The oldest terrestrial material yet found is challenging scientists to redate the birth of life on Earth. It may have happened sooner than they thought.
A team of geophysicists and chemists has dated the formation of this material - a tiny zircon crystal from western Australia - to 4.4 billion years ago. That's only about 100 million years after the planet itself was formed.
Scientists have thought the Earth at that time was still a molten mass pummeled by incoming asteroids - hardly a cradle for primitive life. The new analysis indicates the crystal formed under relatively cool conditions in an environment that already featured a continental-type crust and liquid-water ocean.
Team member John Valley at the University of Wisconsin in Madison notes that this is not direct evidence for early life. But he says if life were going to evolve that early in our planet's development, this is an environment that probably could have supported it.
Dr. Valley explains that the importance of this research lies in opening a window on "the missing period of Earth history" between the planet's formation and the next 600 or so million years. Not even the rocks in which the zircons formed have survived from that time.
The zircons give geophysicists a way to begin filling in the missing record. Scientists have been aware of zircons older than 4 billion years for nearly two decades, but largely ignored them. Now Valley expects "there's going to be a gold rush" to the Australian site.
Valley and his colleagues published the zircon dating today in the journal Nature. A companion paper by Stephen Mojzsis and T. Mark Harrison at the University of California, Los Angeles, and Robert Pidgeon at Curtin University of Technology in Perth, Australia, reports analyses of other Australian zircons. These show evidence of liquid water at Earth's surface 4.3 billion years ago.
Meanwhile, William Peck and colleagues at Colgate University are publishing a related zircon study in the journal Geochimica. The crystal's composition indicates it formed in a low-temperature environment. This suggests the young Earth cooled faster than thought - reaching the 100-degree C (boiling water) range in about 100 million years.
Valley notes that the scenario of a relatively benign early Earth is an imaginative leap to make from the analysis of tiny grains of zirconium silicate (zircon) measuring about the width of two human hairs. These tell nothing about how such an environment could coexist with asteroid bombardment.
He suggests the bombardment may have been episodic, with millions of years of relative quiet between attacks. Geochemist Ariel Anbar of the University of Rochester in New York, Dr. Mojzsis, and Kevin Zahnle at the NASA Ames Research Center in Palo Alto, Calif., have looked into this possibility for a slightly later time in early Earth history. This is the so-called Late Heavy Bombardment about 3.9 billion years ago. It ended the era of major asteroid attacks.
They studied the abundance of iridium in sediments in southern Greenland. These are the same sediments in which Mojzsis found signs of microbial life from 3.85 billion years ago. Iridium is rare on Earth and generally deposited by extraterrestrial objects. The scientists concluded that there were long quiet periods between asteroid bombardments during which life could have developed.
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