Precocious
Earth
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Tiny zircon crystals found in
ancient stream deposits suggest that Earth harbored
continents and liquid water remarkably soon after our
planet formed.
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January 17, 2001 -- Scientists are
drawing a portrait of how Earth looked soon after it
formed 4.56 billion years ago, based on clues within
the oldest mineral grains ever found.
Tiny zircons (zirconium silicate
crystals) found in ancient stream deposits indicate
that Earth developed continents and water -- perhaps
even oceans and environments in which microbial life
could emerge -- 4.3 billion to 4.4 billion years ago,
remarkably soon after our planet formed.
Right: Where the
newly-discovered zircons fit in Earth history. (Complete
timeline) Image by Dan Brennan.
The findings by two research groups,
one in Australia and the other in the United States,
suggest that "liquid water stabilizes early on
Earth-type planets," said geologist Stephen
Mojzsis, a member of the NASA Astrobiology
Institute's University of Colorado, Boulder, team.
"This increases the likelihood of finding life
elsewhere in the universe" because conditions
conducive to life can evidently develop faster and
more easily than once thought.
It also "gives us a new view of
the early Earth, where the Earth cooled quickly"
after gas and dust in the newborn solar system
congealed to form planets, said geologist William
Peck, of Colgate University in Hamilton, New York.
"There were continents and water really early --
and maybe oceans and life -- all to be obliterated
later by meteorites, with almost no record left except
these zircons."
Until roughly 3.9 billion years ago, swarms
of comets and meteorites whacked the young Earth
often enough to occasionally vaporize the surface
zones of the oceans and erase any life residing there.
The earliest
known evidence of microbial life on Earth comes
from carbon isotope patterns investigated by Mojzsis
and colleagues in 3.85-billion-year-old Greenland
sediments.
Now, the zircons from Western
Australia demonstrate that continents and water
existed 4.3 billion to 4.4 billion years ago.
"Life could have had the opportunity to start 400
million years earlier than previously
documented," Mojzsis said.
"Life could have arisen many
times, only to be smashed, and it only gets a hold
once the meteorites taper off," Peck added.
Mojzsis and Peck belong to separate
research teams, one that found a 4.4-billion-year-old
zircon in 1999 and another team that unearthed a pair
of 4.3-billion-year-old zircons last year from the
same area of Western Australia's Jack Hills rock
formation. Both groups published their studies
in the Jan. 11, 2001, issue of the British journal
Nature.
The 4.4-billion-year-old zircon is
"our earliest record of the earliest crust"
on Earth, Peck said. That zircon and the slightly
younger zircon grains measure roughly 250 microns wide
-- less than one one-hundredth of an inch.
"These
zircons have really been through the wringer,"
said Peck.
Left: The Jack Hills
region of Western Australia, where the zircons were
discovered. Photo by Simon Wilde.
Their history began sometime after
Earth formed, when "liquid water interacted with
rocks," he said. That interaction can happen in
one of three ways: when water exchanges with minerals
in rocks, when crystals grow out of solution in ground
water, or when mineral veins are deposited. Exposure
to water increased the rocks' normally low ratio of
the uncommon isotope oxygen-18 to the more-common
isotope oxygen-16, he said.
Later, the rocks were melted
underground -- or perhaps during a meteorite
bombardment -- and the zircons formed as crystals
within molten granite that was cooling to form solid
rock.
The zircon-laden granite eventually
was thrust upward to form mountains, which later
eroded. The granite vanished, but the zircons
ultimately came to rest 3 billion years ago in sandy
Australian stream sediments. These sediments later
hardened into rocks that subsequently were altered by
heat and pressure.
Both research teams used instruments
called ion
microprobes to date and analyze the zircon
crystals, which often contain uranium, rare earth
elements and other impurities. Uranium decays to lead
at a known
rate. Uranium-lead ratios in the zircons showed
they formed as early as 4.4 billion to 4.3 billion
years ago when they crystallized in molten
granite.
Below: Microscopic view
of a zircon (zirconium silicate) crystal determined to
be 4.4 billion years old. Photo by John W. Valley
Continental
crust is different than crust that underlies the
oceans. Granite is a common rock in continents. And
zircons commonly crystallize in granite.
So the zircons indicate granite was
present 4.3 billion to 4.4 billion years ago, while
the granite means continents existed at that time.
Such old granitic rock has not been found; it all has
subsequently been eroded away or otherwise recycled.
The ancient zircons are surviving vestiges of crustal
granite from Earth's early years.
"The fact you have a
4.4-billion-year-old zircon from granite suggests
there had to be the rock of the continental
crust," said geologist Sam Bowring of the
Massachusetts Institute of Technology.
Ion microprobe analysis of
rare-earth elements within the zircon crystals also
found levels typical of continental rocks, Peck
said.
The presence of water on the young
Earth was confirmed when both groups analyzed the
zircons for oxygen isotopes and found the telltale
signature of rocks that have been touched by water: an
elevated ratio of oxygen-18 to oxygen-16.
As a result, "we know there was
liquid water at some point before 4.4 billion years
ago," Peck said. Liquid water had to collect
somewhere, raising the possibility of oceans, he
added.
He said it also is likely oceans
existed because "to make continents, you need to
have water."
Peck said that before there were
oceans, giant plates of Earth's crust already could
have started moving and colliding with each other,
causing large blocks of rock to dive downward in a
process called subduction. Without oceans, that rock
could not have melted to form continental rock like
granite, he said.
Below: Outcrop of the
type of rock where the zircons were discovered. The
hammer shows scale. Photo by Simon Wilde.
Once
there were oceans, however, seawater would have
reacted with and hydrated lava erupting from undersea
volcanoes at the mid-ocean ridges. The lava would then
have cooled and formed new seafloor, which later
subducted. The water trapped in minerals within the
sinking rock lowered its melting point, triggering
volcanic eruptions that probably produced island
chains made of granitic rocks. It is thought that such
"island arcs" ultimately clumped together to
form continents.
"Oceans, atmosphere and
continents were in place by 4.3 billion years
ago," said Mojzsis.
According to Peck, the first oceans
might have formed from water brought to Earth by
comets or have been emitted during early volcanic
eruptions from what became mid-ocean ridges.
The zircons suggest that life could
have existed on Earth 4.3 billion years ago, said
Mojzsis, because three key factors necessary for life
to take hold were present: energy, organic material (from
incoming comets and atmospheric reactions) and --
according to the zircons -- liquid water.
Credits: Discovery of the
4.4-billion-year-old zircon was reported by Peck,
Simon Wilde at the Curtin Institute of Technology in
Australia; John Valley at the University of Wisconsin,
Madison; and Colin Graham of the University of
Edinburgh in the United Kingdom. Wilde found the
4.4-billion-year-old grain in 1999 while dating
zircons from a rock collected in 1984, Peck said.
Mojzsis and colleagues say they found a pair of
4.3-billion-year-old zircons last year from the same
area of Western Australia's Jack Hills rock formation.
Mojzsis worked with geochemist Mark Harrison of the
University of California, Los Angeles, and Robert
Pidgeon of the Curtin Institute of Technology.
Related Links:
NASA's
Astrobiology Institute -- Home page
Zircon
analysis -- describes how researchers at Curtin
University of Technology deduced the ages and
geological histories of the zircon crystals
Evidence
for Life on Earth Before 3,800 Million Years Ago
-- Nature Japan article
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