What may be the oldest known Earth Rock has emerged in a surprising place: the moon. A 2-centimeter chip embedded in a larger stone collected by Apollo astronauts is actually a 4 billion-year-old fragment of our own planet, researchers say.
"It's a very provocative conclusion, but it could be right," says Munir Humayun, a cosmos chemist at Florida State University in Tallahassee. The foundation "helps to paint a better picture of the early earth and the bombardment that changed our planet during the dawn of life," said David Kring, a monthly linguist at the Lunar and Planetary Institute in Houston, Texas, and an author on a study that was published on January 24 in Soil and Planetary Science Letters .
Sometimes after the rock is formed, Kring says that an asteroid effect blew it from Earth. It found its way to the moon, which was three times closer to the earth than it is today. The fragment was later engulfed in a lunar breccia, a modley type rock. Finally, apollo 14 astronauts returned it to the ground in 1971. Although geologists have found meteorites on Earth that came from the moon, Mars and asteroids, "This is the first time a rock from the moon has been interpreted as a terrestrial meteorite," says Elizabeth Bell. geochemist at the University of California, Los Angeles, who was not part of the study.
Several years ago, a team led by Kring discovered detected fragments of asteroids in similar moon racks, so looking for pieces of land was a logical next step.
Trace elements in the stone minerals, which are a granite mixture of quartz, feldspar and zircon crystals, provided traces to its origin. By measuring uranium and its degradation products in the zirconia, the team dated the formation of the rock, while the titanium level helped reveal temperature and pressure at that time. Still other trace elements, such as cerium, pointed to the amount of water likely to be present.
The results, Kring says, show that the rock formed in an aqueous environment at temperatures and pressures equal to either 19 kilometers below the earth's surface, or about 170 kilometers deep in the moon. Craig O & # 39; Neill, a geodynamist at Macquarie University in Sydney, Australia, favors an earthquake because a depth of 170 kilometers would be "crazy" under the moon's crust, where granite rocks could have formed.
The Rock is the Earth's oldest relic: Zircon crystals from Western Australia have been dated as far back as 4.4 billion years, only 150 million years after Earth's formation. But these zirconia were cut off from their parents' stones and reworked into new materials. Here, Kring says that there is no doubt that the rock and its zirconia are formed at the same time. "We're sure it's a complete rock," he says. The clock is about as old as the oldest rocks found on Earth's metamorphic rocks from Canada and Greenland.
Bell says the conservation is not so surprising because the moon lacks weather and geological processes that erase old rocks on the ground. In fact, she says, the moon can be a better place to look for old earth's rocks than Earth itself. Norm Sleep, a geophysicist at Stanford University in Palo Alto, California, agrees. He says that although meteorites from Earth are likely to be a small fraction of the lunar surface material, eons of subsequent asteroid effects have churned them through the earth's soil, making it easier to find a small piece of land in a random sample of the moon.  If stone is really ground-based, it contains traces of an ancient time called Hadean. First, it confirms that the soil was hit by asteroids that are large enough to burst rocks all the way to the moon. It also shows that the granitic rocks that make up the Earth's continents have already formed, says Circle. "It's a great thing."
Circle believes that other scientists will soon fight the Apollo lunar cliffs for bits of early Earth. Only a small portion of the 382 kilograms of rock left by the moonwalkers has been studied, he says, and analytical techniques are constantly improving. "I think we should get a small library of fragments from the early Earth coming up in the coming years," he says.