杏吧原创

IT SOUNDS like a hellish place: a barren, crater-strewn landscape covered in molten lava. But tantalising evidence from one of the world鈥檚 oldest surviving minerals suggests that this conventional picture of the early Earth is wrong: the planet was in fact relatively cool and wet, and may even have supported life.

鈥淕eologists have tended to assume the first 500 million years were hot,鈥� says geologist John Valley of the University of Wisconsin in Madison. The planet formed 4.6 billion years ago, but the oldest rocks on Earth are no older than 4 billion years old. That has led researchers to believe that, after spending about 100 million years taking shape, the early Earth remained a molten mass鈥攑robably because continual bombardment by asteroids kept it too hot for rocks to solidify.

But that view has now been turned on its head by a tiny 4.4-billion-year-old zircon crystal discovered last year at a remote sheep station in Western Australia called Jack Hills. Its extreme age was a surprise, and now Valley and his team have found that the crystal has a high concentration of the isotope oxygen-18, which is thought to be a sign that the original rocks the crystal came from were exposed to liquid water and relatively low temperatures.

This implies that the early Earth was not so hot after all, chilling out at a relatively cool 200 掳C or less. Presumably the rate of asteroid impacts dropped more quickly than was previously thought. The planet could have formed a crust as soon as 160 million years after its birth, says Valley.

Not only that, but the possibility of liquid water means there is also a chance that life existed, says David Kring, a specialist in planet formation at the University of Arizona鈥檚 Lunar and Planetary Laboratory.

So why does so little evidence of this early landscape survive? The explanation may lie in the lunar rocks brought back by Apollo astronauts. Tests show them to be 3.9 billion years old, which suggests the entire Moon may have undergone a late and relatively short-lived 鈥渓unar cataclysm鈥� caused by a surge of asteroid strikes. This could have completely resurfaced it and wiped out traces of its earlier rocks, says Kring.

If so, the Earth must have taken a similar battering at the time. Erosion and plate tectonics have since erased Earth鈥檚 craters, but the zircon crystal appears to have survived the pummelling, offering the first evidence from that mysterious time before the bombardment wiped the geological slate clean.

If the early Earth was cool and wet, Kring says, then life may have appeared 900 million years earlier than the oldest known fossils, which are found in rocks 3.5 billion years old. Perhaps a few very early organisms survived the cataclysm in protected environments far from the surface, living off geothermal heat and chemical energy like the creatures that inhabit deep-sea hydrothermal vents today. Alternatively, life may have been entirely wiped out by the cataclysm, only to begin again shortly afterwards.