As leftover fragments from the formation of the solar system, meteorites can act as time capsules that have some strange stories to tell. Scientists at the EPFL have now cracked open one of these time capsules – a piece of an asteroid known as 2008 TC3 – and found diamonds that could only have come from a large, long-lost planet that roamed our neighborhood billions of years ago.

In October 2008, astronomers discovered a 4-m-wide (13-ft) asteroid on a collision course with Earth. Less than a day later, the space rock 2008 TC3 exploded over the Nubian Desert in Sudan at a height of 37 km (23 mi), showering the landscape in meteorites. About 50 of these fragments, measuring between 1 and 10 cm (0.4 and 4 in) wide, were gathered into a collection named Almahata Sitta, for scientific study.

Most of these meteorites are what are known as ureilites, meaning they have stony compositions and are often loaded with nanoscale diamonds. These tiny diamonds could form in a number of ways, and figuring out how can help unravel their history – and by extension, the history of the solar system.

One such origin story could be the pressure from the shockwaves as the meteor collides with the Earth. This has been put forward as the explanation for an ultra-hard form of diamond known as lonsdaleite, which has a hexagonal atomic structure that makes it 58 percent harder than regular Earthly diamond. Other possibilities include chemical vapor deposition, or the more familiar form of static pressure that gives birth to diamonds deep beneath a planet's surface.

A chemical map showing iron (yellow) and sulfur (red) impurities inside the diamonds(Credit: EPFL / Earth and Planetary Science Laboratory)

To find out how the Almahata Sitta diamonds formed, the EPFL team, along with colleagues in France and Germany, analyzed samples of the meteorites. The researchers used transmission electron microscopes to determine their composition and morphology, and found that the diamonds contained inclusions (impurities) made of chromite, phosphate and iron-nickel sulfides.

These inclusions are common in diamonds formed underground here on Earth, but this marks the first time they've been found in alien rocks. That's interesting enough on its own, but it has much bigger implications – the team calculated that these diamonds could only have formed under pressure of more than 20 gigapascals. That means they must have been born inside a planet at least as big as Mercury, and possibly up to the size of Mars.

But there's still more to the story. The fact these diamonds made it to Earth implies that their home planet, whatever it may have been, is no longer with us, since it would take quite a cataclysm to wrench them out of their birthplace deep underground and fling them into space. Instead, the team believes the diamonds came from a planetary embryo.

A colorized scanning transmission electron microscope image of the meteorite samples, showing the diamond (blue), inclusions (yellow) and the graphite region (grey)

In its early years, the solar system was a far more turbulent place than it is today. After the Sun formed, the huge cloud of dust and gas swirling around it slowly clumped together to form planetary embryos. These in turn smashed into each other fairly regularly, creating bigger and bigger planets until we were left with just the ones we see today. Moons formed in the same way, with particularly huge collisions likely creating our own satellite and the two orbiting Mars.

The researchers say the Almahata Sitta meteorite diamonds provide compelling evidence for the planetary embryo theory. Whatever lost planet these diamonds would have called home, it was most likely torn to shreds several billion years ago.

The research was published in the journal Nature Communications.

Source: EPFL

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