[meteorite-list] Earlier Water on Earth? Oldest Rock Suggests Hospitable Young Planet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:44 2004
Message-ID: <200201092344.PAA05084_at_zagami.jpl.nasa.gov>

Note: This press release is one year old.

Ron Baalke
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http://www.nsf.gov/od/lpa/news/press/01/pr0102.htm

Earlier Water on Earth? Oldest Rock Suggests Hospitable Young Planet
National Science Foundation
January 10, 2001

NSF PR 01-02
Media contact: Cheryl Dybas (703) 292-8070 cdybas_at_nsf.gov
Program contact: Sonia Esperanca (703) 292-8554 sesperan_at_nsf

Geological evidence suggests that Earth may have had surface water --and
thus conditions to support life -- millions 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
Received on Wed 09 Jan 2002 06:44:32 PM PST