordinates from that map of all the objects mentioned in this paper. In Table II are listed the lunar photographs reproduced in the present paper. TABLE I Coordinates of Features Mentioned in This Paper Object x y Albategnius Aliacensis Archimedes Blancanus Catharina Clavius Gassendi Grimaldi Gruemberger Hell Plain Hipparchus Janssen Longomontanus Maginus Mare Crisium Object x y Mare Nubium Mare Serenitatis Pitatus Plato Posidonius Ptolemaeus Purbach Regiomontanus Rutherford Scheiner Schickard Schiller Walter Wilhelm I Plates I, II, and III were taken by the writer with the 60-inch reflector of the Mount Wilson Observatory. All were on Kodak I-N plates with a Pyrex 7-69 infrared filter. Plates IV and V are from the Moore-Chappell series, taken with the 36-inch re- 437

2 438 DINSMORE ALTER fractor of the Lick Observatory. Foreshortening has been partially eliminated. TABLE II Lunar Photographs Plate Date I Nov. 7, 1955 II May 24, 1954 III Aug. 10, 1955 IV Oct. 26, 1937 V Same as Plate IV P.S.T. 3 h 38 m Phase 22 d 16 h 06 n Colongitude 18L The craterlike formations of the moon have for a long time been classified as mountain walled plains, ringed plains, crater rings, craters, craterlets, and crater pits. Probably a better classi- fication could be devised today but new nomenclature would result in much confusion. Furthermore, it appears best to wait intil the present disagreements concerning their nature have been recon- ciled before a change is made. Neison describes mountain walled plains as follows : Walled-plains extend from 40 to 150 miles in diameter, and are seldom surrounded by a single wall, but usually by an intricate system of mountain ranges, separated by valleys, crossed by ravines, and united to one another at various points by cross walls and buttresses ; all usually, however, subordinate to one or two principal ranges, forming a massive crest to the rest. Towards the exterior and interior extend numerous projections and arms, at times rising even above the wall, and at others low, short, and insignificant. Occasionally, as in Schiller and Posidonius, these arms extend throughout the greater portion of the interior, or even divide it into two portions. Towards the exterior, these branching arms and projecting buttresses occasionally unite two or more walled-plains together, and at times these rise into considerable ridges, often enclosing long valleys. The interiors of the walled-plains are as a rule comparatively level, sometimes, as in Plato and Archimedes, only broken by a few mounds, or perhaps by a crater cone or so ; but more usually the interior is interrupted by a number of small irregularities, as ridges, mounds, or crater-

3 THE NATURE OF THE LUNAR PLAINS 439 pits, as in Maginus and Ptolemy ; whilst at times these irregularities assume considerable dimensions, as in Posidonius, Gassendi, and Catharina. Though many are roughly circular in shape, others possess very irregular outlines, appearing more like several confluent plains, or like a space enclosed by intersecting mountain chains rather than as true independent formations. Though commonly classed under the crateriform formations of the moon, the true walled-plains would appear to be related rather to the Mares or plains, more especially to those Mares bordered by great highlands and mountains like the Mares Crisium and Serenitatis, to which certain of the great walled-plains, as Clavius, Maginus, Ptolemaeus, Hipparchus, and Schickhardt, bear a considerable resemblance, though on a smaller scale a circumstance that did not escape Mädler. A close examination of such examples of the walled-plain as these would suggest their being low-lying bright plains surrounded by mountain ranges and extensive highlands, rather than actual independent formations bearing any relation to true volcanoes, and, as Mädler remarks, had Clavius possessed a dark interior, and been nearer the centre, Riccioli would have probably classed it as a Mare, and the same holds good with some of the others. 2 There are certain data which either must be explained or, at least, not contradicted by any satisfactory hypothesis concerning the nature of the typical mountain walled plains. These are : 1. They are very shallow in respect to their diameter. 2. They have little or no external walls. 3. They seldom have central peaks. 4. The walls are not circular. There are rectangles, squares, and especially hexagons. 5. Many small craters are found on their rims. 6. The floors tend toward smoothness. 7. They become inconspicuous under a high sun. 8. They have diameters from approximately 50 to 150 miles. 9. They are found only in mountainous areas. 10. They resemble the maria. 11. Scarps and lines of small craters sometimes are common boundaries to adjoining plains. 12. The walls are not continuous ; gaps are common.

4 440 DINSMORE ALTER 13. They have no ray systems. 14. Some (Ptolemaeus, Plate I, best example) exhibit numerous "ghosts on their floors. 15. In general, if a piece of paper be laid over a photograph so as to just touch the inner edge of the scarp, nothing is seen to indicate that any unusual depression has been covered. The accompanying diagram, Figure 1, attempts to show vertical sections of several of the largest walled plains at a true scale. Heights of the ridges are not known accurately. Usually the wall contains some mountains which are much higher than the average. Often gaps occur in the walls. Plate I, showing Ptolemaeus (74, 82) at sunset, exhibits both of these features. Probably the average heights are even less than those used in the diagrams. This statement applies especially to Longomontanus (83, 118), for which the old measurements appear to be especially excessive. In contrast to those craterlike formations that have considerable external slopes, there is little tendency for the typical walled plains to be circular. Plate II shows Ptolemaeus, which is an almost perfect hexagon, with Albategnius (65, 84), another hexagon, to the west of it. Hipparchus (65, 78), just north of Albategnius, has almost lost its northern wall, but the still remaining southern boundary of the plain is more nearly part of a square than anything else. Directly east of the southern side of Ptolemaeus is an unnamed plain (78, 83), which is nearly a perfect rectangle. Farther south (Plate III) is the string Purbach (71, 98), Regiomontanus (70, 102), and Walter (68, 106). Regiomontanus is a rather good square, except for the fact that Purbach has cut off its northeast corner. Walter also shows somewhat the same tendency. To the east of these three is a very large, very shallow old plain (74, 106) which has not been named. It also roughly approximates a square. Grimaldi (125, 74), near the eastern limb, is one of the largest of the walled plains. Foreshortening disguises its form, but the globe projection method, described in these Publications 3 and in more detail elsewhere, 4 shows it in nearly its true shape. It is revealed as a hexagon with a mutilated northern wall where a

5 Vertical sections of some typical walled plains.

6 PLATE I Sunset on Ptolemaeus.

7 PLATE II Ptclemaeus, Alphonsus, Arzachel, Albategnius, and Hipparchus,

8 PLATE III Rough area from Mare Nubium south to limb

9 PLATE IV Clavius and surroundings corrected for foreshortening.

10 PLATE V Grimaldi corrected for foreshortening

11 THE NATURE OF THE LUNAR PLAINS 441 pass leads to Oceanus Procellarum. Such a photograph is Plate V. Grimaldi is truly a connecting link between the maria and the typical walled plains. At its eastern shoreline can be seen the remains of several craters whose seaward»walls sank in the formation of Grimaldi. Clavius (75, 124), the second largest of the typical walled plains, is found near the top of Plate III. At a glance one observes the many craters on and near the rim, the lack of external wall, the general smoothness of the floor, and the beautiful arc of craters on the floor, starting with Rutherford (72, 125) on the southern wall. A second look reveals that it is a polygon and that there is something peculiar about the southern side. Its true form is more difficult to distinguish because it is near the southern limb of the moon. In Plate IV one of the Lick photographs has been projected on a globe. The floor is observable as a somewhat irregular hexagon. The east and west walls are seen to continue southward with rough ground, which is higher than the floor, between them. The inner east wall of Clavius then becomes an outer west wall for Blancanus (77, 126), and the inner west wall an outer east wall for Gruemberger (70, 127). Each of these is a walled plain more than 50 miles in diameter. These examples of shapes could be multiplied. If the walled plains were due to explosions, either internal or external, we would not expect to find these polygons. If, however, the typical walled plains are somewhat similar to what geologists call a graben, we would expect them to be polygons of varying types. A graben is a sunken area between faults. The geologist Spurr did some excellent research in his investigation of lunar faults and their effects, 5 although it would appear that he attempted to extend the process to too wide a category of features. The evidence favoring a grabenlike explanation of the walled plains is emphasized by a study of the lines along which the sides lie. In Plate II an examination of the northern wall of Ptolemaeus shows it continuing eastward as a line of small craters. Following the line of craters westward from Ptolemaeus, it merges with the northern wall of Albategnius. The coincidence is too great to be accidental. This is perhaps the most striking example of many

12 442 DINSMORE ALTER which show such a relationship between large neighboring walled plains. The western wall of Walter can be observed on Plate III continuing southward#in a conspicuous manner beyond the floor for a distance equal to the diameter of the plain, and can be picked up at intervals for a much greater distance southward. Under a morning lighting Walter appears as the heel of a giant s footprint. Walter s northern wall continues eastward and westward to form a line with the southern walls of Regiomontanus and Aliacensis (65, 105). This line contains somewhat of an excess of crater density. Plate III shows a large area of the rough section of the moon south of Mare Nubium (85, 94). In it there are several places where the rim of a craterlike formation continues conspicuously beyond the depressed area either as a scarp or as a row of craters. The small craters at, or close to, the rims of these plains are too numerous relative to those in the surrounding areas to be accidental. This fact may well be observed by examination of Plate III. The second largest of the walled plains, Clavius, exhibits them near the top of the picture. To the east of Clavius, Scheiner (80, 124) has a nice ring of them. Below and slightly to the left of Clavius, Maginus (72, 119) shows perhaps more of this characteristic than does any other plain. Directly across from Maginus, Longomontanus shows a similar formation. North of Longomontanus, Wilhelm I (84, 114) has a rather delicate ring of small craters. Near the bottom of the picture, the partially ruined Pitatus (83, 103) continues the story. This excess of small craters indicates very definitely weakness in the rock at these loci. Although the discussion of the nature of the small craters belongs in a later paper, it may be noted here that in many cases lines of them appear somewhat to resemble our terrestrial blowhole craters. A line of them must lie along a fracture of the rock. These data indicate strongly that typical mountain walled plains, examples of which are listed below, are not due primarily to explosions either from internal or from external causes. Albategnius (65, 84) Maginus (72, 119) Clavius (75, 124) Ptolemaeus (74, 82)

13 THE NATURE OF THE LUNAR PLAINS 443 Hell Plain (74, 106) Hipparchus (65, 78) Janssen (38, 114) Longomontanus (83, 118) Purbach (71, 98) Scheiner (80, 124) Walter (68, 106) Wilhelm I (84, 114) In searching for a cause, one is tempted to accept the hypothe- sis of collapse of great domes. Such have been postulated on the earth as, for example, the great central plain in California. Their terrestrial existence is, however, controversial. If large domes existed, they were formed probably by great pressure of gas or steam beneath large areas of the surface. Their collapse could be due either to rock fracturing as they were lifted too high or to trigger action by large meteorites. Meteoritic explosions, how- ever, cannot account for the form in which we now observe the great mountain walled plains. At present I can neither accept nor reject the dome hypothesis. The areas appear to be too small to make a lack of isostasy a satisfactory hypothesis for their origin, as it may be in the case of the largest maria. The origins of the craterlike formations that exhibit consider- able external slopes must be entirely different from that of the walled plains. For them some type or types of explosions are indicated. They will be considered in a later paper. The following conclusions are definite concerning the typical mountain walled plains. 1. They are closely related in nature to the maria. 2. Only trigger explosions, if any, occurred in their formation. 3. They are large sunken surfaces in mountainous areas of the moon, and are usually of polygonal form. 4. Often they show definite relationships to their neighbors. 5. Small craters at or near their rims indicate sinkings along fault lines. 1 The first two papers appeared in Pub. A.S.P., 67, 237, 1955 and 68,38, E. Neison, The Moon (London: Longmans, Green and Co., 1876), pp D. Alter and P. E. Roques, Pub. A.S.P., 67, 246, D. Alter and P. E. Roques, Griffith Observer, 20, 50, J. E. Spurr, Geology Applied to Selenology, Vol. II (Lancaster, Pa. : Science Press Printing Co., 1945), p. 80 ff.