Stage set earlier for life on Earth
A scientific quest called "Mission to Really Early Earth" has unearthed evidence that our planet had an ocean, a continent and an atmosphere...
Knight Ridder Newspapers
BOULDER, Colo. — A scientific quest called "Mission to Really Early Earth" has unearthed evidence that our planet had an ocean, a continent and an atmosphere suitable for life 500 million years earlier than previously thought.
Since the requirements for life — land, water and air — were established so soon on Earth, some scientists say the finding makes it more likely that living creatures could have arisen on other worlds as well.
"If it happened so early on Earth, why couldn't it happen elsewhere in the universe as well?" said Stephen Mojzsis, a geoscientist at the University of Colorado in Boulder.
According to the traditional view, Earth formed between 4.5 billion and 4.6 billion years ago from a disc of dust, rocks and gas circling the sun. It then took 700 million years for the young planet to settle down and cool off enough for the first microscopic organisms to appear around 3.8 billion years ago, paleontologists believed.
Researchers, however, now report evidence that conditions were much more benign when the Earth was 150 million to 200 million years old — 3 percent to 4 percent of its present age.
"The stage was set 4.3 billion years ago for life to emerge on Earth," Mojzsis told a conference on astrobiology — the study of life on other worlds — last month.
"There was probably already in place an atmosphere, an ocean and a stable crust within about 200 million years of the Earth's formation," said Mojzsis, chairman of the conference. "Water was gushing out of the Earth."
This picture of a comfortably warm, wet, young world "contrasts with the hot, violent environment envisioned for our young planet by most researchers," Bruce Watson, a geochemist at Rensselaer Polytechnic Institute in Troy, N.Y., declared in the May 6 weekly online edition of the journal Science. "It opens up the possibility that life got a very early foothold."
"If there was surface water, then life presumably could exist," said Don Brownlee, an astronomer at the University of Washington.
"We don't know when life began on Earth," cautioned Mark Harrison, an Australian geoscientist who was at the astrobiology conference. "But it could have emerged as early as 4.3 billion years ago. Within 200 million years of the Earth's formation, all of the conditions for life on Earth appear to have been met."
The evidence for a very young habitable Earth consists of a collection of tiny crystals called zircons dug up in the Jack Hills of western Australia over the past 20 years. New technology pioneered by Mojzsis and John Valley, a geochemist at the University of Wisconsin, Madison, has made it possible to determine how and when they formed.
For example, zircons contain uranium, which decays to lead at a known rate. The Jack Hills zircons also enclose bits of shale, a sedimentary rock that must have been created previously by erosion by liquid water. In addition, the zircons contain a rare type of "heavy" oxygen that forms only in the presence of water.
"These zircons tell us that they melted from an earlier rock that had been to the Earth's surface and interacted with cold water," Mojzsis said. "There is no other known way to account for that heavy oxygen."
Sonia Esperanca, an earth scientist at the National Science Foundation in Washington, D.C., called the Jack Hills zircons "time capsules of processes happening in the earliest times in Earth's history."
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