The Craters of the Moon

Silvana Mandeville
Writer, Ravenclaw

Dear, Readers! Welcome back to Alte Sententiam. This month we will have The Moon as our theme. Of course you would be familiar with the Moon as you have seen it every day. Speaking of the Moon, have you ever wondered why the Moon has many craters? Its surface is basically covered with thousands of craters. 

Craters are roughly circular, excavated holes made by impact events. The circular shape is due to material flyingㅡby asteroids and meteorites colliding with the lunar surfaceㅡout in all directions as a result of the explosion upon impact, not a result of the impactor having a circular shape. 

When an impactor strikes the solid surface of a planet, a shock wave spreads out from the site of the impact. The shock wave fractures the rock and excavates a large cavity (much larger than the impactor). The impact sprays material—ejecta- out in all directions. The impactor is shattered into small pieces and may melt or vaporize.

Sometimes the force of the impact is great enough to melt some of the local rock. If an impactor is large enough, some of the material pushed toward the edges of the crater will slump back toward the center and the rock beneath the crater will rebound, or push back up, creating a central peak in the crater. The edges of these larger craters also may slump, creating terraces that step down into the crater.

Both the Earth and the Moon have been hit many, many times throughout their long 4.5 billion year history. Given its size, Earth has the possibility of getting hit because it's bigger than the Moon. But, why does the Moon have so many craters compared to the Earth? The main difference between the two is that Earth has processes that can erase almost all evidence of past impacts. The Moon does not. Pretty much any tiny dent made on the Moon’s surface is going to stay there.

There are three processes that help Earth keep its surface crater free that the Moon can't do:

The first is called erosion. Earth has weather, water, and plants. These act together to break apart and wear down the ground. Eventually erosion can break a crater down to virtually nothing. The Moon has almost no erosion because it has no atmosphere. That means it has no wind, it has no weather, and it certainly has no plants. Almost nothing can remove marks on its surface once they are made. The dusty footsteps of astronauts who once walked on the Moon are still there today, and they aren’t going anywhere anytime soon.

The second thing is something called tectonics. Tectonics are processes that cause our planet’s surface to form new rocks, get rid of old rocks, and shift around over millions of years. Because of tectonics, the surface of Earth is recycled many times throughout its long history. As a result, very few rocks on Earth are as old as the rocks on the Moon. The Moon has not had tectonics for billions of years. That’s a lot more time for craters to form and stay put.

The third thing is volcanism. Volcanic flows can cover up impact craters. This is a major way impact craters get covered up elsewhere in our solar system, but it is less important than the recycling of crust here on Earth. The Moon once had large volcanic flows way in the past that did cover up many of the bigger earlier impacts, but it has been without volcanism for around three billion years.

The Moon may attract fewer bits of space rock than the Earth, but the Moon is powerless to do anything about it after it has been hit. Once something hits the Moon, that event becomes frozen in time. Earth, on the other hand, simply brushes these impact craters off and moves on with its life. No wonder there are so many craters on the Moon compared to Earth, right? 

Source: 
https://www.lpi.usra.edu/education/explore/shaping_the_planets/impact-cratering/
https://spaceplace.nasa.gov/craters/en/