by Chris Peterson » Fri Aug 24, 2007 4:17 pm
grayness wrote:Why are there so many craters on the moon and how often do they hit?
We don't see as many on other planets do we?
About 4 billion years ago, something happened that flung a lot of material into the inner Solar System. This was the Late Heavy Bombardment, which isn't well understood, but may be related to an orbital resonance between Jupiter and Saturn. In any case, most of the cratering in the Solar System, including that on the Moon, happened back then. It is preserved on the Moon, on Mercury, and maybe on some of Jupiter's and Saturn's large moons because they are airless and geologically stable. We don't see much on Venus, Earth, Mars, and many moons because they have active processes, either tectonic or atmospheric, that reshape the surface over millions or hundreds of millions of years (the process is slower on Mars, so we do see significant cratering, although much less than on the Moon).
Cratering since then has been much more sporadic and rare. Gravitational effects of the planets, especially Jupiter, serve to keep the inner Solar System relatively clear of debris. Small meteoroids hit the Moon regularly, but these are not large enough to make craters visible from Earth. Events large enough to produce craters ~1km across probably happen every few tens of thousands of years. Amateurs and professionals armed with video cameras and small telescopes record multiple impacts on the Moon during many meteor showers.
[quote="grayness"]Why are there so many craters on the moon and how often do they hit?
We don't see as many on other planets do we?[/quote]
About 4 billion years ago, something happened that flung a lot of material into the inner Solar System. This was the Late Heavy Bombardment, which isn't well understood, but may be related to an orbital resonance between Jupiter and Saturn. In any case, most of the cratering in the Solar System, including that on the Moon, happened back then. It is preserved on the Moon, on Mercury, and maybe on some of Jupiter's and Saturn's large moons because they are airless and geologically stable. We don't see much on Venus, Earth, Mars, and many moons because they have active processes, either tectonic or atmospheric, that reshape the surface over millions or hundreds of millions of years (the process is slower on Mars, so we do see significant cratering, although much less than on the Moon).
Cratering since then has been much more sporadic and rare. Gravitational effects of the planets, especially Jupiter, serve to keep the inner Solar System relatively clear of debris. Small meteoroids hit the Moon regularly, but these are not large enough to make craters visible from Earth. Events large enough to produce craters ~1km across probably happen every few tens of thousands of years. Amateurs and professionals armed with video cameras and small telescopes record multiple impacts on the Moon during many meteor showers.