Craters on the moon

Ken Tapping, November 12th, 2014

In the sky this week…

  • Jupiter rises before midnight.
  • Mars lies low in the sunset glow. Mercury lies low in the dawn sky.
  • The Moon reaches Last Quarter on the 14th.

Most of us look at the Moon at least sometimes. We are familiar with the face of the "Man in the Moon", which is due to a combination of lava flows on the lunar surface and the ability of our brains to make human features out of random patterns.

A pair of binoculars or small telescope will show more. Near the lower edge of the disc is a circular feature with paler lines radiating from it, extending far over the Moon's surface. There are similar but less pronounced examples, and countless round structures without the radial lines. These circular features are craters, formed from impacts. The craters with the radial lines or rays were formed from recent impacts, where the rays have not yet been covered by dust. The other craters are older.

You will see mountainous areas and flat lava plains. The mountains on the Moon were not formed by plate motions, as on Earth; they are the accumulated remains of countless craters – so many that a new impact would erase earlier ones. The lava plains are the result of huge impacts, which caused a flow of molten rock over most of the surface. Many craters were drowned; some only partially, so we can see parts of their rims sticking up above the surface of the lava.  Most of the cratering was over by the time these lava flows were made, because after they solidified relatively few craters mark their surfaces.

The Moon and Earth are believed to have been formed about the same time, about 4.5 billion years ago, and received the same savage cratering treatment. This is all part of how planets are born – built up from lots of smaller objects colliding with one another, usually at many kilometres a second. By about 1.5 billion years ago, the planet building had peaked. However it still continues at a low level today.

Almost all the impact craters on Earth have been erased by plate motions and weather, whereas the Moon has no plate motions and no weather, so it still shows all the impact scars.

Lunar craters range enormously in size, from hundreds of kilometres across down to ones too small to see without actually being on the Moon's surface. On average they are circular, with a raised rim and the ground outside sloping upwards to the base of the rim. In many cases there is a peak in the middle. The impact slammed the surface material down hard, and the peak formed when it rebounded. It is similar to what we see when a drop of water falls into a bath. It forms a crater with a rim, and then a spike of water rises in the middle, resembling a crater's central peak.

Another thing you'll notice is that craters are almost always circular. This is one reason that for a long time scientists believed that lunar craters were volcanic. If they were due to impacts we would have oblong craters from things coming in at a low angle. However, laboratory experiments done with a special high-speed gun showed that for high-speed impacts, the crater is always round.

If you have a good enough telescope, it is worth looking carefully at craters. You can see how in many cases material on the inner faces of the crater wall has slumped, forming a series of terraces. In some cases the floor of the crater is cracked from the movement of the rock as it recovered from the impact; in other cases the crater is floored with lava formed from the heat of the impact. It is also worth exploring the lava plains to see craters that are partially buried by the ocean of lava as it swept over the lunar surface as a result of a really huge impact. On Earth the story of our world's birth has been largely erased. On the Moon, our sister world, the story is still there, which is why we are so interested in it.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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