Comets Scarred the Face of Lady Moon

A new study suggests comets gouged out the vast majority of craters on the moon (Image: NASA)

Icy comets and not rocky asteroids, launched a dramatic assault on the Earth and moon around 3.85 billion years ago, a new study of ancient rocks in Greenland suggests. The work suggests much of Earth's water could have been brought to the planet by comets.

"We can see craters on the moon's surface with the naked eye, but nobody actually knew what caused them – was it rocks, was it iron, was it ice?" says Uffe Gråe Jørgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark. "It's exciting to find signs that it was actually ice."

Evidence suggests that the Earth and moon had both formed around 4.5 billion years ago. But almost all the craters on the moon date to a later period, the "Late Heavy Bombardment" 3.8 to 3.9 billion years ago, when around 100 million billion tonnes of rock or ice crashed onto the lunar surface. The Earth would have been pummelled by debris at the same time, although plate tectonics on our restless planet have since erased the scars.

To find out whether asteroids or comets were the main culprits for the bombardment, Jørgensen decided to measure levels of the element iridium in ancient terrestrial rocks. Iridium is rare on the Earth's surface because almost all of it bound to iron and sank into the Earth's core soon after the planet had formed. But iridium is relatively common in comets and meteorites.