Why does the Moon have so many craters?

Our Moon is a common sight in the night sky, a familiar friend that even many children can draw early on. Its familiar white light guides us during the night and if we take a moment to observe it, we easily spot dark areas and craters marring its surface. It serves as almost a sister or child to Earth and yet it is so different. 

Why, in comparison to Earth, is it pockmarked with craters? Shouldn’t Earth also be? To understand this, we need to delve into the early formation of the solar system, the history of both Earth and its Moon, and the differences between the two.

Introduction

The Moon has been our constant companion throughout human history, inspiring various forms of art and storytelling such as myths and legends. Every month we see it slowly increase in size from the nonexistent new Moon and crescent Moon through the fat gibbous phase until it is fully illuminated in the night sky and then slowly disappears again as it orbits around us.

The moon is tidally locked, meaning it rotates at exactly the same rate as it orbits around us, and we only ever see one side of the Moon.

This familiar face of the Moon is mainly white with several darker areas, but why is it so covered with craters as compared with the Earth?

The lunar surface is soft and so even micrometeorites will leave a notable mark.

The Moon's lack of an atmosphere

First, let’s discuss the obvious fact. The Earth has a protective layer of gas, primarily nitrogen and oxygen, surrounding it which stretches into space up 10,000 kilometers (6,214 miles) above the surface which is vital to life but also provides a protective shield. This layer of gas, called the atmosphere, burns up most incoming objects such as meteors.

The Moon does not have this protective layer, meaning no air for us to breathe. It does have a very thin layer of gases including argon-40, helium-4, oxygen, methane, nitrogen, carbon monoxide, and carbon dioxide (detected from equipment left by the Apollo missions) on its surface that could almost be called an atmosphere, but is more accurately an exosphere.

An exosphere has gas molecules, but they are very very spread out, so much that they rarely interact with one another. There are only about 100 molecules per cubic centimeter compared to Earth's atmosphere at sea level of about 100 billion billion molecules per cubic centimeter. 

This is very different than Earth’s atmosphere and is not nearly thick enough and therefore protective enough to burn up approaching objects such as fragments of asteroids, meaning many more make it to the surface of the Moon, creating craters on impact.

The lack of an atmosphere is the first reason for our Moon having so many more craters than us, but to understand the full story, we will need to go deeper, into the history of it and the solar system and to understand the make-up and processes of the little rock that orbits us and how it is different from the rock we live on.

The Moon's history of intense meteorite impacts and tectonic activity

The early solar system was a very chaotic place as the gas, dust, and material leftover from previous stars collided together as they orbited the early Sun. For the first 600 million years of our Moon’s existence, asteroids, comets, and other debris continued to bombard its surface, along with the surface of the other planets, including Earth. Due to its location in orbit around the Earth, it is also likely that it was hit by debris more frequently than the Earth.

By about 3.8 billion years ago, much of the debris left in the solar system had figured itself out, either being swept up into rings or pushed out into stable orbits separate from astronomical bodies. Impacts from debris became less and less frequent. In fact, the first signs of life on Earth (microorganisms) are from shortly after the end of the heavy impacts.

In addition to our atmosphere, Earth has another natural process that helps to eliminate past changes to its surface: tectonics. As the tectonic plates move around on the sea of magma beneath the crust, mountains are formed where plates collide, rifts are formed where they separate, and volcanoes and new land are formed when one sinks under another and releases magma to the surface. 

In the past, the Moon was still volcanically active as pockets of the mantle would regularly seep to the surface, forming the dark areas of basalt we see on the Moon today, primarily between 3.0 and 3.8 billion years ago. However, for the past 1 billion years, the Moon has been inactive geologically except for meteorite impacts every once in a while, meaning these craters are preserved.

The Moon's ancient age

But one of the true keys to understanding our Moon’s differences from Earth which leads to it having more craters than us is understanding its history, its ancient age.

The sun ignited nuclear fusion and the planets formed about 4.5 billion years ago. The current theory of how the Moon formed, known as the Giant Impact, happened shortly after. Only a few million years after the beginning of our solar system, a sister Mars-sized object we call Theia impacted the Earth, and the resulting debris from the collision formed the astronomical bodies we know as Earth and Moon today.

While the prevailing theory estimated that it took months to years after the initial impact for the Moon to form its current shape, new research (from October 2022) indicates the Moon may have formed much quicker, within possibly hours after impact, from a blob of material that was ejected into orbit and not reabsorbed by Earth. 

Check the below simulation from NASA for a more detailed visual explanation.

This new research adds to the Giant Impact theory, and it is fascinating that in a field where change usually occurs very slowly, the formation of a Moon could have formed that quickly. It’s not the only event that causes massive change quickly, but it’s still fascinating and makes our Moon even older than we originally thought, even if it is only by years or months.

With this crucial information, we are better able to understand our Moon. It is very old, though not older than Earth. It was also formed violently and very quickly with debris from the collision collecting together and coming into a stable orbit around Earth. The debris making up our Moon then went through a process called differentiation just like planets do early in their formation in which the materials separated into layers with the heavier iron sinking to create its core with the outer layer being a magma ocean of liquid rock which eventually cooled and formed the features we see today such as the lunar highlands.

Now, we can combine this information with the other facts we have learned about our Moon, its differences from the Earth.

Without tectonic activity and other erosion processes from weather like rain, the craters on the Moon have been preserved over the eons. In fact, the lack of these processes means our Moon holds the best record of the early solar system: the formation, differentiation of different layers, and bombardment from asteroids and other debris that occurred on all the rocky terrestrial planets, making it crucial to understanding Earth’s formation and history as well as that of the solar system.

Conclusion

While there are many other moons in our solar system and ours doesn’t hold many records besides being the biggest in comparison to the size of its planet (at about a quarter the size of Earth), our Moon is unique due to its ancient history and recording of that history, including the bombardment of asteroids and debris. 

The lack of recent tectonic activity along with the lack of atmosphere means that history was not erased as it was on the other planets and bodies in our solar system. Our Moon’s pristine record of the early solar system and the time since then holds the key to our understanding of how our and other solar systems form, how planets form, and so much more. 

While samples from the Apollo missions and data from other instruments such as the Lunar Reconnaissance Orbiter have helped us so far, further missions to the moon will help us to better understand this history. We look forward to future discoveries that can help us in our quest for this knowledge visible in the craters in our Moon.