Tour the moon!

Hell Q crater (the Moon) shows off pristine impact melt that lined the crater walls and pooled in the bottom, now solidified into rock.  Ejecta was thrown out several crater radii, and dark impact-melt streamers that formed late in the impact process crossed over the early emplaced ejecta.
Hell Q, a relatively recent impact crater on the Moon.
Credit:NASA/GSFC/Arizona State University

On a clear night, take a close look at the Moon and you’ll see that it’s dotted with impact craters. Craters form when an asteroid or a comet hits a planet or moon, typically creating a bowl-shaped hole. Asteroids and comets travel at such high speeds that, immediately after they hit, they make the solid ground beneath to behave like a fluid. Rock that has been obliterated by the impact springs back and ejects upwards out of the crater, covering the surrounding area in a blanket of debris and dust. This material is called ejecta. Most ejecta material on the Earth is either impossible to see or has long-since eroded away. On the Moon, ejecta from ancient impacts is still perfectly preserved as if it was formed yesterday.

Smash hits

The marks of asteroid impacts such as craters, ejecta, and crushed or intensely heated rocks litter the rocky bodies across our solar system. Compared to the Moon, planets like Earth and Venus have surprisingly few visible impact sites. This is partly because Earth and Venus’s thick atmospheres help to break up asteroids before they hit. On Earth, when asteroids strike, the impact structures are broken down over time by wind, water and other physical processes.

Conversely, the Moon has no atmosphere and no flowing water to erase the marks of impact sites, meaning they remain intact for billions of years. It therefore represents a geological archive of asteroid and comet activity in the solar system. We think that there was a period of intense activity called the Late Heavy Bombardment 3.9 billion years ago, when the planets of our solar system were being pelted with impacts at a dizzying rate. Luckily for us, today the collision rate is much lower.

Aerial view of an impact crater on mars depicted in false colour to highlight steep scree slopes in blue and crater edges in orange.
A relatively recent impact crater on Mars with a sharp rim
Credit: NASA/JPL-Caltech/Univ. of Arizona
Aerial view of Barringer Crater, a well-preserved sandy impact crater in the USA with steep and dark edges.
Barringer Crater is well preserved thanks to aridity in the USA
Credit: NASA Earth Observatory

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