Earlier Water on Earth? Oldest Rock Suggests Hospitable Young Planet (Forwarded)
Andrew Yee
Washington, D.C.
Media contact: Cheryl Dybas, (703) 292-8070, cdy...@nsf.gov
Program contact: Sonia Esperanca, (703) 292-8554, sesp...@nsf.gov
Embargoed until 2 p.m. EST, January 10, 2001
NSF PR 01-02
Earlier Water on Earth? Oldest Rock Suggests Hospitable Young Planet
Geological evidence suggests that Earth may have had surface water -- and
thus conditions to support life -- billions of years earlier than previously
thought. Scientists reconstructed the portrait of early Earth by reading the
telltale chemical composition of the oldest known terrestrial rock. The
4.4-billion-year-old mineral sample suggests that early Earth was not a
roiling ocean of magma, but instead was cool enough for water, continents,
and conditions that could have supported life. The age of the sample may
also undermine accepted current views on how and when the moon was formed.
The research was supported in part by the National Science Foundation (NSF),
and is published in this week's issue of the journal Nature.
"This appears to be evidence of the earliest existence of liquid water on
our planet," says Margaret Leinen, assistant director of NSF for geosciences.
"If water occurred this early in the evolution of earth, it is possible that
primitive life, too, occurred at this time."
By probing a tiny grain of zircon, a mineral commonly used to determine
the geological age of rocks, scientists from the University of Wisconsin-
Madison, Colgate University, Curtin University in Australia and the
University of Edinburgh in Scotland have found evidence that 4.4 billion
years ago, temperatures had cooled to the 100-degree Centigrade range,
a discovery that suggests an early Earth far different from the one
previously imagined.
"This is an astounding thing to find for 4.4 billion years ago," says
John Valley, a geologist at UW-Madison. "At that time, the Earth's
surface should have been a magma ocean. Conventional wisdom would not
have predicted a low-temperature environment. These results may indicate
that the Earth cooled faster than anyone thought." Previously, the oldest
evidence for liquid water on Earth, a precondition and catalyst for life,
was from a rock estimated to be 3.8 billion years old.
The accepted view on an infant Earth is that shortly after it first formed
4.5 to 4.6 billion years ago, the planet became little more than a swirling
ball of molten metal and rock. Scientists believed it took a long time,
perhaps 700 million years, for the Earth to cool to the point that oceans
could condense from a thick, Venus-like atmosphere. For 500 million to 600
million years after the Earth was formed, the young planet was pummeled by
intense meteorite bombardment. About 4.45 billion years ago, a Mars-size
object is believed to have slammed into the Earth, creating the moon by
blasting pieces of the infant planet into space.
The new picture of the earliest Earth is based on a single, tiny grain of
zircon from western Australia found and dated by Simon Wilde, of the School
of Applied Geology at Curtin University of Technology in Perth, Western
Australia. Valley worked with William Peck, a geologist at Colgate
University, to analyze oxygen isotope ratios, measure rare earth elements,
and determine element composition in a grain of zircon that measured
little more than the diameter of two human hairs. Colin Graham's
laboratory analyzed the zircon to obtain the oxygen isotope ratios.
Graham is a contributor to the paper and geochemist at the University
of Edinburgh.
"What the oxygen isotopes and rare earth analysis show us is a high oxygen
isotope ratio that is not common in other such minerals from the first
half of the Earth's history," Peck says. In other words, the chemistry of
the mineral and the rock in which it developed could only have formed from
material in a low-temperature environment at Earth's surface.
"This is the first evidence of crust as old as 4.4 billion years, and
indicates the development of continental-type crust during intense
meteorite bombardment of the early Earth," Valley says. "It is possible
that the water-rock interaction (as represented in the ancient zircon
sample) could have occurred during this bombardment, but between
cataclysmic events."
Scientists have been searching diligently to find samples of the Earth's
oldest rocks. Valley and Peck say such ancient samples are extremely rare
because rock is constantly recycled or sinks to the hot mantle of the
Earth. Over the great spans of geologic time, there is little surface
material that has not been recycled and reprocessed in this way.
The tiny grain of zirconium silicate or zircon found by Wilde in western
Australia was embedded in a larger sample containing fragments of material
from many different rocks, Valley says. Zircons dated at 4.3 billion years
were reported from the same site a decade ago, but the new-found zircon
grain is more than 100 million years older than any other known sample,
giving scientists a rare window to the earliest period of the Earth.
"This early age restricts theories for the formation of the moon," Valley
says. "Perhaps the moon formed earlier than we thought, or by a different
process." Another intriguing question is whether or not life may have
arisen at that early time. Low temperatures and water are preconditions
for life. The earliest known biochemical evidence for life and for a
hydrosphere is estimated at 3.85 billion years ago, and the oldest
microfossils are 3.5 billion years old.
"It may have been that life evolved and was completely extinguished
several times" in catastrophic, meteorite-triggered extinction events
well before that, Valley says. The research conducted by Valley, Peck,
Graham and Wilde was also supported by the U.S. Department of Energy,
the U.K. Natural Environment Research Council and a Dean Morgridge
Wisconsin Distinguished Graduate Fellowship.
-NSF-
Note to reporters and editors
For images of the rock and crystal, see:
http://www.nsf.gov/od/lpa/news/press/01/pr0102.htm
Editors: A high-resolution image is available for downloading at:
http://www.news.wisc.edu/newsphotos/zircon.html.
See also: Time line graphic at
http://www.news.wisc.edu/newsphotos/oldrock.html
IMAGE CAPTIONS:
[Image 1: http://www.news.wisc.edu/newsphotos/zircon.html]
Cathodoluminescence image of the oldest known material from the Earth, a
single crystal of zircon from the Jack Hills metaconglomerate, Western
Australia. Concentric, magmatic growth zoning is shown about the crystal
core. The crystallization age of 4.40Ga (4004 +/- 4Ma) was determined from
the circled area by ion microprobe. The arrow points to an inclusion of
quartz. The high oxygen isotope ratio from this sample suggests that low
temperature surficial processes including liquid water were important for
the formation of protoliths to this magma."
[Image 2: http://www.nsf.gov/od/lpa/news/press/01/pr0102.htm]
False color backscattered electron image of the 4.40Ga zircon. Ion
microprobe pits are visible as are 3 inclusions of quartz (black spots,
10-20 microns in diameter).
[Image 3: http://www.nsf.gov/od/lpa/news/press/01/pr0102.htm]
False color cathodoluminescence image of the 4.40Ga zircon showing
concentric, magmatic growth zoning.
--
Andrew Yee
ay...@nova.astro.utoronto.ca
Doug O'Neal
> Geological evidence suggests that Earth may have had surface water -- and
> thus conditions to support life -- billions of years earlier than
previously
> thought.
> The accepted view on an infant Earth is that shortly after it first
formed
> 4.5 to 4.6 billion years ago, the planet became little more than a
swirling
> ball of molten metal and rock. Scientists believed it took a long time,
> perhaps 700 million years, for the Earth to cool to the point that oceans
> could condense from a thick, Venus-like atmosphere.
700 million doesn't qualify as "billions" of years in my numeration system.
Doug
RC
s.net>, Doug O'Neal <don...@cobweb.net> writes
When I got to the bit about life having been wiped out several times I
had to check the calendar on my computer.
Surely the crystal in question could have been made on a meteorite?
--
Reason
http://www.earthpoetry.demon.co.uk
RC
Rob Carr
r...@earthpoetry.demon.co.uk wrote:
> Surely the crystal in question could have been made on a meteorite?
By looking at the non-Uranium and non-lead isotope ratios, it would be
easy to determine if it was of meteoric origin or not. Now, one would
hope they actually tested for this.
Anyone know?
--
Rob
I'm running for my life!
Rome Marathon to raise money for the ADA
Sent via Deja.com
http://www.deja.com/
George Dishman
news:X9tLXIAT...@earthpoetry.demon.co.uk...
> In article <EF52486B20BB7CC8.500522FF...@lp.airnew
> s.net>, Doug O'Neal <don...@cobweb.net> writes
> >Geez ... another wonderfully accurate press release.
> When I got to the bit about life having been wiped out several times I
> had to check the calendar on my computer.
Its easy. It was known that water first formed about 3.8b yr ago. The
first evidence of life was at 3.6b yr so it took 0.2b yr to get going.
Now we know there was water at 4.6b yr, the life must have appeared
at 4.4b yr, so since there is no evidence before 3.6b yr, it must have
been wiped out. In fact it was wiped out at 4.4b, 4.2b, 4.0b and 3.8b yr
as well.
:-P
--
Give me a small laser and I'll move the sun.
Weep not for those who were, rather for those who may never be.
Seti: 2.4!#@2.5*#