Exploring the moon

Ken Tapping, 29nd March, 2016

In the sky this week…

  • Jupiter is in the southeastern sky after sunset.
  • Mars and Saturn rise around 2am.
  • The Moon will reach Last Quarter on the 31st.

One of the easiest alien worlds to explore is the Moon. It is close – about 384,000 km away, which is nothing in cosmic terms, and is easy to observe. You can see things with your unaided eyes, see more with binoculars, and what you can see with a telescope, even a small one, can be stunning.

With our unaided eyes we see a bright disc with some darker, irregular patches, which form the face of the "Man in the Moon". Binoculars will show the Moon to be pockmarked with craters, the result of ancient impacts. Binoculars or a telescope show the darker and lighter areas are markedly different. The light areas are rougher and highly cratered; the darker areas are smoother and less cratered. Our astronomical ancestors thought those darker areas were seas, and gave them names like the "Sea of Tranquillity" (in Latin, Mare Tranquillitatis), "Sea of Rains" (Mare Imbrium) and the "Sea of Troubles" (Mare Crisium). There are features in these "seas" such as the "Bay of Rainbows" (Sinus Iridum), and the "Bay of Dew" (Sinus Roris). Considering that the Moon has been a dry, airless ball of rock for billions of years, these names are highly imaginative.

The Moon's surface is heavily cratered, the result of billions of years of meteoric bombardment. The Earth has received the same treatment, but most of its craters have been erased by the weather and by the continual recycling of the Earth's surface by plate tectonics. The Moon has no atmosphere and therefore no weather. If plate tectonic processes were ever active on the Moon, they tailed off billions of years ago. The erosion on the Moon is due mainly to micrometeorites, large temperature changes and of course the occasional big impact. This is a much slower process, so today we can see craters that are billions of years old.

Look carefully at the lunar mountain ranges. In most cases they are clearly the remains of the walls of overlapping craters. Many of the craters show central peaks, rather like what we see in photographs of water droplets falling into water. A short-lived depression forms, with a pushed-out crown of droplets forming a ring around the depression. Then the rebounding water drives up a spike in the middle. This happens in rock too. An impact blasts material away and pushes the surface down. This bounces back to form a mountain in the middle of the crater. A close look at the edges of craters will show some of them have terraces. Over time the material thrown up by the impact to form the walls of craters often starts to slump back. Some of the craters have "rays" extending hundreds of kilometres out from their parent crater. This is debris ejected by the huge explosion caused by the impact. These craters are fairly recent, because those rays will eventually disappear. There are also craters with other craters overlapping them or even inside them.

The Moon's "seas" are really huge lava flows, believed to be the result of huge impacts. Huge amounts of molten rock flooded across the surface, burying mountains and craters. We can see some of the flooded craters today, as rings and curves of rock sticking up through the lava.

The Full Moon is not the best time to explore, because we are seeing the Moon with the light coming from behind us, so there are no shadows to outline the surface features. A partially sunlit lunar disc is better, especially along the terminator – the boundary between the sunlit and unlit parts of the disc, where it is sunrise or sunset. The low lighting angle really shows things up, especially tops of mountains and other features that are sticking up out of the darkness into the sunlight. Of course, when you are looking you are going to see other interesting things; just get out the telescope.

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

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