Biggest impactor after formation of Earth and beginning of life may have been pretty big. The system may have calmed down since those early days, but no doubt more big impactors are out there just biding their time.

Brimstone pickled Permian

18 September 2001
PHILIP BALL

Two hundred and fifty million years ago, life on Earth nearly ceased. A giant meteorite, six times larger than the one that did away with the dinosaurs almost two hundred million years later, may have caused the massive extinction at the end of the Permian period, researchers now suggest.

Kunio Kaiho of Tohoku University, Japan, and his colleagues have found evidence in southern China that a massive impact converted huge amounts of solid sulphur into sulphur-rich gases1.

The released sulphur could have consumed 20-40 per cent of the atmosphere's oxygen, and generated enough acid rain to raise the acidity of the ocean's surface waters temporarily to that of lemon juice. Ocean life would have been pickled.

The fossil record shows that 95 per cent of all species disappeared in the mass extinction that ended the Permian period. The event was more dramatic even than the perishing of 70 per cent of species - including the dinosaurs - at the boundary of the Cretaceous and Tertiary periods 65 million years ago....

Now Kaiho's team has found sulphate in end-Permian limestone, marl and shale rocks formed from shallow sea-floor sediments. The rocks also have a nickel-rich layer, which could have been carried by an impacting meteorite. Moreover, in the nickel-rich layer, the researchers detect a sudden change in the relative amounts of different sulphur isotopes (whose atoms have slightly different masses).

If a giant meteorite impact vaporized a large area of sulphur-containing rock where it struck the seabed, it would probably have ejected the lighter of sulphur's two common natural isotopes into the air, changing the isotope ratio of the remaining rocks.

From the size of isotope ratio shift, Kaiho's group estimates that the meteorite could have been up to 60 kilometres across. The Cretaceous-Tertiary meteorite was probably less than 10 km across.

© Nature News Service / Macmillan Magazines Ltd 2001