A New Picture of the Early Earth

The New York Times | Science
By KENNETH CHANG
Published: December 1, 2008

The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek name, Hell.

That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.

Scars on the surface of the Moon record a hail of impacts during what is called the Late Heavy Bombardment. The Earth would have received an even more intense bombardment, and the common thinking until recently was that life could not have emerged on Earth until the bombardment eased about 3.85 billion years ago.

Norman H. Sleep, a professor of geophysics at Stanford, recalled that in 1986 he submitted a paper that calculated the probability of life surviving one of the giant, early impacts. It was summarily rejected because a reviewer said that obviously nothing could have lived then.

That is no longer thought to be true.

“We thought we knew something we didn’t,” said T. Mark Harrison, a professor of geochemistry at the University of California, Los Angeles. In hindsight the evidence was just not there. And new evidence has suggested a new view of the early Earth.

Over the last decade, the mineralogical analysis of small hardy crystals known as zircons embedded in old Australian rocks has painted a picture of the Hadean period “completely inconsistent with this myth we made up,” Dr. Harrison said.

Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30 percent less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice.

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