Analysis of a tiny zircon crystal suggests the Earth had oceans and continents just 50 million years after a giant impact was supposed to have melted the planet and formed the Moon.
The 200-micron crystal comes from ancient rocks in the remote Jack Hills area of Western Australia. Radiometric dating puts its age at 4.404 billion years, over 100 million years older than the next-oldest known fragment of the Earth.
More surprisingly, the ratio of oxygen isotopes indicates the material that formed the zircon had reacted with liquid water 鈥� suggesting the Earth had oceans much earlier than anyone had expected.
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鈥淭he conventional model is that the top 1000 kilometres of the Earth was molten rock at the time,鈥� says John Valley, a geologist at the University of Wisconsin in Madison involved in the analysis.
The Earth formed 4.55 billion years ago, but the impact of a Mars-sized body about 4.45 billion years ago was thought to have melted the planet again and splashed material into space which formed the Moon.
The discovery presents 鈥渞eal problems鈥� for the lunar impact theory, says Allan Treiman of the Lunar and Planetary Institute in Houston.
On impact, 鈥渁n incredible amount of heat gets dumped into the Earth 鈥� all the heat of the impact, plus the heat of the object鈥檚 core鈥� he says.
Cooling would take a long time. With only 50 million years between the supposed impact and the time the zircon formed, 鈥測ou鈥檙e running out of time to create the Moon and have the Earth cool down enough to have oceans,鈥� Treiman told New 杏吧原创. Although a magma ocean might lose heat quickly, cooling would slow once a solid crust covered the liquid.
The ancient zircon suggests the Earth鈥檚 surface was solid rock 4.4 billion years ago, and had even developed continents. Zircons normally form in granites or andesites and these rocks result from the melting of crustal rock and subsequent eruption on continents.
The concentration of 18O in the zircon indicates the rocks that melted had previously reacted with liquid water. This implies that surface temperatures were not above 100 掳C at the time.
The original rock in which the zircon formed eroded away billions of years ago, but the hard grain survived and was incorporated in a sedimentary rock in the ancient heart of Australia.
Source: Nature (vol 409, p175, p178)