This meteorite’s diamonds hint that it was born in a lost planet | Science News

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This meteorite’s diamonds hint that it was born in a lost planet

The space rock could come from a Mars-sized protoplanet in the early solar system

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2:30pm, April 18, 2018
illustration of early solar system collisions

HERE TODAY  Violent collisions in the early solar system, illustrated here, probably broke apart would-be planets — but diamonds forged in those planets’ cores may have made it to Earth as meteorites.

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A chunk of space rock may have been forged inside a long-lost planet from the early solar system. Tiny pockets of iron and sulfur embedded in diamonds inside the meteorite probably formed under high pressures found only inside planets the size of Mercury or Mars, researchers suggest April 17 in Nature Communications.

The parent planet no longer exists, though — it was smashed to smithereens in the solar system’s violent infancy.

“We probably have in our hands a piece of one of these first planets that have disappeared,” says Philippe Gillet of École Polytechnique Fédérale de Lausanne, or EPFL, in Switzerland.

EPFL physicist Farhang Nabiei, Gillet and their colleagues analyzed minuscule fragments of the Almahata Sitta meteorites. These meteorites are famous for coming from the first-ever asteroid tracked from orbit to ground as it streaked to the Nubian desert in Sudan in 2008 (SN: 4/25/09, p. 13).

Rough in the diamond

Tiny pockets of sulfur and iron (yellow) inside a diamond (blue) inside a meteorite suggest the meteorite was once part of a long-lost planet in the early solar system. The gray part is graphite, which wasn’t compressed as much as the diamond.

sulfur and iron pockets inside a diamond found in a meteorite

The meteorites belong to a class called ureilites, which have compositions different from those in any of the known stony planets in the solar system. These ureilites contain 100-micrometer diamonds — too large to have been formed in the shock of two asteroids colliding. Such diamonds could form, however, inside asteroids that are at least 1,000 kilometers in diameter, where pressures would be high enough to compress carbon. 

But the researchers discovered an oddity that made them question whether the gems came from an asteroid at all: The diamonds had grown around even smaller crystals of iron and sulfur, which normally would repel each other like oil and water, says EPFL physicist Cécile Hébert.

Those crystals would be stable only at pressures above 20 gigapascals, almost 200,000 times atmospheric pressure at sea level on Earth. “That can only be at the center of a very large planet” the size of Mercury, about 4,900 kilometers wide, or in the core-mantle boundary of a planet as large as Mars, about 6,800 kilometers wide, Hébert says.

Such planets probably roamed the early solar system some 4 billion years ago. But only a few survived to become the four rocky planets that exist today. Simulations of the early solar system suggest most of these early planets crashed into each other and broke apart in the first 100 million years.

“We are confirming the existence of such former planets,” Gillet says.

Those planets’ existence alone isn’t surprising, says cosmochemist Meenakshi Wadhwa of Arizona State University in Tempe. “This is the first time, though, that there is direct meteoritic evidence for the existence of a large protoplanetary body in the early solar system that is no longer in existence,” she says.

transmission electron micrscopy image of diamond crystal

Not so fast, says cosmochemist Martin Bizzarro of the Natural History Museum of Denmark in Copenhagen. The protoplanet explanation isn’t the only one possible.

"They've done very careful work," he says, but more needs to be done. Testing for remnant magnetic fields could reveal if the meteorites were once within a large planet's molten core, for instance. Whether the meteorites came from a protoplanet is “still an open question.”


Editor's note: This story was updated April 25, 2018, to correct the lower limit of the pressures at which the iron and sulfur crystals are stable.

Citations

F. Nabiei et al. A large planetary body inferred from diamond inclusions in a ureilite meteorite. Nature Communications. Published online April 17, 2018. doi: 10.1038/s41467-018-03808-6.

Further Reading

T. Sumner. Competing ideas abound for how Earth got its moon. Science News. Vol. 191, April 15, 2017, p. 18.

S. Barazesh. Asteroid tracked from space to Earth. Science News. Vol. 175, April 25, 2009, p. 13.

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