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Lunar Orbiter: Impact Basin Geology

Much of the geologic history of the Moon is controlled by the distribution of large impact structures across its surface. As impact structures increase in size, they become increasingly complex, changing from craters with central peaks or groups of peaks to features surrounded by two or more rings. This transition occurs for structures with diameters between about 180 and 300 kilometers. Features larger than 300 kilometers in diameter are termed impact basins. More than 40 such basins have been identified on the Moon. These basins control much of the Moon's geology. For example, mare basalts are mainly confined in the topographic depressions produced at the centers of these basins. Such large impacts also produce faulting and other deformations over large portions of the Moon.

Finally, material ejected from these basins is distributed over wide regions. These materials serve as useful markers in analyzing the geologic history of the Moon. For example, if a crater or other structure is formed on top of such ejecta, then the crater is younger than the impact basin. On the other hand, if a feature is partially buried by the ejecta blanket, the feature must be older than the impact basin. By analyzing such relationships (termed superposition relationships by geologists) across the Moon, it is possible to derive a good overview of the Moon's geologic history.

Mare Orientale

The Orientale Basin occurs near the western limb of the lunar nearside and is only partially visible from telescopes on Earth. Lunar Orbiter images provided our first good look at this basin, which is 930 kilometers in diameter. Material from this basin was not sampled by the Apollo program, so the basin's precise age is not known. However, it is the freshest impact basin on the Moon and is believed to be slightly younger than the Imbrium Basin, which formed about 3.85 billion years ago.

This image provides an overview of the Orientale Basin. Unlike most other basins on the Moon, Orientale is relatively unflooded by mare basalts, exposing much of the basin structure to view. As a result, study of the Orientale Basin is important to our overall understanding of the geology of large impact basins. There are three prominent basin rings in this image. From the inside out, they are the Inner Rook Mountains, the Outer Rook Mountains, and the Cordillera Mountains. The Cordillera Mountains are regarded as the rim of the basin, defining the basin's 930-kilometer diameter. (Lunar Orbiter image IV- 187M.)

This oblique view of Orientale emphasizes the distribution of ejecta material on the southeast side of the basin. Portions of all three major basin rings are visible. Basin ejecta begins just outside the Cordillera Mountains and extends up to 500 kilometers beyond the base of the mountains. This ejecta has a rough, hummocky texture and contains linear patterns that point back at the center of Orientale. (Lunar Orbiter image IV-172M.)

This image illustrates details of the Outer Rook (left) and Cordillera Mountains (right). The shadows on the western sides of both mountain ranges emphasize the large topographic relief of these mountains. Most of the region between the two basin rims is relatively rough, with only a few smooth regions. These smooth regions are thought to be material that was melted by the energy of the impact and emplaced as sheets on top of the other impact ejecta. (Lunar Orbiter image IV-181H2.)

This image shows the southern portion of Mare Orientale. The Inner Rook Mountains cross the center of the image and the Outer Rook Mountains cross the bottom of the image. The central portion of Mare Orientale (top of picture) is covered by a thin layer of mare basalt. This layer is probably less than 1 kilometer deep, much less than in other nearside mare basins. (Lunar Orbiter image IV-195H1.)

Clementine images of Mare Orientale.
These images, based on data obtained by the Clementine spacecraft in 1994, illustrate the rock composition and topography of the Orientale Basin.

Mare Humorum

The Humorum Basin is 825 kilometers across. It was not sampled by the Apollo program, so a precise age has not been determined. However, geologic mapping indicates that it is intermediate in age between the Imbrium and Nectaris Basins, suggesting an age of about 3.9 billion years.

This image provides an overview of Mare Humorum and the surrounding region. Unlike the Orientale Basin, the Humorum Basin is filled with a thick layer of mare basalt. In the center of the basin, the basalt is believed to exceed 3 kilometers in thickness. On the north edge of Mare Humorum is the large crater Gassendi, which was considered as a possible landing site for Apollo 17. (Part of Lunar Orbiter image IV- 143M.)

When a load of basalt several kilometers thick is emplaced on the Moon's surface, it causes the underlying material to sag under the extra weight. The resulting motion can cause the surface to buckle, producing a series of ridges on the mare surface. This image of the southeastern portion of Mare Humorum shows several such ridges in the upper left corner. These structures are frequently termed wrinkle ridges and are common in many mare regions, including Mare Serenitatis and Mare Imbrium. Geologically, this type of ground motion occurs on a thrust fault. (Lunar Orbiter image IV-137H1.)

While the basalt load causes the central portion of the mare basin to subside, more distant regions may actually be flexed upward by the resulting ground motion. This stretches the crust and may produce graben, which are narrow valleys surrounded on both sides by faults. This image lies to the east of the previous image, at a greater distance from the center of the Humorum Basin. Several examples of graben are clearly seen on the left side of the image. A few such structures are also present at the bottom right of the previous image. (Lunar Orbiter image IV-132H1.)

Mare Imbrium

The Imbrium Basin is the largest basin on the nearside of the Moon, with a diameter of 1160 kilometers (the South Pole-Aitken Basin on the farside is twice as large). The Imbrium Basin is also the second youngest basin on the Moon. Based on samples returned by Apollo 15, it formed about 3.85 billion years ago.

This image provides an overview of the Mare Imbrium region, which occupies the upper left portion of the image. Part of Mare Serenitatis is visible in the upper right. Imbrium and Serenitatis are separated by the Apennine Mountains, which form part of the main basin ring of the Imbrium Basin. On the northeast side of Imbrium are the Alpes Mountains, which are another part of the main Imbrium Basin ring. The Alpine Valley cuts through the Alpes Mountains near the 1 o'clock position around the Imbrium Basin. Copernicus Crater is prominent in the central portion of the image, just below Mare Imbrium. (Lunar Orbiter image IV-121M.)

This image shows details of the Apennine Mountains along the southeastern rim of Mare Imbrium. The Apennines reach an elevation of 4 kilometers above the mare and are highest immediately adjacent to Mare Imbrium. In the center of the image is the rough terrain of the Apennines backslope, composed of material ejected when the Imbrium Basin formed. Mare Serenitatis is on the right. The sinuous channel in the upper left is Hadley Rille, a volcanic channel that was the site of the Apollo 15 landing. (Lunar Orbiter image IV-102H3.

This image shows details of the Alpine Valley, which runs through the Alpes Mountains in the northeastern portion of the Imbrium Basin. This oblique image was taken looking south, with a portion of Mare Imbrium visible at the top of the image. The Alpine Valley is about 10 kilometers wide and is probably a graben, produced by faulting at the time of the Imbrium Basin-forming impact. The valley was subsequently filled by volcanic material, and includes a small rille running down the center of the valley. Similar valleys are occasionally seen at other large impact basins on the Moon. (Lunar Orbiter image V-102M.)

Material ejected by the formation of an impact basin is generally redeposited in the region outside the basin. Near the basin rim, the ejecta blanket can be quite thick and completely cover the preexisting terrain. At greater distances, the ejecta is thinner and does not completely blanket the terrain. However, even at these larger distances, the ejecta can substantially modify the terrain. This image is of a region about 1000 kilometers southeast of the Imbrium Basin rim. This region has been strongly scoured by Imbrium Basin ejecta, as seen by the pronounced lineations that run nearly vertically through this image. This type of terrain is known as Imbrium sculpture. In places, large secondary craters can be seen among the sculpture. (Lunar Orbiter image IV-101H2.)

Walter S. Kiefer

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