What if the Moon had no craters?

Look at the Moon with binoculars. You will see lots of round holes. They are called craters.

What if the Moon had no craters at all? What would it look like?

Through binoculars or a small telescope, the Moon's surface is crowded with round craters, from tiny pits to giant bowls hundreds of kilometers wide. NASA says space rocks of all sizes keep striking the Moon.

So what if the Moon had no craters? It would look very different, and we would lose a record of billions of years of history. This is a thought experiment: we use real science about craters to imagine a smooth Moon.

The Moon's surface is saturated with impact features, from small simple craters to basins over 300 km across. NASA calls them a record of the process that built and shaped the solar system.

This thought experiment removes them and asks what else would disappear, why Earth has so few craters in comparison, and how crater counts let scientists date planetary surfaces. Each step separates fact from idea.

A crater forms when a space rock hits the Moon very fast. It leaves a round hole with a raised edge.

Some craters are small. Some are huge!

A lunar crater forms when an asteroid, meteoroid or comet slams into the Moon, usually faster than the speed of sound. The impact blasts out a round hole and throws out rock called ejecta. Small craters are simple bowls; bigger ones, around 15 kilometers or more, have flat floors and peaks in the middle.

Tycho crater is about 85 kilometers wide and about 110 million years old, young for the Moon. Bright streaks of ejecta, called rays, spread out from young craters like Tycho.

Impactors typically strike faster than sound; OpenStax gives the Moon's escape velocity, 2.4 km/s, as the minimum impact speed, often with 10 km/s or more added. The excavated crater is generally 10 to 15 times the projectile's diameter. Simple craters are bowls up to about 10–15 km; complex craters show central peaks and terraces; basins exceed 300 km.

Tycho, about 85 km across, is about 110 million years old. Most major basins formed about 3.9 billion years ago and later filled with lava, forming the dark maria.

With no craters, the Moon would look smooth. It might not have its dark patches either!

The dark patches are giant old craters filled with cooled lava.

Imagine the Moon with no craters at all. It wouldn't just lose its pits and bowls. The dark patches you see with your own eyes, called maria, are huge impact basins that later filled with lava. Without impacts, those 'seas' might never have formed, and the Moon's familiar face could look quite different.

The Moon's powdery gray soil comes from billions of years of impacts grinding the surface down. A Moon with no impacts might not have that dust. These are ideas, because a real Moon could never avoid every space rock.

Removing impacts removes more than craters. NASA identifies the maria as impact basins filled with basaltic lava between 4.2 and 1.2 billion years ago, and the regolith as the product of billions of years of impacts grinding the surface. A crater-free Moon would therefore also lack, or differ in, its maria and its regolith.

These are reasoned ideas. Impacts are unavoidable for an airless body; a crater-free Moon would require either no impactors or continuous resurfacing, neither of which applies to our Moon.

The Moon has no wind, no rain and no rivers. Nothing wears its craters away.

On Earth, wind and water slowly rub craters out.

On the Moon there's no liquid water, no wind and almost no air, NASA explains, so craters stay crisp for billions of years. On Earth, wind, water and the slow movement of the ground (plate tectonics) gradually erase craters, so Earth has very few left.

The Moon's thin air also can't slow space rocks down, so even small ones reach the surface. That's why the Moon is covered in craters of all sizes.

NASA attributes the preservation of lunar craters to the Moon's lack of liquid water, wind and other active surface processes. On Earth, erosion and plate tectonics erase impact structures; OpenStax notes that impact craters are extremely rare on Earth, which is why lunar craters were long assumed to be volcanic.

The Moon's exosphere is too thin to impede impactors, so a steady rain of asteroids, meteoroids and comets reaches the surface. Over billions of years, this 'impact gardening' churns and grinds the regolith.

More craters means an older surface. It is like snow piling up on a path nobody has swept.

Scientists count craters to find out how old different places are.

Scientists count craters to work out how old a surface is. OpenStax compares it to a sidewalk in a snowstorm: a path swept recently has little snow, while one left alone is deep in snow. A surface with lots of craters hasn't been 'swept clean' by lava for a long time.

On the Moon, the bright highlands have about ten times more craters than the dark maria, so they're older. Rocks brought back by the Apollo astronauts showed the maria are 3.3 to 3.8 billion years old.

Crater density measures time since a surface was last reset, because the impact rate has been roughly constant for several billion years. Counts give relative ages; radiometric dating of Apollo samples supplied absolute ones: maria 3.3–3.8 billion years, highlands about 4.2 billion.

The highlands carry about ten times the maria's crater density yet are only a little older, which implies a much higher impact rate before about 3.8 billion years ago, the period of 'heavy bombardment'. OpenStax estimates a 1-km lunar crater forms about every 200,000 years today.