| Ultimi arrivi - The Lunar Surface in HR |
APOLLO 16 AS 16-1973.jpgAS 16-1973 - The "Davy Crater Chain"59 visiteThe Davy Crater Chain (arrow) is one of the most spectacular chains of craters on the Moon. It extends for about 50 Km across the floor of the large, very old crater Davy "Y" and onto its eastern rim. The chain may be related in origin to the pair of irregular craters Davy "G" and Davy "GA", 75 Km from the furthest end of the chain.
Two origins have been proposed: some lunar geologists believe it is a chain of secondary impact craters and others believe it is a line of volcanic craters. The simple geometry of the Davy Chain, the symmetry and uniform spacing of its individual craters and its alinement with Davy "G", strongly support, in my opinion, a volcanic origin. Also arguing against a secondary impact origin is the fact that the Davy Chain is a lone feature. There are no other similar chains with this trend in the area. Secondary crater chains tend to occur in large numbers within the belt of secondary craters surrounding a large primary crater.
On Earth some rocks from deep within the crust have been brought to the surface through volcanic orifices, thus providing a means of studying material that would otherwise be inaccessible. For this reason the Davy area was once seriously considered as a landing site. However, when the originally planned number of Apollo missions was reduced, the Davy area was one of those eliminated.Ago 18, 2006
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APOLLO 17 AS 17-3107.jpgAS 17-3107/3105/3103 - Euler Crater (HR)54 visiteParts of 3 frames from the Apollo 17 PanCam were mosaicked to form this HR view of the crater Euler, in South-Western Mare Imbrium (an exceptionally fine example of a young mediumsized crater). 27-Km in diameter, Euler has most of the features that typify young craters in this size range. Its sharp rim shows little evidence of rounding. A solid blanket of ejecta is visible for approximately 1/2 crater diameter outside the rim and the radial pattern of secondary craters, crater clusters, ridges and grooves is visible outward to a full crater diameter.
Terraces formed by slumping of the steep crater walls, probably contemporaneously with the formation of the crater, are clearly evident. The steepness of the walls and the fact that the crater floor is below the level of the surrounding mare surface indicate that relatively little erosion and infilling have occurred. Other features typical of medium-sized craters are the central peak and the level floor surrounding the central peak. The pattern of ejecta around Euler is notably asymmetric because the area was later flooded by mare lavas that inundated parts of the ejecta blanket and other ejecta features.Ago 17, 2006
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APOLLO 16 AS 16-4530.jpgAS 16-4530 - Seconday impact craters, faults and rifles56 visiteLinear features of external and internal origin are contrasted in this area of Southernmost Mare Tranquillitatis.
The North-trending line of overlapping, very irregular craters along the East edge of the picture is clearly a chain of secondary impact craters. Its trend is radial to Theophilus, a large crater of early Copernican age that lies about 105 Km south of this area. The flaring shapes of some of the craters and their state of preservation also suggest that Theophilus is the primary crater. The narrow, straight rifle or graben that extends westward across the picture is clearly of internal origin. It formed when tensional forces ruptured the crust, causing the floor of the rifle to subside along faults.
Straight rifles are commonly the sites of volcanic cones or of blankets of volcanic ejecta; however, there are no signs of volcanism here that can be related to this rifle.Ago 17, 2006
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APOLLO 17 AS 17-2321.jpgAS 17-2321 - Crater Chain56 visiteThis crater chain in Southern Mare Serenitatis is clearly of internal origin because it is lined up parallel to several fault valleys or grabens.
The craters in the chain do not appear to have any rims; consequently, they may have formed by collapse and not by the explosive ejection of volcanic material.
The large crater in the right side of this scene, however, has a rim and so cannot be the result of collapse alone.
The finely lineated texture across the left side of the photograph is caused by ejecta from the crater Dawes to the south.Ago 17, 2006
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APOLLO 17 AS 17-149-22838.jpgAS 17-49-22838 - Crater Chain (Far-Side)55 visiteThis oblique view taken with the Hasselblad camera shows a crater chain on the Far-Side, about 500 Km North of Tsiolkovsky. For an idea of the scale, the large crater near the upper left corner is about 26 Km wide. The origin of this chain is controversial. To some geologists, the irregular shape of many of the craters suggests that the chain was formed by the impact of a stream of ejecta from a large primary crater. The presence of herringbone ridges would have strengthened this interpretation, but none are visible; perhaps the high Sun angle and the oblique viewing angle of this scene have obscured them. To others the simple geometry of the chain suggests a volcanic origin. However, there is an apparent lack of faulting to control the alinement of the craters and an apparent absence of a blanket of volcanic ejecta.
The origin of this chain may not be decipherable until, and unless, additional photography becomes available.Ago 17, 2006
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APOLLO 16 AS 16-4653.jpgAS 16-4653 - Secondary Craters' Clusters54 visiteThese 3 clusters of secondary craters (see arrows) are on the East flank of the larger crater Ptolemaeus near the center of the Moon's Near-Side. Each cluster has a ridged and hummocky appearance. The primary crater has not been identified in this case, but the configuration of the clusters tells us that it must be to the South of Ptolemaeus. Note that the South-facing side of each cluster is more sharply defined than the North-facing side.
This is a consequence of the oblique trajectory of impacting fragments that causes the ejecta of the secondary craters to be propelled away (down range) from the primary crater. Observations of manmade impact craters have shown that the individual fragments within a cluster of secondary debris strike the surface nearly simultaneously.
In the process, ejecta from one secondary collides and interferes with ejecta from adjacent craters, producing a ridged and hummocky surface.Ago 17, 2006
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APOLLO 16 AS 16-4502.jpgAS 16-4502 - The beautiful Isidorus "D"56 visiteThis oblique view of the crater Isidorus "D" was taken with the PanCam on Apollo 16. Isidorus "D" is about 15 Km in diameter and is located in the highlands between Mare Tranquillitatis and Mare Nectaris.
Evidence of avalanching (Howard, 1973) and of other types of downslope movement of material are clearly visible on the inner walls of the crater. The streaks resembling shooting stars on the left wall appear to be avalanche scars. The avalanches probably were spearheaded by large blocks followed by fine-grained material. On the near wall (arrow) a larger landslide terminates in a straight line against the relatively flat crater floor. In the shadowed part of the crater wall many short irregular benches or narrow terraces mark the tops of masses of slumped material. The brightness of the avalanche scars is an indication of their freshness; in general, freshly exposed lunar materials are brighter than undisturbed materials nearby.Ago 17, 2006
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APOLLO 16 AS 16-4559.jpgAS 16-4559 - Kant "P" Crater and its "inner" crater57 visiteKant "P" is a medium-sized crater located in the central highlands on the Moon's Near-Side. About 5,5 Km in diameter, its overall shape is not in the least unusual. However, the younger, small pear-shaped crater on Kant "P"'s North wall is an excellent example of the controlling effect that topographic relief plays on the shape of an impact crater. Because the small crater was formed on a steeply sloping surface, its ejecta was deposited chiefly downslope and formed a broad rim. The original rim and wall on the upslope side have been obliterated by slumping.
The slumping has left a landslide scar and has caused talus and scree to be deposited in the lower part of the crater.Ago 16, 2006
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APOLLO 15 AS 15-9287.jpgAS 15-9287 - A "Middle-aged" Crater56 visiteRemarkable detail is shown in this enlargement of a small part of a panoramic camera frame. In most respects, the crater itself is typical of a great many craters its size-about 1,2 Km. Because it does not have rays, it is believed to be older than most other Lunar craters. Its rounded rim crest and slightly raised rim (extending outward to the arrow, on the West side) also point to its greater age. On the other hand, it is young enough that some of the original dunelike texture of the ejecta blanket is preserved (especially to the West), a great many large blocks of ejecta are still visible, and the original depth of the crater has not been greatly lessened by infalling debris. The largest blocks, which are about 30 mt in size, occur near the rim. The terrace (T) extending partly around the wall about 100 mt below the surface probably marks the top of a resistant rock layer. However, if there were other signs of bedrock stratification within this crater, they have been obscured by the movement of debris down the walls. The very smooth floor is the only unusual feature of this crater. It may consist of a solidified pool of rock melted by heat generated from the impact.Ago 16, 2006
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APOLLO 15 AS 15-0018.jpgAS 15-0018 - On the rim of Gibbs Crater55 visiteImpact craters with asymmetric ray patterns and profiles can be caused by conditions other than the angle of trajectory. This 5-Km crater was formed when a meteoroid impacted on the North-East rim crest of Gibbs, a very much larger and older crater near the Moon's East limb. In this restricted view, Gibbs' rim is the dark area in the North half of the picture, and its wall is the light area in the south half. The rim crest extends from arrow to arrow. Discrete rays of both light and dark ejecta are well developed around the North half of the small crater where they were deposited on a relatively level surface. They are poorly developed around the South side of the small crater, probably having been partly destroyed by mixing as the ejected materials cascaded down the much steeper wall of the Crater Gibbs. Subsequent erosion has further destroyed the original pattern. The configuration of the small crater's rim has also been affected by topography.
It is sharply defined along the North side but is barely discernible along the south side where large volumes of material have slumped down the wall of the older crater.Ago 16, 2006
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APOLLO 17 AS 17-2744.jpgAS 17-2744 - Crater's "Rays"55 visiteThis is an oblique view of another crater that probably was formed by a meteoroid following a relatively low-angle trajectory. This crater, 4 Km in diameter, is located in the highlands East of Mare Serenitatis. Compared to the crater described in AS 15-9524, this one is less elliptical and its bilobate ray pattern is much less pronounced. The differences may be attributed to a higher trajectory angle of the impacting body that formed this crater as it struck the surface.
H. J. Moore (1976), in his study of craters formed by impacting missiles at White Sands Missile Range, recognized a characteristic asymmetric profile along the axis of trajectory for craters formed in this manner.
The wall beneath the missile trajectory is typically less steep than the opposite or down-trajectory wall, and its rim crest is lower and more rounded. These observations, when applied to the lunar crater in this photograph, indicate that the impacting body was traveling toward the East when it struck the Moon.Ago 16, 2006
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APOLLO 15 AS 15-9254.jpgAS 15-9524 - Bright and Big "Streaks"55 visiteThis elliptical crater is 1 Km long with an unusual, winglike pattern of rays.
This ejecta pattern is similar to those around some small experimental impact craters produced by missiles traveling along low-angle trajectories at White Sands Missile Range, N. Mex.
From the shape of the crater and the distribution of the rays, it is difficult to tell whether the meteoroid was traveling from North to South or South to North.
The higher albedo (brightness) of the North wall and the concentration of high albedo ejecta on the North-West and North-East flanks suggest that it traveled from South to North.Ago 16, 2006
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