EARTH may have become the blue planet just 200 million years after it formed, making it a welcoming home for life hundreds of millions of years earlier than we thought.
Earth's first 600 million years are called the Hadean – for good reason. "It is traditionally seen as a period of Earth history when our hot, young planet was hellish and uninhabitable," says Judith Coggon at the University of Bonn, Germany.
But hell may actually have been relatively short-lived. Coggon and her colleagues have found that rocks in Greenland contain a chemical signature from the mantle 4.1 billion years ago – just 400 million years after our planet was born. That signal suggests conditions at the time may have been more like they are today than we expected.
Models suggest some "iron-loving" metals like gold and platinum – which dissolve in molten iron – should all have sunk into the iron-rich core as it formed. Because they are relatively abundant in the mantle today, it has been suggested that meteorites and comets smashing into Earth about 3.9 billion years ago replenished the stock. This hypothesised event, known as the Late Veneer, is also thought to have given Earth most of its water, delivered as ice.
But Coggon's rock samples suggest that Earth's mantle had already been topped up with iron-loving minerals by 4.1 billion years ago, meaning the Late Veneer, and the birth of Earth's oceans, must have occurred earlier. It may actually have been before 4.3 billion years ago, says Coggon: rocks of that age discovered last year hint that, at the time they formed, Earth's mantle was already rich in iron-loving minerals (Nature Geoscience, doi.org/njz).
If so, Earth gained its oceans little more than 200 million years after it formed – which also pushes back the date for the earliest possible origin of life, says Coggon.
This article appeared in print under the headline "The planet Earth was blue long before we knew"
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