Analysis of a tiny zircon crystal suggests the Earth
had oceans and continents just 50 million years after a giant impact was
supposed to have melted the planet and formed the Moon.
The 200-micron crystal
comes from ancient rocks in the remote Jack Hills area of Western Australia.
Radiometric dating puts its age at 4.404 billion years, over 100 million years
older than the next-oldest known fragment of the Earth.
More surprisingly, the
ratio of oxygen isotopes indicates the material that formed the zircon had
reacted with liquid water - suggesting the Earth had oceans much earlier than
anyone had expected.
"The conventional
model is that the top 1000 kilometres of the Earth was molten rock at the
time," says John Valley, a geologist at the University of Wisconsin in
Madison involved in the analysis.
The Earth formed 4.55
billion years ago, but the impact of a Mars-sized body about 4.45 billion years
ago was thought to have melted the planet again and splashed material into
space which formed the Moon.
Hot problem
The discovery presents
"real problems" for the lunar impact theory, says Allan Treiman of
the Lunar and Planetary Institute in Houston.
On impact, "an
incredible amount of heat gets dumped into the Earth - all the heat of the
impact, plus the heat of the object's core" he says.
Cooling would take a
long time. With only 50 million years between the supposed impact and the time
the zircon formed, "you're running out of time to create the Moon and have
the Earth cool down enough to have oceans," Treiman told New Scientist.
Although a magma ocean might lose heat quickly, cooling would slow once a solid
crust covered the liquid.
Early
developer
The ancient zircon
suggests the Earth's surface was solid rock 4.4 billion years ago, and had even
developed continents. Zircons normally form in granites or andesites and these
rocks result from the melting of crustal rock and subsequent eruption on
continents.
The concentration of 18O
in the zircon indicates the rocks that melted had previously reacted with
liquid water. This implies that surface temperatures were not above 100 °C at
the time.
The original rock in
which the zircon formed eroded away billions of years ago, but the hard grain
survived and was incorporated in a sedimentary rock in the ancient heart of
Australia.
Nature