Early Earth Marred by Acid Rain

The climate of early Earth was no day at the beach, with stinging acid rains and an intensely warm surface, a new study suggests. These harsh conditions could explain why geologists today have found no rocks more than 4 billion years old: They were all weathered away.

The fate of all those rocks from the first 500 million years after Earth formed has been a longstanding question in geology. Scientists have floated various explanations for the missing rocks, including destruction by barrages of meteorites and the possibility that the early Earth was a sea of red-hot magma in which no rocks could form.

The analysis in the new study suggests a different scenario.

Clues from oldest crystals

Geologists from the University of Wisconsin-Madison examined zircon crystals, the oldest known materials on Earth, to shed light on the fate of rocks from the early Earth. Zircons, which are smaller than a speck of sand, can offer a window back in time to about 4.4 billion years ago, when the Earth was a mere 150 million years old because they are extremely resistant to chemical changes.

The research team analyzed the ratios of different isotopes of lithium (which have different atomic weights and number of neutrons per atom) in zircons from the Jack Hills in Western Australia. They compared the lithium fingerprints of those zircons to those from continental crust and rocks similar to those found in Earth's mantle, the molten layer sandwiched between the crust and core.

The results of the analysis, detailed in a recent online issue of the journal Earth and Planetary Science Letters, provide evidence that the young Earth already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.

But the lithium signatures also hold signs of rock exposure on Earth's surface and breakdown by weather and water, suggesting that early rocks were destroyed by intense weathering.

"Extensive weathering earlier than 4 billion years ago actually makes a lot of sense," said study team member John Valley. "People have suspected this, but there's never been any direct evidence."

Acid rain and greenhouse effect

The early Earth is thought to have had extremely high levels of carbon dioxide — perhaps 10,000 times as much as today. Carbon dioxide in the atmosphere can combine with water to create acid rain.

"At [those levels of carbon dioxide], you would have had vicious acid rain and intense greenhouse [effects]. That is a condition that will dissolve rocks," Valley said. "If granites were on the surface of the Earth, they would have been destroyed almost immediately — geologically speaking — and the only remnants that we could recognize as ancient would be these zircons."

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