Earth's crust first formed at least 4.4
billion years ago, just 160 million
years after the formation of our solar system,
a new study has found. A
time-line of the history of our planet places the formation of the Jack Hills
zircon and a "cool early Earth"
at 4.4 billion years, researchers said. With the help of a tiny fragment of
zircon extracted from a remote rock outcrop in Australia, the picture of how
our planet became habitable to life is coming into sharper focus. An
international team of researchers led by University
of Wisconsin-Madison geoscience Professor John Valley reveals data that
confirm the Earth's crust first formed at least 4.4 billion years ago, just 160
million years after the formation of our solar system. The work shows that the time when our planet
was a fiery ball covered in a magma ocean came earlier.
"This confirms our view of how the Earth
cooled and became habitable. This may also help us understand how other
habitable planets would form," said Valley. The study confirms that zircon crystals from
Western Australia's Jack Hills region crystallised 4.4 billion years ago,
building on earlier studies that used lead
isotopes to date the Australian zircons and identify them as the oldest
bits of the Earth's crust. The microscopic zircon crystal used by Valley and
his group is now confirmed to be the oldest known material of any kind formed
on Earth.
The study, according to Valley, strengthens
the theory of a "cool early Earth," where temperatures were low
enough for liquid water, oceans and a hydrosphere not long after the planet's
crust congealed from a sea of molten rock. "The study reinforces our
conclusion that Earth had a hydrosphere
before 4.3 billion years ago," and possibly life not long after, said
Valley. The study was conducted using a new technique called atom-probe tomography that, in
conjunction with secondary ion mass spectrometry, permitted scientists to
accurately establish the age and thermal history of the zircon by determining
mass of individual atoms of lead in the sample. The clusters
of lead atoms formed 1 billion years after crystallisation of the zircon,
by which time the radioactive decay of uranium had formed the lead atoms that
then diffused into clusters during reheating. "The zircon formed 4.4
billion years ago, and at 3.4 billion years, all the lead that existed at that
time was concentrated in these hotspots," Valley said. The study was
published in the journal Nature Geoscience.
