June 3, 2005
Scientists Confirm Liquid Water on Early Earth
NASA -- Research funded partly by NASA has confirmed the existence of liquid water on the Earth's surface more than 4 billion years ago.
Scientists have found that the Earth had formed patterns of crust formation, erosion and sediment recycling as early as 4.35 billion years ago. Their findings came during a study of zircon crystals formed during the earliest period of Earth's history, the Hadean Eon (4.5 billion to 4.0 billion years ago).
"NASA is interested in how early the Earth had abundant liquid water. If oceans form early in a planet's history, then so can life," said Carl Pilcher, senior scientist for astrobiology at NASA Headquarters, Washington. "Learning how early oceans formed on Earth will help us understand where else oceans and perhaps even life may have formed in this solar system and in planetary systems around other stars."
"This work provides direct evidence that the Earth was probably habitable within a hundred million years of its formation," said Bruce Runnegar, director of the NASA Astrobiology Institute (NAI) at NASA Ames Research Center, Moffett Field, Calif., which provided some of the study's funding.
Published in the May 6, 2005, edition of Science, the research was conducted by T. Mark Harrison of the Research School of Earth Sciences, Australian National University, Canberra and the University of California, Los Angeles; and E. Bruce Watson of the Department of Earth & Environmental Sciences at Rensselaer Polytechnic Institute, Troy, N.Y. Field research was completed in Western Australia's Jack Hills, which preserve a record of the Hadean Eon.
Watson and Harrison devised a new method of determining the temperatures at which the rocks formed. The team extracted and examined more than 50,000 zircons, crystals about the width of a human hair, which have been exposed through natural erosion in the Jack Hills. From the 50,000 zircons, only a couple of hundred were older than 4.2 billion years. Measuring the temperature at which the rocks melt gives an indication of the conditions in which they formed.
"Rocks formed as a result of the thermal energy from meteorite impacts would be bone dry and melt at greater than 900 degrees Celsius," said Harrison. "In contrast, our study has found that Hadean rocks melted at a consistent average temperature of 690 degrees Celsius. Water, which is a very powerful catalyst, must have been present in very large amounts for rocks to melt at such a relatively low temperature."
This discovery supports the proposal by Harrison's group four years earlier that a heavy oxygen isotope signature in the Hadean zircons is evidence for liquid water at or near the Earth's surface by 4.3 billion years ago.
The NAI, founded in 1997, is a partnership between NASA, 16 major U.S. teams and five international consortia. NAI's goal is to promote, conduct and lead integrated multidisciplinary astrobiology research and to train a new generation of astrobiology researchers.
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