APOLLO OVER THE MOON: A VIEW FROM ORBIT (NASA SP-362)
Chapter 6: Rimae (3/3)
Straight Rimae
[198]
Figure 208. Location of photographs of straight rilles; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society]
[199] In many places the lunar surface is broken and a portion is downdropped, forming trenchlike features known as straight rimae or rilles. Some of these rilles are large enough (tens of kilometers across) to be visible on Earth based photographs; others are so small (a few meters across) that they are barely visible on the highest resolution orbital pictures.
Some of these trenches cut across the surrounding plains, uplands, and craters and may record preferred directions of breakage of the lunar crust caused by internal stresses (the so called lunar grid). Others ring crater floors and may be related to uplift of the floor caused by crustal readjustment after impact. A few contain low rimmed dark halo craters that are interpreted to be volcanic vents. Many trenches are curvilinear; some appear to be transitional between straight rilles and sinuous rilles.-H.M.
[200]
[For a high resolution picture- click
here]
AS8-13-2225 (H)
FIGURE 209 [above].-This oblique view looks westward over the crater Goclenius (large crater in the foreground) at the western edge of Mare Fecunditatus. The area is typical of the edge of shallow mare basins in that ejecta deposits and interiors of older craters along the margins are partially flooded by mare material. The pitted portions of the intercrater areas in the background are crater deposits that have not been flooded by the smoother mare material. Both the craters and maria are cut by linear rilles (Rime Goclenius I and II); rilles are also seen along other shallow mare basin margins. In this picture, several linear rilles, each about 1 km wide, can be seen crossing from right to left across the mare into the floor of Goclenius. The rilles are thought to be similar to fault bounded troughs on Earth (grabens) and may have originated when the central portion of the mare basin settled. J.W.H.
[201]
[For a high resolution picture- click
here]
AS15-93-12641 (H)
FIGURE 210 [above].-This picture of the interior of the crater Humboldt (approximately 200 km in diameter) was taken looking southward. The terraced crater wall lies in the background, and the central peaks are visible in the lower right hand portion of the picture. The crater floor is typical of those that exhibit both radial and concentric cracks, or fissures. The fissures and cracks appear to be related to the uplift of the crater floor subsequent to the formation of the crater. The deformation may be related to upwelling of portions of the crust in an attempt to reach isostatic equilibrium, or it may be coincident with intrusion of lavas below the crater floor. Small patches of dark mare material can be seen along the edge of the crater floor in the lower left and middle right hand portions of the picture.-J.W.H.
[202]
[For a high resolution picture- click
here]
AS16-2478 (M)
FIGURE 211 [above].-This oblique metric photo shows part of the lunar highlands where the ancient crust is saturated with large craters. Portrayed here are the crater Alphonsus (middle ground) and the ancient crater Ptolemaeus (foreground). The floor of Alphonsus is broken by faults that form a polygon roughly parallel to the walls. Dark halo craters lie along these faults. The rims of the dark halo craters fill in the fault troughs. This relationship indicates that the craters must have been formed by material ejected from the central vents rather than by collapse of material into the cracks. However, unlike impact craters with their hummocky ejecta and lines of secondary craters, the smooth rimmed deposits have been interpreted as finegrained volcanic ejecta.
Lunar transient events have been observed many times in the crater Alphonsus. Red glows have been documented and spectra have been recorded by Kozyrev (1971) that apparently confirm the existence of gaseous emissions. These events are thought to be related to orbital parameters; when gravitational stresses are high, the crust shifts and gas escapes from the interior at regular intervals. If this is true, a low level of activity still continues to affect the lunar crust and interior.
General and detailed contour maps have been made (Wu et al., 1972) of Alphonsus using metric and panoramic photography obtained by Apollo 16. (See figs. 212 to 215. Fig. 212 is outlined in this figure, and fig. 213 is outlined in fig. 212.)-H.M.
[203]
FIGURE 212 [above].-This map, compiled of the floor of Alphonsus at a scale of 1:300000 shows an array of faults, volcanic centers, and impact craters. The smooth flanks and youthful appearance of the central peak are apparently the result of the downslope movement of fragmental material that has filled in any impact craters as fast as they formed.-H.M.
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