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Chapter 4: The Maria (2/3)

[78]



[For a high resolution picture- click here]

AS16-2839 (M) | AS16-2836 (M)


FIGURE 68 [above].-Although this mosaic covers only a small part of Oceanus Procellarum, it shows a large number of different features that typify the mare surfaces of the Moon. All these features except the ubiquitous craters superposed on the mare are identified in the accompanying sketch map [below]. The rugged terrae in the lower left corner mark the edge of Oceanus Procellarum. Similar terra materials project through the mare in many other places suggesting that the mare fill is thin here. Broad gentle arches, visible only in very low Sun pictures such as these, are numerous and seem to be independent of the even more numerous mare ridges or wrinkle ridges. The ridges are alined mostly northwest and to a lesser degree north-northeast. The alinement suggests-but does not prove-control by tectonic disturbance within the crust. A large forked rifle emanates from three elongate depressions located on or....


[79]


....very near two of the largest ridges. This led Young (Young et al., 1972) to postulate that these rilles are lava channels or tunnels formed during "upwelling and outpouring of lava beneath a thin or viscous crust to form the mare ridges." The large rifle extends southward for 135 km beyond the picture. From the depression at A, a smaller rifle meanders about 100 km northward before disappearing at B. Note that this rifle is interrupted by a small crater (C) and by the two mare ridges at D and E. The crater is obviously younger than the rifle and, almost as certainly, so are the two ridges. Many small sinuous and arcuate rilles, small grabens, and faults are present. Although it is difficult to distinguish between some of them, most of these linear features are located on or near high areas of terra islands, mare arches, or mare ridges. Also identified on the map are the small possible volcanic domes.-G.W.C.





[For a high resolution picture- click here]

AS16-120-19244 (H)
[80] FIGURE 69 [above].-This cluster of features in Mare Cognitum was photographed in color with a handheld camera and 250-mm lens. Its location on a mare ridge is shown in figure 91. Thomas K. Mattingly, the astronaut who took the picture, noted the taluslike skirt of material around the base of this and many other positive relief areas in the western maria. In this photograph the apparent difference in tone (and in color on the original negative) between the hills and the slightly darker skirt of material around their base is largely, if not entirely, due to differences in slope-a more steeply sloping surface appears brighter, as in the walls of craters in the adjacent mare. However, a real color difference has been found at the base of other prominences. The suggestion has been made that in some mare basins the original level to which lava filled the basin has receded and that skirts such as these are, in effect, "bathtub rings" recording the higher level. The two hills pictured here are probably of volcanic origin.-M.C.M.

[81] FIGURE 70 [below].-These two pictures were taken of the same mare area near the southeastern edge of Oceanus Procellarum, south and east of the crater Kunowsky. The lower left (facing page [bottom below]) picture is part of a Lunar Orbiter 4 high-resolution photograph taken when the Sun was at a moderate elevation of 18 to 20°. The picture below is a mosaic of Apollo 14 frames taken when the Sun was exceptionally low-0° to 2°. Douglas Lloyd designed the special experiment by which these near-terminator photographs were obtained, using very high-speed film in the Hasselblad camera.
The density of craters (more properly, the number of craters per unit area of surface) has been used by geologists as a tool to determine the relative age of rock units on the Moon's surface. The method has been applied principally in mare areas because crater populations generated on the irregular highland surfaces cannot be accurately measured. These two pictures illustrate some of the problems encountered when applying the method.
The number of craters that can be seen and hence counted is affected by the Sun angle. For example, many more craters are visible in the mosaic of low Sun Apollo pictures than in the Lunar Orbiter picture, and a detailed count of all craters in each picture would result in two radically different relative ages for the same area. Further, comparison of the two views shows that the apparent discrepancy in abundance of craters exists only among the very small craters-those a few hundred meters or less in diameter. The number of craters a kilometer or more in diameter is the same in both pictures. The explanation is that most small craters can be recognized as such only by the shadows they cast. The materials on their walls and rims are commonly indistinguishable from those of the surrounding terrain. For each picture there is, depending on the angle of Sun elevation, a threshold value of slope below which no shadow is cast. In the picture at lower left that value has been calculated to be about 5°. Consequently, those craters that have been degraded so that their slopes are less than 5° are not visible. In contrast, craters with slopes as gentle as 0.25° are visible in the picture below.
An observation immediately follows. Small craters have a relatively short lifespan. That is, once formed, they are rapidly degraded. Their rims are eroded, and their interiors are filled with debris from the continuing bombardment of the surface by other impacting bodies. By actual count about 80 percent of the small craters in this area have been so degraded that their slopes no longer exceed 5°.
It has also been shown (Soderblom and Lebofsky, 1972) that the small crater population here-and in most mare areas-is in a steady state. In other words, the rate of formation of new craters and the rate of destruction of existing craters (either by superposition of other craters, or by gradual erosion by much smaller craters) are balanced. It is fruitless, therefore, to count small craters because such counts will result in the same false age.
Fortunately, the crater counting method does yield satisfactory results when applied to larger craters. The same number of craters larger than about 1 km is visible in both pictures. This means that no craters this size or larger have been degraded to the extent that their slopes are less than 5°-as were 80 percent of the small craters. We may assume then that all the larger impact craters that ever formed on the upper part of the mare surface have been retained and that their relative abundances in different areas are a measure of the relative age of those areas.
With the careful application of this method, it has been possible to assign relative ages to most of the mare areas of the Moon. Using the absolute ages that have been determined for samples returned from the Apollo landing sites in the maria (Apollos 11, 12, 15, and 17), the relative time scale now has a quantitative base so that relative ages can be converted to absolute ages.-L.A.S.

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