On October 7, 2008, a car-sized asteroid entered Earth’s atmosphere and exploded into hundreds of pieces over the Nubian Desert in Sudan, which is in the eastern reaches of the Sahara. The recovered chunks, known collectively as the Almahata Sitta meteorite, have been identified as “ureilites,” a rare category of diamond-rich extraterrestrial rock that scientists think may be the last corporeal remains of a world called the Ureilite Parent Body (UPB).
This ill-fated planet, so the theory goes, was destroyed billions of years ago by the tumultuous collisions that defined the solar system’s early history. Its fractured vestiges now haunt our stellar neighborhood and occasionally end up falling to Earth.
As if that premise isn’t mind-boggling enough, new research published Tuesday in Nature Communications has found that this lost world was no small shakes protoplanet—it was somewhere between Mercury and Mars in size.
“Until now, people had many suggestions ranging from 200 kilometers up to 1,000 kilometers in size for the Ureilite Parent Body,” lead author Farhang Nabiei, a materials scientist at the École Polytechnique Fédérale de Lausanne in Switzerland, told me over the phone. “Now we are claiming that no, it was much much bigger than that.”
Nabiei and his co-authors estimate that the meteorite hails from a bygone planet some 6,000 kilometers in diameter, which is roughly half the size of Earth. The team was able to reconstruct this scale by meticulously studying a meteorite sample, called Almahata Sitta MS-170, using transmission electron microscopy. This technique allowed researchers to discern the chemical composition and structures within “inclusions,” or pockets of minerals trapped inside the extraterrestrial diamonds.
“Normally when we [study] planets, you expect people to talk about telescopes, not microscopes,” Nabiei commented. But because these tiny inclusions measure around 50 nanometers—about 40,000 times shorter than the width of a human hair—it was necessary to use sophisticated microscopic imagery to analyze them.
Nabiei and his colleagues learned that the grain size of the nanodiamonds was too large to have been formed by a shock impact, which ruled out a major explanation for previous ureilite properties. The only way the observed nanodiamond features could have been formed was “in a large planetary body,” the team found, at pressures greater than 20 gigapascals.
In other words, the diamonds were forged either at the core-mantle boundary of a Mars-sized world, or at the center of a Mercury-sized planet. This body would have formed alongside the current planets within the first ten million years of the solar system, but in a twist of cosmic fate, it was wiped out by the intense bombardment of that era.
Though Earth persevered through this period, it was not unscathed—many scientists think the Moon was created after our planet collided with a Mars-sized body like the UPB.
For Nabiei, who hadn’t worked with ureilite meteorites before this study, the discovery was “really surprising and really cool.” He and his colleagues are eager to follow up by studying other ureilite fragments to corroborate the results.
“There are many theories about the minerals and composition and all the processes going on inside the UPB that are compatible with the [previously estimated] smaller size,” he told me. “To make our model stronger, we have to look at the meteorites to have a general overview that can explain both this large size, and those other properties of these materials. That’s what we are doing right now.”
In the meantime, take a moment to consider how wild it is that diamond-encrusted remnants of an ancient planet that died billions of years ago are floating around in space, and that the cosmic powers-that-be occasionally gift one of these gems to its habitable surviving sibling: Earth.
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