012 hartmann comm_lpl_1962
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No. 12 CONCENTRIC STRUCTURES SURROUNDING LUNAR BASINS by W. K. Hartmann and G. P. Kuiper June 20, 1962 1. Introduction STUDENTS of the moon have been familiar with the magnificent Altai Mountains, arching for about one quadrant around Mare Nectaris; and the continuation of this arc, as a broken series of ridges spanning about another quadrant, with roughly the same radius of curvature. Well known also are the several mountain arcs and depressions surrounding Mare Imbrium, though the outward resemblance between the Imbrium system and the Nectaris arcs is not close. The systematic study of lunar photographs projected on a large white globe, with the result ing "rectification" of geometrical relationships, has brought to light several additional structures of the Nectaris class. This Communication contains a first report on these features, and gives a comparison with their prototype, Mare Nectaris, which itself is found to have a greater degree of complexity than appears to have been noted before. A selec tion of rectified photographs is reproduced with this report. They are shown with north up to make them readily comparable with topographic maps now in production, and with east and west used in accord ance with the conventions adopted by the Interna tional Astronomical Union (1962) at Berkeley, in August, 1961. Supplementary direct photographs (not rectified) with north up also, are added to show specific detail. The present report discusses certain gross features of the ring structures. More detailed tectonic studies of each ring system are to follow. 2. Well-Knoum Structural Features of Lunar Basins The discussion of the basins may start with a brief description of Mare Crisium, which well shows the features normally attributed to lunar basins. A rectified photograph of this mare is found in Plate 12.1. It is apparent that the mare and its surrounding mountain wall resemble a giant crater, such as Cleo- medes just north of the mare. The shape of the mare's ring wall is not strictly circular but somewhat elliptical, with the major axis E-W, or more accu rately, hexagonal, with rounded corners. But the mare shelf shown in Plate 12.1, presumably out lining the deep basin formed by the impact, is almost a perfect circle. The breach in the mountain wall to the east is apparently caused by local subsidence and melting, while a smaller similar disturbance occurs at the center of the west wall. As Plate 12.1 shows, some narrow lava basins north of the mare are roughly parallel to (and con centric with) the mountain wall; several radial structures are also seen. This relationship is not un like that shown by the Apennines bordering on Mare Imbrium (Plates 12.18ff). These peripheral basins seem to be due to subsidence and melting or flood ing, possibly resulting from the overburden of the wall and the availability of lavas at moderate depths after the Crisium impact. The walls surrounding Mare Crisium show no signs of lava splashes, even by visual observation with large telescopes. Appar ently the impact was a "dry" one, with the flooding having occurred later. This classical concept of a crater-type mare may now be contrasted with the prototype of the structures here described, Mare Nectaris (Plates 12.2-11). 3. Mare Nectaris Plates 12.2 and 12.3 are a matching pair, one morning and one afternoon illumination, both recti fied, showing Mare Nectaris and surrounding struc tures. Of these structures, the Altai Scarp (SW quadrant) is the most prominent. It continues north- 51
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Transcript of 012 hartmann comm_lpl_1962
No. 12 CONCENTRIC STRUCTURES SURROUNDING LUNAR BASINS
by W. K. Hartmann and G. P. KuiperJune 20, 1962
1. Introduction
STUDENTS of the moon have been familiar withthe magnificent Altai Mountains, arching forabout one quadrant around Mare Nectaris; and thecontinuation of this arc, as a broken series of ridgesspanning about another quadrant, with roughly thesame radius of curvature. Well known also are theseveral mountain arcs and depressions surroundingMare Imbrium, though the outward resemblancebetween the Imbrium system and the Nectarisarcs is not close.
The systematic study of lunar photographsprojected on a large white globe, with the resulting "rectification" of geometrical relationships, hasbrought to light several additional structures of theNectaris class. This Communication contains a firstreport on these features, and gives a comparisonwith their prototype, Mare Nectaris, which itselfis found to have a greater degree of complexitythan appears to have been noted before. A selection of rectified photographs is reproduced with thisreport. They are shown with north up to make themreadily comparable with topographic maps now inproduction, and with east and west used in accordance with the conventions adopted by the International Astronomical Union (1962) at Berkeley, inAugust, 1961. Supplementary direct photographs(not rectified) with north up also, are added to showspecific detail. The present report discusses certaingross features of the ring structures. More detailedtectonic studies of each ring system are to follow.
2. Well-Knoum Structural Featuresof Lunar Basins
The discussion of the basins may start with abrief description of Mare Crisium, which well showsthe features normally attributed to lunar basins. A
rectified photograph of this mare is found in Plate12.1. It is apparent that the mare and its surroundingmountain wall resemble a giant crater, such as Cleo-medes just north of the mare. The shape of themare's ring wall is not strictly circular but somewhatelliptical, with the major axis E-W, or more accurately, hexagonal, with rounded corners. But themare shelf shown in Plate 12.1, presumably outlining the deep basin formed by the impact, is almosta perfect circle. The breach in the mountain wall tothe east is apparently caused by local subsidence andmelting, while a smaller similar disturbance occursat the center of the west wall.
As Plate 12.1 shows, some narrow lava basinsnorth of the mare are roughly parallel to (and concentric with) the mountain wall; several radialstructures are also seen. This relationship is not unlike that shown by the Apennines bordering on MareImbrium (Plates 12.18ff). These peripheral basinsseem to be due to subsidence and melting or flooding, possibly resulting from the overburden of thewall and the availability of lavas at moderate depthsafter the Crisium impact. The walls surroundingMare Crisium show no signs of lava splashes, evenby visual observation with large telescopes. Apparently the impact was a "dry" one, with the floodinghaving occurred later.
This classical concept of a crater-type maremay now be contrasted with the prototype of thestructures here described, Mare Nectaris (Plates12.2-11).
3. Mare NectarisPlates 12.2 and 12.3 are a matching pair, one
morning and one afternoon illumination, both rectified, showing Mare Nectaris and surrounding structures. Of these structures, the Altai Scarp (SWquadrant) is the most prominent. It continues north-
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C o n c e n t r i c S t r u c t u r e s S u r r o u n d i n g L u n a r B a s i n s 65
TABLE 2CHARACTERISTICS OF LUNAR BASINS
ITEM EXAMPLES EXCEPTIONS
1. The large circular maria are the sites of depressionscalled basins.
2. The basins tend to be surrounded by more than oneconcentric ring of mountains which set them apartfrom ordinary craters. For the larger basins eachring tends to be about \XA times larger than thering interior to it. For the smaller basins two ringsare observed and the ratio of the radii tends to be1.9-2.2; the difference may be due to the invisibilityhere of the weaker intermediate ring shown by thelarger basins (cf. Table 1).
3. Basins with concentric rings tend to be larger thanordinary craters.
All Cases
cf. Table 1 SerenitatisNubium
All but the smallest examplesmentioned in text
4. The slopes of the inner faces of the concentric ringstend to be steeper than those of the outer faces.
5. Flooding by dark material, apparently lava, is mostprobable within the inner-most ring and decreasesoutward.
6. In the outer parts flooding is favored in the low-lyingzones between the mountain rings, especially at thefoot of the inner face of scarp-like rings, makingsmall, arc- like, satellite maria.
7. Flooded surfaces tend to have systems of "wrinkleridges," roughly concentric with the ring systems.
8. The basins tend to have concentric rille systems.
The basins tend to be surrounded by radial-valley,ridge, and fault systems.
10. The inner rings tend to be made up of more isolatedpeaks as opposed to the outer rings which appearlocally as fault scarps.
11. The concentric rings tend to detour along the linesof crater walls, rather than cut through them. As inthe discussion of Grimaldi, points 10 and 11 aretaken to indicate that the outer rings are primarilythe result of local faulting.
12. There are many cases where craters have damagedthe basin walls, and have themselves been damagedby flooding. Therefore, the formation of basins tendsto precede the time of their flooding by considerableintervals. The evidence is strong that the flooding didnot accompany the basin-forming process. There isno evidence for post-mare basins.
13. The basins with the best-developed concentric-ringsystems appear to be the youngest. Basins whichseem to be very old tend to have only traces of outerrings. These older basins must have formed in theearly stages of the crater-forming process, as theyare heavily damaged by later craters.
Nectaris (Altai scarp)Imbrium (Apennines)Orientale (outer rings)All cases
Orientale — Aestatis;Imbrium — FrigorisNectaris
ImbriumHumorumNectarisHumorumNectarisImbriumImbriumNectarisOrientaleHumboldtianumImbriumOrientaleGrimaldiOrientaleGrimaldiObject near SchillerLeibnitz scarps
Nectaris (Fracastorius, etc.)Humorum (Gassendi, etc.)
Imbrium (young)Orientale (younger than next)SE Limb Basin (old)Basin near Schiller (old)Janssen (old)
J i '1
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Plate 12.1. Mare Crisium, rectified photograph. The white east-west line is a star trail (addedfor selenodetic purposes). North up in all plates, unless otherwise indicated. Y369.
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Plate 12.11. Same region as 12.10, sunset, not rectified, W97.
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Plate 12.21. Mare Imbrium, E. part, rectified, Wl 15.
Plate 12.31. Mare Orientale scarp system, rectified, M74.
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Plate 12.45. Southeast Basin, rectified, Yl614.
Plate 12.61. Basin near Schiller, rectified, morning, Y1011.
