Tycho (crater)

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Tycho is a prominent lunar impact crater located in the southern lunar highlands, named after the Danish astronomer Tycho Brahe (1546-1601). To the south is the crater Street; to the east is Pictet, and to the north-northeast is Sasserides. The surface around Tycho is replete with craters of various sizes, many overlapping still older craters. Some of the smaller craters are secondary craters formed from larger chunks of ejecta from Tycho.

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Age and description

Tycho is a relatively young crater, with an estimated age of 108 million years (Ma), based on analysis of samples of the crater rays recovered during the Apollo 17 mission. This age suggests that the impactor may have been a member of the Baptistina family of asteroids, but as the composition of the impactor is unknown this is currently conjecture. However, simulation studies give a 70 percent probability that the crater was created by a fragment from the same break-up that created asteroid 298 Baptistina;[1] a larger asteroid from the same family may have been the impactor responsible for creating Chicxulub Crater on Earth 65 million years ago (mya), and causing the extinction of the dinosaurs.

The crater is sharply defined, unlike older craters that have been degraded by subsequent impacts. The interior has a high albedo that is prominent when the Sun is overhead, and the crater is surrounded by a distinctive ray system forming long spokes that reach as long as 1,500 kilometers. Sections of these rays can be observed even when Tycho is illuminated only by earthlight.

The ramparts beyond the rim have a lower albedo than the interior for a distance of over a hundred kilometers, and are free of the ray markings that lie beyond. This darker rim may have been formed from minerals excavated during the impact.

Its inner wall is slumped and terraced, sloping down to a rough but nearly flat floor exhibiting small, knobby domes. The floor displays signs of past volcanism, most likely from rock melt caused by the impact. Detailed photographs of the floor show that it is covered in a criss-crossing array of cracks and small hills. The central peaks rise 1.6 kilometers above the floor, and a lesser peak stands just to the northeast of the primary massif.

Infrared observations of the lunar surface during an eclipse have demonstrated that Tycho cools at a slower rate than other parts of the surface, making the crater a "hot spot". This effect is caused by the difference in materials that cover the crater.

The rim of this crater was chosen as the target of the Surveyor 7 mission. The robotic spacecraft safely touched down north of the crater in January 1968. The craft performed chemical measurements of the surface, finding a composition different from the maria. From this one of the main components of the highlands was theorized to be anorthosite, an aluminium-rich mineral. The crater was also imaged in great detail by Lunar Orbiter 5.

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