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Where did the materials that make up the Earth and moon come from—and when did they arrive?
Most scientific models contend the Earth formed gradually by addition from an assortment of moon- to Mars-sized masses that had a vast array of isotopic characteristics. New research published Jan. 26 in Nature maintains the Earth, as well as the moon and certain meteorites, were formed from materials that were more similar, holding almost indistinguishable isotopic characteristics.
“The Earth accreted from an isotopically homogenous reservoir,” said Nicolas Dauphas, the Louis Block Professor in Geophysical Sciences, the study’s author. “In terms of colors, you could say that it was not ‘green, blue, red,’ but rather ‘green, green, green.'”
Dauphas used the isotopic similarities he found in select elements to record the stages of Earth’s formation. Soon after the Earth formed 4.5 billion years ago and as its core grew, the core attracted elements that had strong affinities for metal. As core formation was almost complete, however, such elements—as they continued to arrive from space—were left to reside in the mantle.
This helps explain the age of parts of the Earth and the role they played in forming our planet, Dauphas said. “For example, I can tell you that the coin in your pocket with the image of Jefferson on it contains no nickel from the first 60 percent of the Earth’s accretion because the core scavenged that early-to-arrive nickel.”
“Before this work, the question of the nature of the Earth’s accreting material through time was mostly rhetorical,” said Dauphas. “By studying high-precision measurements, we have shown that the Earth, the moon and meteorites with a high concentration of the mineral enstatite have almost indistinguishable isotopic compositions.”
The formation of the moon
“The moon is isotopically similar to the Earth,” Dauphas said. “Therefore the giant impactor that struck the Earth soon after it was created, thereby forming the moon, most likely had a similar isotopic composition to the Earth.”
Dauphas’ method offers “an elegant approach” for sourcing the materials that made up the Earth, Stevenson concluded.