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Early Earth had a cool crust


The Earth's crust, which would eventually form the continents we know today, cooled much earlier than we think (Image: NASA)

Early Earth had a cool crust that could have harboured life, say scientists who have dug up tiny zircon crystals that were formed right after the birth of the planet.

Most remnants of the very early crust, formed more than 4 billion years ago, are gone, recycled as part of the steady ongoing process known as plate tectonics.

But the little zircon crystals from ancient Australian deposits survived, say Assistant Professor Stephen Mojzsis of the University of Colorado and colleagues.

Mojzsis says the key is a rare metal element known as hafnium, which is found with the zircons.

"This is one of the few Earth materials that we know of that is capable of surviving recycling of the crust," Mojzsis says.

Clues in ancient rocks

Mojzsis and his colleagues found their clues in sedimentary rocks from the Jack Hills in Western Australia, which date to almost 4.4 billion years ago.

"First we determined how old these little grains were," Mojzsis says.

Their composition suggested they were formed at the right temperatures for making crust, and in the presence of water, the researchers repot online today in the journal Science.

"It was thought that the [early] world looked more like a lunar blasted landscape than a water-filled world with a continental landscape," Mojzsis says.

But he says the finding now showed that within a few hundred million years of its creation, the planet had the three necessary elements for life: water, energy and organic compounds.

"Now we see that was probably true back to 4.5 billion years ago," Mojzsis says.

"This helps us explain how the Earth became the habitable world it is."

It also might mean that life can occur easily, if the conditions can be right so soon after a planet forms, he says.

"We don't know how life got started. But if it happens so quickly, that might mean it's common, that it's easy," he says. "And that looks well for finding habitable planets outside Earth."

Earliest signs of life

The earliest signs of life on Earth are 3.5 billion-year-old filaments and tubes in lava deposits in South Africa that bacteria appear to have left.

In 2001 Mojzsis and colleagues published a study showing evidence of water on Earth's surface 4.3 billion years ago.


"The evidence indicates that there was substantial continental crust on Earth within its first 100 million years of existence," he adds. "It looks like the Earth started off with a bang."

Tags: environment, geology, palaeontology