Alex McColgan

Well, we aren't sure.

Both of these craters are named as one, the Bell E crater.

This peculiar type of crater is known as a donut or concentric crater.

It is possible that they are the result of two impactors aligning up nicely, but further investigation suggests otherwise.

If they were the result of chance collisions, then there should be a random distribution of concentric craters around the surface of the moon.

However, that is not the case.

Have a look at this.

The population of concentric craters actually clump up around certain areas, especially around the edge of this region of the moon here, called Oceanus Procellarum.

Another factor to consider is that most of these craters are of similar ages.

Looking for clues in the crater itself also reveals something interesting.

This outer crater should be around twice as deep as it currently is when comparing it to other similar sized craters around the Moon.

Now, while a few concentric craters on the Moon will certainly be the result of double impacts, the location, age and depth of most craters means that something else must be at play.

One theory is that some of these impacts occurred during a time when the surface of the moon in this region was in a state in between solid and liquid, with a consistency similar to cool lava or honey.

As the impact happened, it caused ripples which propagated outwards, but then stopped and never smoothed off until it was fully cooled and frozen in place, although this is seen as an outside possibility.

The most likely theory

is that when the Moon was more geologically active, craters in the region were pushed up from beneath by magma trying to escape onto the surface.

This would explain the shallowness of the crater, and why we see concentric craters mainly around specific points on the Moon.

However, while this is the best theory we have at the moment, we don't know for sure.

What do you think it could be?

Now apart from the occasional meteor, you probably think the surface of the Moon barely changes at all.