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Figure 70

 Slightly better NASA image
 Click here for lunar chart showing location

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 image below 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 at the top of this page 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. (FULL IMAGE)

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.

Report Source: NASA SP-362, Page 81, Figure 70.

This web page was created by Francis Ridge for The Lunascan Project

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