Diamonds from the heart of a lost planet

Diamonds in a meteorite recovered from Sudan’s Nubian Desert have revealed traces of a lost planet, possibly as large as Mars, smashed to rubble early in the solar system’s history.

Planetary scientists have long believed the solar system once teemed with such bodies, which, during its chaotic infancy, either merged into larger planets, fell into the sun, were flung into interstellar space, or were dashed to bits by catastrophic collisions. But this is the first direct evidence that any such lost world truly existed.

The diamonds come from a meteorite called Almahata Sitta, which made headlines in 2008 when astronomers tracked a 4.1-metre asteroid into the Earth’s atmosphere and watched it explode in the skies above Sudan. Fragments collectively weighing about 10.5 kilograms were subsequently recovered and named for a railroad station between Khartoum and Wadi Halfa.

Almahata Sitta proved to be part of a family of meteorites called ureilites, of which several hundred are known.

“They are interesting meteorites, with strange properties,” says Farhang Nabiei, a materials scientist at École Polytechnique Fédérale de Lausanne, Switzerland.

Among other things, they include diamonds — enough to pose challenges to researchers.

“Ureilites are hard to cut and grind for thin sections because of the diamonds,” says Melinda Hutson, curator of the Cascadia Meteorite Laboratory at Portland State University in the US.

Nabiei and his team are the first to study these diamonds in detail.

Diamonds can be produced in space by a number of processes, but the ones in Almahata Sitta are too large to have been formed by most of them, Nabiei says.

They are not so large that thieves will be plundering them for gemstones. They are only 100 microns (0.1 millimetre) in size — barely large enough to be seen without a magnifying glass — and are badly cracked by subsequent events, such as impact shocks.

But they are large enough, Nabiei says, that they must originally have formed deep inside a protoplanet, just as Earth’s diamonds formed far below its the surface.

How deep can be determined by studying materials trapped within them.

To jewellers, such materials, dubbed “inclusions”, would be considered defects, but to planetary scientists they are the true gems. Nabiei calls them “direct samples” of the places where the diamonds formed.

Based on measurements of about 30 inclusions, he says, it appears that they could only have formed at pressures above 20 gigapascals (roughly 200,000 atmospheres).

“So the body should have been large enough to have had 20 gigapascals pressure inside its mantle,” Nabiei explains.

That means it must have been at least as large as Mercury, he says, which has a diameter of 4,900-kilometres, and possibly as big as Mars, with a 6,800 kilometre diameter. The difference depends on whether the Almahata Sitta diamonds formed all the way at the planet’s centre, or not quite as far down.

Hutson, who was not part of the study team, notes that Nabiei’s lost planet isn’t the only protoplanet that may have been destroyed in collisions.

Studies of a different class of meteorites, known as iron meteorites, she says, indicate that they may have been formed in protoplanets hundreds to thousands of kilometres across, “implying [other] large objects that have broken apart”. These, however, would not have been as large as the source of Nabiei’s diamonds.

Fragments of such bodies, she says, could have helped produce the large impact basins we see today on the moon, Mercury, Mars, and Jupiter’s moon Callisto. They may also have crashed into the Earth or Venus, where we can no longer see their imprint. And, she adds, “A lot of material could have been ground down to small pieces in [subsequent] collisions [and] ejected from the solar system by a number of processes that remove sand and dust-sized particles.”

The next step for Nabiei, whose work is published in the journal Nature Communications, is to look at more ureilites, seeking additional information about how their minerals formed — and from that, additional information about the lost world from which they originated.

For example, he says, these meteorites have some characteristics of materials that formed in the inner solar system, but they also contain large amounts of carbon, similar to things that formed in its outer regions.

“There are things we don’t understand,” he says, “and that’s where we learn.”