"Early inspections of the meteorite revealed it to be a ureilite, an unusual composition that does not match other space rocks known to have come from the moon or Mars. The finding led some scientists to speculate that it may have had a more exotic origin.
That suspicion grew when researchers noticed little diamonds in the meteorite material. While other meteorites are known to contain diamond crystals, they are generally far smaller. Typical meteorite diamonds are only a few millionths of a millimetre across and are thought to form in collisions with other space rocks that send brief but intense shockwaves through the carbon-rich asteroids.
In 2015, researchers at the Swiss lab teamed up with Japanese scientists to argue that the diamonds in the Almahata Sitta meteorite, which are up to 100 micrometres long, were much larger than could be formed in collisions with other asteroids. They speculated at the time that the meteorite might have come from a lost planet, but sought more evidence to bolster their theory.
Writing in the journal Nature Communications, the Swiss team now describe fresh analyses that show that diamonds in the meteorite contain specks of an iron-sulphur compound that is thought only to form at pressures greater than 20 gigapascals. They conclude that the diamonds formed with the specks inside them, deep beneath the surface of an unknown world."