Star Astronomy

Depending on the size, speed and angle at which the falling objects hits the moon’s surface will determine the size and shape of moon craters.

Objects which are relatively small and travelling at slow speeds when they hit the moon’s surface form what are known as simple craters. The majority of the craters on the moon are simple craters with dimensions of less than fifteen kilometres and a bowl shaped form. The crater known as Moltke is a simple crater with a diameter of seven kilometres.

Larger impact craters are formed as a result of collisions between an asteroid, comet or meteorite travelling at different speeds and colliding with the moon The moon’s surface is covered with millions of this types of craters. Unlike the earth there is no atmosphere on the moon and therefore it has no protection from being hit by these impactors, the moon also has no erosion caused by wind or rain and once the moon craters have been made they remain in exactly the same place and form until another impactor changes it. Some impact craters can hundreds of kilometres in diameter, however, the very largest craters have been filled with lava and only the outlines of them are visible.

The Maria seas have the least amount of craters, this is due to these areas being formed more recently than other parts of the moon’s surface and have had less time to be hit.

Enormous impacts can also cause secondary cratering as eject debris falls back onto the surface of the moon forming new craters or a series of small craters. Aristarchus is a huge circular impact crater with a diameter of forty kilometres and a depth of over three and half kilometres from the floor to the rim.

Craters which have a diameter in excess of fifteen kilometres have more complex forms than smaller craters; they are often seen with shallow flat floors which are made of solid lava, with a peak or multiple peaks and terraces on the inner rim of the walls. Euler is an example of a complex crater which has a diameter of twenty eight kilometres and a depth of two and a half kilometres.

The moon’s surface also has many lines of mountainous cliffs or scarps which are known as rupes. These are the remains of the rims of ancient moon craters. The Rupes Altai is a mountain range on the moon which is some fifty kilometres long.