Scientists have discovered the oldest mineral ever found on Earth, provingthat the Earth had oceans and continents just 50 million years after a giantimpact melted the entire planet. An asteroid the size of Mars broke off apiece of the Earth that became the Moon over 4 ? billion years ago. The Moonmade higher forms of life possible on Earth, since its counter-rotationslows down the high-speed winds that would normally sweep along the surface.

Analysis of a tiny zircon crystal found in the ancient and remote hills ofWestern Australia shows that it is a little less than 4 ? billion years old,over 100 million years older than the next-oldest known fragment of Earth,and that it formed only about 50 million years after the asteroid hit.

The ratio of oxygen isotopes in the fragment indicates that the materialforming the zircon had reacted with liquid water, suggesting the Earth hadoceans-and therefore a climate that could support life-much earlier thanpreviously thought.

“The conventional model is that the top [600 miles] of the Earth was moltenrock at the time,” says John Valley, a geologist at the University ofWisconsin.

The discovery poses “real problems” for the lunar impact theory, says AllanTreiman of the Lunar and Planetary Institute in Houston. After such a largeimpact, it would take the Earth a long time to cool down. With only 50million years between the date of the impact and the time the zircon formed,”you’re running out of time to create the Moon and have the Earth cool downenough to have oceans,” he said.

Rather than being melted by the impact, the zircon suggests that the Earth’ssurface was solid rock 4 ? billion years ago, and had even developedcontinents. Zircon crystals form in the kind of rock that is made from themelting of the Earth’s crust. The concentration of oxygen in the zirconindicates that the Earth’s surface had been in contact with liquid water,implying that temperatures were below the boiling point at the time.

If we recalculate the period when the Earth was able to sustain life back toan earlier date, it could mean that traces of the past might be found of atype not previously seen. It could also lead to an earlier dating forintelligent life on our planet.

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