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Cool world. Minute fragments of the mineral zircon, shown here in false-color images, give clues of early Earth's geology.

Was Early Earth a Cool World?

The young Earth cooled quickly after forming, and life may have been able to arise soon after, a new study suggests. The analysis, using shards of Earth's oldest material, bolsters a relatively new view that Earth acquired oceans and formed tectonic plates much earlier than previously thought.

Soon after it coalesced from matter swirling around the Sun about 4.57 billion years ago, Earth was likely covered with seas of molten rock. The long-prevailing view, based largely on models of planet formation, was that Earth stayed hellishly hot for 500 million years, the Hadean Eon. But the oldest minerals found--hardy zircons from Australia's Jack Hills, some of which formed when Earth was only 200 million years old--have suggested a much different picture. Studies of oxygen isotopes in these crystals indicated they formed at relatively low temperatures, and the only known way for that to happen is in the presence of liquid water. Also, these zircons include fragments of other minerals, such as mica, which provide circumstantial evidence that they formed from sedimentary rocks--suggesting the Hadean Earth had a crust and oceans to boot.

Now a new study, looking again at the Jack Hill zircons but using a different approach, bolsters the view that Earth became placid relatively quickly. Geochemist Bruce Watson of Rensselaer Polytechnic Institute in Troy, New York, developed a new way of telling the temperature at which a zircon crystal formed. As zircon crystallizes, titanium atoms can replace zirconium or silicon atoms in the mineral. But once crystallized, the titanium is locked in for good, and the proportion of titanium reflects the temperature at which the zircon formed.

With this in mind, Watson and geochemist Mark Harrison of the Australian National University in Canberra re-examined the ancient rock shards. Their findings, published today in Science, show signs the crystals formed at low temperatures, around 700°C. This could happen only in water-saturated sediments packed and heated until they formed rock, the researchers reason, so the low crystallization temperatures support the idea of a cool early Earth. "The Jack Hill zircons are providing the first glimpses of what was actually going on," at these early times, Watson says. "These results open up the possibility that complex organic molecules were stable, and maybe even life began, much earlier than previously thought."

The new titanium thermometer method is "very exciting," says petrologist William Peck at Colgate University in Hamilton, New York. "This is a significant and independent piece of evidence supporting the cool early Earth" hypothesis, he says.

Related sites
Bruce Watson's homepage
How to date the oldest bits of Earth