Three of them called Archimedes, Galilaei and Kepler. We would have to look for the smallest of them with a magnifying glass. The larger ones are with a diameter of more than 200 kilometers, and we can see them from Earth with the naked eye. We are talking about lunar impact craters that cover the moon. Since the moon has no protective atmosphere, numerous asteroids, comets and meteors have struck it over the past billion years and created tons of craters. On the side facing us alone, researchers count around 300,000 craters with a diameter of more than one kilometer.
The moon rotates on its own axis about as fast as it orbits the earth. As a result, we only see one side of the moon from Earth. Most of the other side is never visible from the Earth. It is therefore called the “dark side of the Moon”. Current measurements show that the moon has approximately the same number of impact craters on both sides.
However, some craters on the earth-facing side are significantly larger than those on the far side of the moon. For example, the front of the moon has eight craters with a diameter of more than 320 kilometers, while the back has only one of this size. Why is that? Why does the moon have bigger craters on the front?
How does researchers determine the diameter of moon craters?
Based on data obtained by NASA’s GRAIL mission in 2010, approximately 90 percent of all lunar craters were formed during the large asteroid bombardment 4.2 to 3.7 billion years ago. The researchers suspected that the crust on the earth-facing side at that time was hotter, softer, and thinner due to volcanic activity. They tested this theory using computer simulations. They virtually had an asteroid struck a moon crust and varied the thickness and temperature of the crust.
The result: At first, the craters were the same size on all surfaces. But then the crust have collapsed inwards on the cooler and firmer side, this made the crater smaller. In the meantime, the reverse was true on the warmer and thinner side: the mantle bulged upwards, causing the crust to expand outwards, thereby doubling the size of the crater.
Recently created moon impact craters
Asteroids are constantly striking the moon today – however, they are relatively small. In September 2013, an asteroid the size of a small car struck the front of the moon, creating a crater. The crater itself is small, measuring 18.8 meters in diameter. Nevertheless, it caused more than 200 related surficial changes in a radius of 30 kilometers. A remarkable impact crater size compared to more recent ones.
The crater has no name yet. It can take several years for the International Astronomical Union to choose a name. Most of the craters were named after deceased astronomers and astronauts, but other famous personalities such as Sigmund Freud or Immanuel Kant were also immortalized on the moon.