[阅读小分队] 【揽瓜阁 外刊精读8.0】Day9 2021.06.24【自然科学-天文、地质】

What may be the oldest-known Earth rock has turned up in a surprising place: the moon. A2-centimeterchip,embedded in a larger rock collected by Apollo astronauts, is actually a 4-billion-year-old fragment of our own planet, scientists say.“ It’s a very provocative conclusion but it could be right,” says Munir Humayun, a cosmochemist at Florida State University in Tallahassee. The finding “helps paint a better picture of early Earth and the born[1]bardment that modified our planet during the dawn of life,” says David Kring, a lunar geologist at the Lunar and Planetary Institute in Houston, Texas, and an author on a study published on 24 January in Earth and Planetary Science Letters

Sometime after the rock formed, Kring says, an asteroid impact blasted it from Earth. It found its way to the moon, which was three times closer to Earth than it is today. The fragment was later engulfed in a lunar breccia, a motley type of rock. Finally，Apollo 14 astronauts returned it to Earth 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, a geochemist at the University of California, Los Angeles, who was not part of the study.

"Trace elements in the rock's minerals, which are a granitelike mix of quartz, feldspar, and zircon crystals, provided clues to it’s origin. By measuring uranium and its decay products in the zircons, the team dated the formation of the rock, while titanium levels helped reveal the temperature and pressure at the time. still other trace elements, such as cerium, pointed to the amount of water likely to have been present.

The results，Kring says, indicate that the rock formed in a water-rich environment at

temperatures and pressures corresponding to either 19 kilometers beneath the surface of Earth，or about 170 kilometers deep in the moon. Craig O'Neill, a geodynamicist at Macquarie University in Sydney，Australia, favors an Earth origin because a depth of170 kilometers would be "crazy""—way be low the moon's crust, where granitic rocks could have formed.

The rock isn't Earth's oldest relic: Zircon crystals from western Australia have been dated to as far back as 4.4 billion years, only150 million years after Earth's formation. But these zircons were stripped from their parent rocks and reworked into new mate-rials. Here, Kring says, there's no doubt that the rock and its zircons formed at the same time. "we're sure it's a complete rock" he says. The rock is about as old as the oldest rocks found on Earth—metamorphic rocks from Canada and Greenland.

Bell says its preservation is not so surprising because the moon lacks the weather and geologic processes that erase ancient rocks on Earth. In fact, she says, the moon might be a better place to look for ancient Earth rocks than Earth itself. Norm Sleep, a geo-physicist at Stanford University in Palo Alto, California, agrees. He says that although meteorites from Earth probably constitute a tiny fraction of the moon's surface material, eons of subsequent asteroid impacts have churned them throughout the lunar soil, making it easier to find a small piece of Earth in a random sample of moon.

If the rock is truly terrestrial, it holds clues about an ancient time, called the Hadean. It confirms Earth was being hit by asteroids big enough to blast rocks all the way to the moon. It also shows that the granitic rocks that make up Earth's continents were already forming, Kring says. "That's a big thing."

Kring believes other scientists will soon be combing the Apollo moon rocks for bits of early Earth. Only a small fraction of the382 kilograms of rocks brought back by the moon walkers have been studied, he says, and analytical techniques are constantly improving." "I think we are going to get a little library of fragments of the early Earth emerging in the next few years" he says.