Q: Why does the moon have craters?

— Yusuf Masood, Madison Community Montessori School

A: One reason the moon has craters is because it gets hit by objects — small pieces of rocks that come from outer space. These are pieces of asteroids and comets that are flying around in the solar system. When they hit the surface, there’s an impact.

The moon has no atmosphere, so even a tiny rock will create a crater. Typically when you see an impact crater, the size of the crater is much larger than the size of the rock that created it. The rock is usually at least three to five times smaller, depending on the amount of energy generated.

You can detect recent craters now that scientists have such wonderful high-resolution pictures of the moon. You can detect craters formed in the last 10 or 20 years.

One famous crater on the moon is called Tycho. A picture from a lunar orbiter shows a top-down view in which you can see a circular ring, the rim of the crater. Inside that ring, the floor is slightly depressed, and right in the center is a shadowed region called the central peak or impact peak.

If you see that structure in the center of a crater, it’s a telltale sign that it’s a crater made from an impact. When a rock from outer space hits the surface, it has so much energy that it actually liquefies the surface and ejects some other material into space. Since the moon has no atmosphere, the material just flies out and lands some distance away from the initial collision.

Some of the liquefied material first gets depressed from the energy of the force, then rises up because it cannot be compressed beyond a certain limit. When it rises up, it cools quickly and leaves behind the central peak.

The moon also has craters because in its past it had volcanoes, and volcanoes can create craters. When a volcano erupted, debris flew out from the erupting point and fell some distance away. Volcanic craters are also round and look similar to impact craters, so it’s not always easy to tell the craters and their peaks apart.

Sanjay Limaye is a planetary scientist at the Space Science and Engineering Center at the University of Wisconsin-Madison.