 |
|
|
|
|
|
Mark Harrison, Professor Emeritus of Geology |
|
 | |
| Mailing Address: | Department of Earth and Space Sciences
University of California, Los Angeles
595 Charles Young Drive East,
Box 951567
Los Angeles, CA 90095-1567 |
| Office: | Geology 6710 |
| Telephone: | (310) 825-7970 |
| Fax: | (310) 825-2779 |
| E-mail: | tmh@argon.ess.ucla.edu |
|
|
|
Scientists from UCLA and Australia Find Evidence of Water on Earth 4.3
Billion Years Ago
Scientists from UCLA and Curtin University of Technology
in Perth, Australia have found strong evidence for liquid water at or near the
Earth’s surface 4.3 billion years ago - research that pushes back our knowledge
of the presence of liquid water on Earth some 400 million years.
“We don’t
know when life began on Earth yet, but it potentially could have emerged as early
as 4.3 billion years ago because we infer that all three required conditions for
life existed then,” said T. Mark Harrison, professor of geochemistry at UCLA,
who directs UCLA’s W.M. Keck Foundation Center for Isotope Geochemistry.
“There
was a source of energy: the sun; a source of raw minerals: complex organic compounds
from meteorites or comets; and our inference that liquid water existed at or near
the Earth’s surface. Within 200 million years of the Earth’s formation, all of
the conditions for life on Earth appear to have been met.”
Stephen J. Mojzsis,
a former UCLA postdoctoral scholar in Harrison’s laboratory, who is now an assistant
professor of geology at the University of Colorado at Boulder and the lead author
of the Nature paper, goes even further.
“The stage was set 4.3 billion
years ago for life to emerge on Earth,” said Mojzsis, who is also a member of
the University of Colorado’s NASA-funded Astrobiology Institute. “There was probably
already in place an Earth with an atmosphere, an ocean, and a stable crust within
about two hundred million years of the Earth’s formation.
“Many geochemists
believe that maintaining stable liquid water on a planetary surface that early
is the most difficult of the three conditions,” Mojzsis said. “The conditions
for life were established very early on Earth, and this suggests that such conditions
might not be uncommon in the universe. If it happened so early on, why couldn’t
it happen elsewhere in the universe as well? Life may not be so difficult to form
when these three conditions are met.”
The scientists analyzed a rock from
Western Australia that was more than three billion years old with UCLA’s high-resolution
ion microprobe - an instrument that enables scientists to date and learn the exact
composition of samples - which Mojzsis described as the “world’s best instrument”
for this research. The microprobe shoots a beam of ions - charged atoms - at a
sample, releasing from the sample its own ions that are analyzed in a mass spectrometer.
Scientists can aim the beam of ions at specific microscopic areas of a sample
and analyze them without destroying the object.
The scientists learned
that while the rock was deposited about three billion years ago, it contains ancient
mineral grains - zircons - that were much older; two of the zircons were 4.3 billion
years old, and nearly a dozen others were older than four billion years. The Earth
is 4.5 billion years old. In addition, the researchers learned that the zircons
contained a unique and revealing ratio of oxygen isotopes.
“We were stunned
to discover a very distinctive oxygen isotopic signature in this rock - a rock
that significantly predates the Earth’s oxygen atmosphere - which tells us that
it interacted with cold water at temperatures appropriate to the Earth’s surface,”
Harrison said. “Many scientists did not think rocks older than two billion years
could provide this information. Was there liquid water at the Earth’s surface
4.3 billion years ago? We have not had any way to answer that question before
until these measurements, which suggest that the answer is yes.”
The telltale
sign is the ratio of the very common O-16 to the much rarer and heavier O-18.
“The
ratio of these isotopes reveals whether water has interacted with a rock,” Harrison
explained. “If a rock has been to the Earth’s surface and interacted with water,
it will be significantly ‘heavier’ and more enriched in O-18, which is precisely
what we have found in these ancient zircons.”
Zircons are heavy, durable
minerals related to the synthetic cubic zirconium used for imitation diamonds
and costume jewelry. The zircons studied in the rock are about twice the thickness
of a human hair.
“These zircons tell us that they melted from an earlier
rock that had been to the Earth’s surface and interacted with cold water,” Harrison
said. “There is no other known way to account for that heavy oxygen.”
The
ion microprobe is the first instrument that allows high-resolution isotope analysis
of inorganic and biological material only a few millionths of a meter in diameter,
Harrison said.
“The microprobe is a fantastic instrument in its sensitivity,
its accuracy and its versatility,” Mojzsis said. “With the microprobe, we can
determine the oxygen isotopic composition of individual spots within the tiny
zircons, and measure with enormous precision the ages of these spots. We can determine
when the zircons formed and how they formed.”
Without the ion microprobe,
the scientists would have been able to learn only the average age of the zircons
in the rock, not the ages of the various zircons, which varied substantially,
the scientists said.
Harrison and Mojzsis’ colleague on the research is
Robert Pidgeon, a professor of applied geology at Curtin University of Technology
in Perth, Australia, who first discovered the very ancient zircons in the rock.
The
research was funded by the National Science Foundation and NASA’s Center for Astrobiology.
The
oldest known rocks are about four billion years old, but Harrison suspects that
older rocks could be found that would reveal significant information about the
Earth’s evolution - including perhaps the source rocks that first contained the
4.3 billion-year-old zircons - if a coordinated effort to search for ancient rocks
were undertaken.
"Zircons are forever," Harrison noted.
|
|
