DID comets made mainly of water ice launch an assault on the young Earth and its fledgling moon?
The two worlds were battered by debris about 3.85 billion years ago, during the so-called Late Heavy Bombardment (LHB), but it is unclear whether rocky asteroids or icy comets were the key culprits.
Uffe Gr氓e J酶rgensen at the in Copenhagen, Denmark, and colleagues reasoned that the element iridium could provide a clue. Comets and asteroids both contain iridium, but comets would leave less iridium on the Earth and hardly any on the moon, compared with asteroids, says J酶rgensen. That鈥檚 because the higher speed of comets and the high volatility of their constituents would create giant plumes on impact, so more of the iridium would escape into space, compared with impacts by rocky asteroids.
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If comets were to blame, the team calculated, we would expect to see iridium levels of roughly 130 parts per trillion in ancient terrestrial rocks. Sure enough, they measured 150 parts per trillion in 3.8-billion-year-old rocks from Greenland. The calculations also predicted lunar rocks should have iridium levels of 10 parts per trillion or less 鈥� a figure that moon rocks returned by NASA鈥檚 have already confirmed (Icarus, ).
The team calculates that if most of the impacts during the LHB came from comets, about 3400 tonnes of ice would have hit each square metre of the Earth. That roughly tallies with the amount of water in Earth鈥檚 oceans today.