Why Does the Moon Have Lots of Craters?

Why does the moon have lots of craters from impacts, and the earth doesn't? - Question posed by Matty G.

Craters, on any astronomical body, are the scars formed when small bodies hit big bodies (when similarly sized bodies hit each other they tend to either stick together or just melt, depending on things like size and collision speed).

Early in the solar system's formation (about four billion years ago) there was a lot of stuff orbiting the Sun. There were some big bits of stuff - planets and moons- that were settling into their familiar positions, and some smaller bits of stuff - asteroids, comets and the like - that were crashing into the big bits of stuff. This period is known as the Late Heavy Bombardment, and the Moon's craters are excellent evidence for this. In fact, radiometric dating of rock samples collected by the Apollo missions was instrumental in proposing this bombardment period in the first place.

Much of the Moon's scar-tissue comes from these impacts, with some of it caused by later impacts. But...


Why doesn't the Earth have lots of craters, too?

The Earth was around when the Moon was being bombarded, so why wasn't the Earth hit too? Well... it was. It got just as much of a beating as the Moon did*, but we don't see as much scarring.

The answer to "why not?" is fairly straightforward: Earth is alive. Actually, that's a bit dramatic. It's not alive in the sense that it gets a little upset when someone doesn't respond to its Facebook Chat messages, but it is active in a geological sense: we have earthquakes and volcanoes, and they are the short-term reminders that there are longer-term processes going on. During a volcanic eruption, melted rock flows out from the Earth's interior, smoothing out the impact-damage scars. It's a kind of planet-wide resurfacing that happens over the course of millions, even billions of years - just like re-surfacing roadworks, only on a larger scale and slightly quicker**.

This doesn't happen on the Moon because the Moon is geologically dormant: it's cold, and there's no mechanism to drive the resurfacing works: the Moon's road-menders are on a permanent tea-break, so if a crater forms, it stays there.

There is evidence of more recent impacts on Earth, though. One of the most imaginatively named meteor craters is Meteor Crater in Arizona in the United States of America. There's one called Chicxulub, which is a better name, and is widely thought to be evidence of the impact that killed the dinosaurs. Even these more recent craters are less obvious on Earth, though. Why? Weather, largely, eroding the nice, crisp edges that you see on the Moon's craters, and filling them in, too. Also, we have things like plants and trees and cities that grow all over a lot of them so it takes us a while to realise they're even there.




* Actually, it's likely that the Moon shielded us from some impacts.
** I jest, but only just. Some of the road-workers near me seem to work on near-geological timescales.

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