Moon's Craters

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The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

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