London, July 28: A new study of ancient rocks in Greenland has suggested that icy comets – not rocky asteroids – launched a dramatic assault on the Earth and moon around 3.85 billion years ago, thus causing the lunar surface to become scarred.
“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?” Uffe Grae Jorgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark, told New Scientist.
“It’s exciting to find signs that it was actually ice,” he said.
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.
To find out whether asteroids or comets were the main culprits for the bombardment, Jorgensen 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.
His team calculated the amount of iridium that asteroids would leave on the Earth and moon compared to comets.
Because comets have more volatile elements and higher impact speeds due to their more elongated orbits around the sun, they would create giant plumes on impact, allowing more iridium to escape into space than during asteroid impacts.
The team predicted that asteroid bombardment would leave iridium levels of 18,000 and 10,000 parts per trillion in rocks on the Earth and moon respectively, while the same figures for comet bombardment would be about 130 and 10.
Ancient moon rocks returned by NASA’s Apollo missions have already confirmed that the lunar iridium levels are 10 parts per trillion or less.
To find out the terrestrial value, Jorgensen’s team sampled some of the world’s oldest rocks from Greenland, aged 3.8 billion years, and asked a Japanese laboratory to assess their iridium levels more accurately than ever before.
They contained iridium levels of 150 parts per trillion, which strongly suggests comets, rather than asteroids, caused the violent bombardment.
First Published: Tuesday, July 28, 2009, 14:07