Diamond-Bearing ‘Almahata Sitta’ Meteorites Came from Long-Lost Protoplanet

The Almahata Sitta meteorites — diamond-bearing space rock fragments that rained down on the Nubian Desert in Sudan in 2008 — are remnants of a Mercury- to Mars-sized protoplanet, a new study suggests.

The Almahata Sitta meteorite. Image credit: Peter Jenniskens.

On October 7, 2008, a 13-foot (4.1 m) diameter asteroid entered Earth’s atmosphere and exploded 23 miles (37 km) above Sudan’s Nubian Desert.

When the asteroid, officially known as 2008 TC3, exploded, it scattered multiple fragments across the desert. About 600 fragments, ranging in size from 0.4 to 4 inches (1-10 cm), were eventually gathered.

“The recovered meteorites, called Almahata Sitta, are mostly ureilites, a rare type of stony meteorite that often contains clusters of tiny diamonds (100-microns in diameter),” said study lead author Farhang Nabiei of the Ecole Polytechnique Fédérale de Lausanne in Switzerland and colleagues.

“These diamonds can form in three ways: (i) enormous pressure shockwaves from high-energy collisions between meteorite’s ‘parent body’ and other space objects, (ii) deposition by chemical vapor, or (iii) the ‘normal’ static pressure inside the parent body, like most diamonds on Earth.”

Nabiei and co-authors studied diamond inclusions in Almahata Sitta meteorites using a combination of advanced transmission electron microscopy techniques.

HAADF-STEM image of diamond inclusions in the Almahata Sitta ureilite. Image credit: Nabiei et al, doi: 10.1038/s41467-018-03808-6.

The analysis showed that the diamonds had chromite, phosphate, and iron-nickel sulfides embedded in them. These have been known for a long time to exist inside Earth’s diamonds, but are now described for the first time in an extraterrestrial body.

“The particular composition and morphology of these materials can only be explained if the pressure under which the diamonds were formed was higher than 20 GPa,” the researchers said.

“This level of internal pressure can only be explained if the planetary parent body was a Mercury- to Mars-sized planetary ‘embryo,’ depending on the layer in which the diamonds were formed.”

Many planetary formation models predict that these planetary embryos existed in the first million years of our Solar System, and this new study offers compelling evidence for their existence.

“Many planetary embryos were Mars-sized bodies, such as the one that collided with Earth to give rise to the Moon,” the scientists explained.

“Other of these went on to form larger planets, or collided with the Sun or were ejected from the Solar System altogether.”

“Our study provides convincing evidence that the ureilite parent body was one such large ‘lost’ planet before it was destroyed by collisions some 4.5 billion years ago.”

The study was published this week in the journal Nature Communications.

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Farhang Nabiei et al. 2018. A large planetary body inferred from diamond inclusions in a ureilite meteorite. Nature Communications 9, article number: 1327; doi: 10.1038/s41467-018-03808-6