The large Apollo 14 sample called “Big Bertha” holds a 2-centimeter chip thought to be from Earth.
NASA
Ancient Earth rock found on the moon
Rocha antiga da terra encontrada na lua
What may be the oldest-known Earth rock has turned up in a
surprising place: the moon. A 2-centimeter chip 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 bombardment 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.
Several years ago, a team led by Kring detected fragments of
asteroids in similar moon rocks, so looking for pieces of Earth was a
logical next step.
Trace elements in the rock’s minerals, which are a granitelike mix of
quartz, feldspar, and zircon crystals, provided clues to its 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
of 170 kilometers would be “crazy”—way below the moon’s crust, where
granitic rocks could have formed.
A rocha não é a relíquia mais antiga da Terra: os cristais de zircônio do oeste da Austrália foram datados de 4,4 bilhões de anos, apenas 150 milhões de anos após a formação da Terra. Mas esses zircônios foram arrancados de suas rochas originais e transformados em novos materiais. Aqui, Kring diz, não há dúvida de que a rocha e seus zircônios se formaram ao mesmo tempo. "Temos certeza de que é uma rocha completa", diz ele. A rocha é tão antiga quanto as rochas mais antigas encontradas na Terra - rochas metamórficas do Canadá e da Groenlândia.
Se a rocha é verdadeiramente terrestre, ela contém pistas sobre um tempo antigo chamado Hadeano. Para começar, confirma que a Terra estava sendo atingida por asteróides grandes o suficiente para explodir rochas até a lua. Também mostra que as rochas graníticas que compõem os continentes da Terra já estavam se formando, diz Kring. "Isso é uma coisa grande."
Kring believes other scientists will soon be combing the Apollo moon
rocks for bits of early Earth. Only a small fraction of the 382
kilograms of rocks brought back by the moonwalkers 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.
doi:10.1126/science.aaw8383
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