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The Nature of the Typical Lunar Mountain Walled Plains

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The Nature of the Typical Lunar Mountain Walled Plains 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 χ 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 repro- duced in the present paper. TABLE I Coordinates of Features Mentioned in This Paper Object χ y Object χ y Albategnius 65 84 Mare Nubium 85 94 Aliacensis 65 105 Mare Serenitatis .. 52 42 Archimedes 75 39 Pitatus 83 103 Blancanus 77 126 Plato 76 18 Catharina 44 93 Posidonius 42 36 Clavius 75 124 Ptolemaeus 74 82 Gassendi 108 90 Pur bach 71 98 Grimaldi 125 74 Regiomontanus ... 70 102 Gruemberger 70 127 Rutherford 72 125 "Heir Plain 74 106 Scheiner 80 124 Hipparchus 65 78 Schickard 104 113 Janssen 38 114 Schiller 93 119 Longomontanus .. 83 118 Walter 68 106 Maginus 72 119 Wilhelm I 84 114 Mare Crisium 15 50 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 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 438 DINSMORE ALTER fractor of the Lick Observatory. Foreshortening has been par- tially eliminated. TABLE II Lunar Photographs Plate Date P.S.T. Phase Colongitude I Nov. 7, 1955 3h38m 22d16h06m 18Π2 4 04 4 32 4 57 II May 24, 1954 2 33 21 14 11 172.2 III Aug. 10, 1955 4 07 21 16 33 175.8 IV Oct. 26, 1937 5 41 22 01 43 173.8 V Same as Plate IV The craterlike formations of the moon have for a long time been classified as mountain walled plains, ringed plains, crater rings, craters, crater lets, 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, subordi- nate to one or two principal ranges, forming a massive crest to the rest. Τowards the exterior and interior extend numer- ous 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 por- tions. Towards the exterior, these branching arms and pro- jecting 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 Ar- chimedes, 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 cráter- es) Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System THE NATURE OF THE LUNAR PLAINS 439 pits, as in Maginus and Ptolemy ; whilst at times these irregu- larities 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 independ- ent formations. Though commonly classed under the crateriform forma- tions 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 sur- rounded 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. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 440 DINSMORE ALTER 13. They have no ray systems. 14. Some (Ptolemaeus, Plate I, best example) exhibit nu- merous ^ghosts" on their floors. 15. In general, i£ 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 ver- tical 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 aver- age 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 consider- able 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 hexa- gon, to the west of it. Hipparchus (65, 78), just north of Alba- tegnius, 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 Ptole- maeus is an unnamed plain (78, 83), which is nearly a perfect rectangle. Farther south (Plate III) is the string Purbach (71, 98), Re- giomontanus (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* 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 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System Vertical sections of some typical walled plains. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PLATE I © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PLATE II Ptclemaeus, Alphonsus, Arzachel, Albategnius, and Hipparchus. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System Rough area from Mare Nubium south to limb. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System Clavius and surroundings corrected for foreshortening. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PLATE V Grimaldi corrected for foreshortening. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 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.
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