Why are lunar craters round?

The next couple of weeks will be a good time to look at the Moon through binoculars or a telescope.

The best time to do this is not when the Moon is full. At that time, we are looking at it with the sunlight coming from behind us. We see no shadows, little detail and the glare can be awful. It is better to look at the Moon is when it is waxing or waning, so that part of the disc is illuminated, and we can see the terminator, the line between the lit and unlit parts of the disc.

If we were on the Moon, standing at the terminator, we would see the Sun rising or setting. Everything sticking up would be casting long shadows across the landscape. Along the terminator is the best place to look for detail.

The most common and eye-catching features on the lunar landscape are the craters—rings of mountains, often with a central peak. These are produced by impacts, rocky objects colliding with the Moon at extremely high speed—many kilometres per second. Some show radial streaks of material thrown out during the impact explosion.

Some areas are more or less saturated with craters, where they are crowded so densely that the impacts making new craters partially or totally obliterate existing ones. In other places there are dark, flat plains with relatively few craters. These areas are where huge lava flows buried or partially buried the existing craters, so the cratering process could start over.

In some places, lava filled the crater but left the rim. In other places only part of the rim was buried, leaving a C-shaped bay. There are also ring-shaped discolourations in the lava where craters were totally buried.

The Earth and Moon are about the same age, so our world must have been bombarded just as intensely in its youth. However, the continuous recycling of the surface by plate tectonic activities has erased most of them. What we see on the Moon, and also on the Earth, raises a very interesting question; why are craters generally circular?

Imagine a large stone ball, referred to as the impactor, hitting a rock face at a few hundred kilometres an hour, that is, at a subsonic speed. Basically, on impact the impactor tries to push the rock it is ploughing into out of the way. That is helped by shock waves moving ahead of it, which makes the rock crack and shatter.

Similarly, the impact sends shock waves back through the body of the impactor causing that too to shatter. If the impact is head-on, we get a round hole. If it is oblique, the mark is different, and the impactor might just bounce off. If we accelerate the impact to say 20 or 30 kilometres a second, or more, the story changes dramatically.

Because the impactor is now travelling far faster than sound, there are no shock waves launched ahead of the body when it hits. The material has no time to move out of the way, and there are no cracks moving back through the cannonball. The result is that instead of a mechanical impact, almost all the energy is converted into heat. Most of the impacting body and the material it has hit is heated to an extremely high temperature and vaporized. The result is a ball of very hot rock vapour under extreme pressure, which then explodes outwards in all directions, making a circular crater no matter at what angle the impactor came in.

Most of the impacts happened in the early years of the Solar System, while the planets and moons, including ours, were being built. There was a big asteroid impact 65 million years ago, which sealed the fate of the dinosaurs, there was a major impact in Arizona some 50,000 years ago and another in Tunguska in 1908, and they all left round craters or impact features. It might be that the age of frequent impacts is over, but they still happen.

•••

• The Sun crosses the equator, heading south, on Sept. 23, marking the autumn equinox and the official end of summer.

• Saturn rises around 8 p.m., Jupiter around 9 p.m., Venus around 4 a.m and Mercury, low in the dawn glow, at 6am. The Moon will reach first quarter on Sept. 22.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.