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 Scientists using two different age-determining techniques have
shown that a tiny zircon crystal found on a sheep ranch in western
Australia is the oldest known piece of our planet, dating to 4.4
billion years ago.
Writing in the journal Nature Geoscience on Sunday, the researchers
said the discovery indicates that Earth's crust formed relatively
soon after the planet formed and that the little gem was a remnant
of it.
John Valley, a University of Wisconsin geoscience professor who led
the research, said the findings suggest that the early Earth was not
as harsh a place as many scientists have thought.
To determine the age of the zircon fragment, the scientists first
used a widely accepted dating technique based on determining the
radioactive decay of uranium to lead in a mineral sample.
But because some scientists hypothesized that this technique might
give a false date due to possible movement of lead atoms within the
crystal over time, the researchers turned to a second sophisticated
method to verify the finding.
They used a technique known as atom-probe tomography that was able
to identify individual atoms of lead in the crystal and determine
their mass, and confirmed that the zircon was indeed 4.4 billion
years old.
To put that age in perspective, the Earth itself formed 4.5 billion
years ago as a ball of molten rock, meaning that its crust formed
relatively soon thereafter, 100 million years later. The age of the
crystal also means that the crust appeared just 160 million years
after the very formation of the solar system.
The finding supports the notion of a "cool early Earth" where
temperatures were low enough to sustain oceans, and perhaps life,
earlier than previously thought, Valley said.
This period of Earth history is known as the Hadean eon, named for
ancient Greek god of the underworld Hades because of hellish
conditions including meteorite bombardment and an initially molten
surface.
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"One of the things that we're really interested in is: when did the
Earth first become habitable for life? When did it cool off enough
that life might have emerged?" Valley said in a telephone interview.
The discovery that the zircon crystal, and thereby the formation of
the crust, dates from 4.4 billion years ago suggests that the planet
was perhaps capable of sustaining microbial life 4.3 billion years
ago, Valley said.
"We have no evidence that life existed then. We have no evidence
that it didn't. But there is no reason why life could not have
existed on Earth 4.3 billion years ago," he added.
The oldest fossil records of life are stromatolites produced by an
archaic form of bacteria from about 3.4 billion years ago.
The zircon was extracted in 2001 from a rock outcrop in Australia's
Jack Hills region. For a rock of such importance, it is rather
small. It measures only about 200 by 400 microns, about twice the
diameter of a human hair.
"Zircons can be large and very pretty. But the ones we work on are
small and not especially attractive except to a geologist," Valley
said. "If you held it in the palm of your hand, if you have good
eyesight you could see it without a magnifying glass."
(Reporting by Will Dunham; editing by Meredith Mazzilli)
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