Lunar Orbiter: Impact Crater Geology
Copernicus | Tycho | Theophilus
Impact craters are produced by the collision of a meteorite or comet with the surface of the Moon, which ejects material and leaves behind a crater. Small craters, less than about 15 kilometers in diameter on the Moon, have a relatively simple, bowl-shaped morphology. Larger craters have more complex forms, including flattened floors, central peaks, terraces, and slump blocks along the inside of the crater rim. Here we illustrate the overall structure of three large, prominent craters: Copernicus, Tycho, and Theophilus.
The crater Copernicus, 93 kilometers in diameter, is one of the most prominent features on the Moon's nearside. It is a relatively fresh crater, believed to have formed less than 1 billion years ago. Its system of bright rays is quite prominent at full Moon.
In this overview image, Copernicus is at the lower left. Just beyond the rim of the crater, ejecta from the crater is present in the form of a continuous ejecta blanket. At greater distances from the crater, ejecta occurs as discontinuous clumps. Chains of secondary impact craters are visible in the central part of the image and are shown in greater detail in one of the following images. (Lunar Orbiter image IV-121H2.)
This image provides a close-up of the interior of Copernicus. The prominent structures near the center of the image are the central peaks, which are common in craters of this size. Some consideration was given to having an Apollo mission land to the north of the central peaks with the objective of sampling one of the peaks, which may be material thrust up from deep in the Moon's crust. The inner part of the crater walls are broken up into a series of terraces. (Lunar Orbiter image V-151M.)
This north-looking oblique image illustrates the hummocky or uneven texture of much of the crater floor and the terraces and debris flows along the crater wall. Although this image was hailed as the "Photo of the Century" when it was returned in 1966, a 1972 Apollo 17 photo of Copernicus proved even more spectacular. (Lunar Orbiter image II-162H3.)
Material ejected from a crater sometimes produces additional craters nearby, called secondary craters. The overlapping chain of craters in this image is an example of such secondary craters. These craters are part of the crater chains shown in the center of the first Copernicus image. (Lunar Orbiter image V- 144M. Apollo 17 view of Copernicus ejecta.)
The crater Tycho, 85 kilometers in diameter, is the youngest large impact crater on the Moon's nearside. Ejecta from this crater was spread across much of the nearside of the Moon and is visible in the form of bright rays at full Moon. One such ray crosses the Apollo 17 landing site, 2000 kilometers from Tycho. Arrival of this material from Tycho is believed to have triggered a landslide from the mountains surrounding the Apollo 17 landing site. Laboratory analysis of samples from this landslide suggest that Tycho's age is about 100 million years.
This image clearly shows both the central peak and terracing in the walls of Tycho. Tycho is in the lunar highlands, and the terrain surrounding the crater is quite rugged. The crater floor is also fairly hummocky. Multispectral images obtained by the Clementine spacecraft show that the central peak has a different composition than the surrounding material, presumably because the central peak is composed of material that originated at greater depths in the Moon's crust. (Lunar Orbiter image V-125M.)
This image shows a close-up of the rugged terrain in the Tycho ejecta deposit to the north of the crater. The scoured appearance, with generally north-south- trending structures, is due to the deposition of ejecta from the crater. Only a few flat regions are visible. Some consideration was given to landing one of the later Apollo missions in this region. However, despite the safe landing of the unmanned Surveyor 7 in this region, a manned landing was deemed too dangerous. (Lunar Orbiter image V-128H2.)
This oblique view of the crater Theophilus, 100 kilometers in diameter, clearly shows both the central peak and terracing on the inner side of the crater wall. (Lunar Orbiter image III-78M.)
Walter S. Kiefer
Lunar Missions
Lunar Orbiter
Lunar Prospector
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