The Moon Guide
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courtesy
U. S. N. O.
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The phase of the Moon right now

Phase
 
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Moltke
Möltke – a small bowl-shaped
crater
Sabine and Ritter
Sabine and Ritter (right) are larger, more flattened craters
Crater with central peak

A large crater with central
peak
About impacts

The process of impact cratering is extremely dynamic - the fastest known geological process, no less. Geologists are used to working on time frames of millions of years, but an impact crater can be carved out of solid surface in just a few seconds to few hours. Blink and you’d miss the event!

Most craters have a circular outline, with a few more elliptical shapes as a result of the impact event occurring at a highly oblique angle. At the initial point of contact of an asteroid or comet with the planetary surface, such as the Moon, a huge amount of energy is transferred to the surface because the impactor would have been travelling at tens of kilometres per second. Temperatures of hundreds if not thousands of degrees and pressures of hundreds of gigapascals would easily be achieved.

Two shock waves are created as the projectile hits the target, one which goes into the target and one which goes into the impactor. The one which travels into the impactor is reflected off its rear surface, causing it, in some cases, to melt or even vaporise.

Once the shock wave in the projectile hits the surface it starts to excavate the crater in what amounts to an explosion, and the size of the crater is determined by things like the energy of the impactor, the type and strength of the target and the gravity of the planet.

On the Moon, small craters just a few kilometres in diameter have a simple, bowl-like form, with smooth sides and a rounded floor. For higher energy impacts this simple bowl shaped crater is obliterated and a much wider but shallower crater is formed.

More and more complex features appear with larger and larger impact events. First, a central peak appears; this is formed when material from underneath the crater rises up in response to having so much material blasted away from the surface, and combined with the force of material flowing down the sides of the crater towards the centre. For even larger events still, this central peak overshoots its limit of stability, cascading down into a ring, called a peak-ring crater. The largest crater of all is in fact called a basin, and can exhibit multiple rings outside the main rim of the crater.

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Lunar geologist

Lunar geology

Moon rocks on Earth

Moon quakes

How the Moon was formed

Geology of the Moon's features

What is the Moon made of?

Lunar gardening

About impacts

Ray craters

Ageing wrinkles

Meandering channels

 
spacerMaintained by SPA Webmaster: Last modified 6 November 2008
 
International Year of AstronomySociety for Popular AstronomySociety for Popular Astronomy