Ken Tapping, January 07, 2010

When Galileo first pointed his telescope at the Moon, he was surprised to find it covered with mountains, craters and great plains. We now know the Moon has too many craters to count easily, and that every solid object in the Solar System, including the Earth, has craters on its surface. Moreover, these features are powerful tools for better understanding the nature of the other worlds, and ours.

Here on Earth we have identified three main classes of crater. There are impact craters, due to bodies from elsewhere in the Solar System hitting the Earth's surface at many kilometres a second. The Barringer Crater in Arizona, the crater at Manicouagan, Quebec and the Sudbury Basin are examples. Then there are two types of volcanic crater. Near subduction zones, where one tectonic plate is sliding under another, as is the case in Western North America, we get steep-sided volcanoes such as Mount Saint Helens, which erupt sticky lava and often explode. Over hot spots in the Earth's mantle we get leakage of runny lava that produces very gently sloping craters called shield volcanoes. The Hawaiian Islands are a string of shield volcanoes formed as the Pacific Ocean seabed moves over a hot spot.

On Earth some 174 confirmed impact craters have been found. They are not uniformly distributed over our planet because erosion and subduction are constantly recycling its crust. Understandably, most of the craters are on the oldest rocks; because they have been recording the ongoing bombardment from space the longest.

The Moon is an interesting contrast. There are the lunar highlands, which are very heavily cratered, and great plains that are much less cratered. The Moon was already old when great lava outflows filled the lower-lying areas, burying the craters and forming a new, flat surface. This idea was supported by age determinations made on rock samples brought back from the Moon by the Apollo astronauts. There are no signs of subduction volcanoes or plate movements, unlike the situation here on Earth.

Mars is interesting. The Northern Hemisphere is much less cratered than the Southern Hemisphere, suggesting something, maybe a large impact, recycled the land in the Northern Hemisphere at some time in the past. Once again there are no subduction volcanoes, so no plate motions. There are huge shield volcanoes like Olympus Mons. With no plate motion, the volcano stayed over the hot spot, and just kept growing.

Venus is different again. There are about 1000 impact craters and shield volcanoes more or less evenly distributed over its surface. This is quite a small number, suggesting the surface is being recycled, but there are no signs of plate motions. It is likely that volcanic eruptions are still happening on Venus, and the surface is being continually renewed by eruptions of runny lava from the many shield volcanoes.

Jupiter sets during the early evening, as Mars rises in the east. Saturn rises around midnight. The Moon will reach Last Quarter on January 7 and will be New on January 14.