If we look at any body in the Solar System having a surface we can observe, we see craters. These circular depressions are the relics of high-speed impacts by objects from space. We see them on Earth too, but on our world erosion and plate tectonics have erased many of them. Craters are also evidence of the process of world-building, where large bodies grow from the accretion of many smaller ones. If you want to see some for yourself, on the next clear night where the Moon is in the sky, get out your telescope or binoculars.
The Moon’s surface is divided between light areas, which are heavily cratered, and dark areas, which are very much less so. There are a few large craters where the impacts threw out rays of ejected material splattering out over the lunar surface for many kilometres. A good example is Tycho, located in the lunar Southern Hemisphere. As you might imagine, these interesting landmarks have been attracting a lot of scientific interest ever since Galileo pointed his telescope at the Moon. Much of this research has involved careful study of individual craters, high-energy impact experiments in the laboratory, and computer modelling. However, we can learn a lot just by counting them.
Imagine a featureless, smooth planet which comes under bombardment with meteoric bodies and asteroids. As this bombardment proceeds, more and more of the planet’s surface will become cratered, until eventually almost all the new impacts fall on top of old ones, and the total number of observable craters more or less stops growing. The surface of the planet is now termed “saturated”.
When we look at the Moon, we see that the light-coloured areas are pretty well saturated with craters, but the dark areas definitely not. Obviously something happened in the Moon’s past where huge lava flows erased or drowned large areas, forming a brand new surface. The rock samples brought back from the Moon by the Apollo astronauts show that the light areas are up to 4.5 billion years old, dating back to when the Moon first solidified. The darker areas are younger, averaging about three billion years. Obviously most of the impacts associated with world building happened in the first billion years. If we assume that the bodies producing the impacts date back to the formation of the Solar System, some 4.5 billion years ago, and that these bodies have been progressively swept up, we can make the estimate a bit better. It looks as though the bombardment was particularly savage for the first 500 million years, which must have been the main construction phase for the Solar System. After that things became more peaceful, making it possible for living things to develop and evolve. However, there are still some big lumps of stuff orbiting out there. Tycho was made by an impact that occurred about 65 million years ago, and has been proposed to have been caused by a partner asteroid fragment to the one that hit the Yucatan peninsula and contributed to a mass extinction of life here on Earth. The dinosaurs are among the more well-known victims. Planet building is not over yet, whether we like it or not.
Jupiter lies low but still conspicuous in the southwestern sky in the evenings. Uranus still lies very close to it. Saturn rises around midnight, and Venus around 5am.The Moon will reach Last Quarter on the 26th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory.