Strong evidence for liquid water at or near the Earth's surface 4.3 billion years ago is presented by a team of scientists in the cover story of the Jan. 11 issue of the journal Nature. The scientists — from UCLA and Curtin University of Technology in Perth, Australia — present 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, and is a co-author of the Nature paper. "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 16O to the much rarer and heavier 18O.
"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 18O, 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.