Dr. F. Nabiei / Dr. E. Oveisi / Prof. C. Hébert, EPFL, Switzerland
(Photo: Colored STEM image of the meteorite, which probably originates from a destroyed protoplanet and shows the diamond phase (blue), inclusions (yellow) and the graphite region.)
Something fell in 2008 from the sky in the Nubian Desert of Sudan and could be a key to understanding lost planets from the early formation of our solar system.
Research Now Published in Nature Communications A group of planetary researchers provides evidence that a fragment of a meteorite could be a piece of a protoplanet destroyed when the solar system formed. This early formation period was a chaotic and violent time, and the four planets of the inner solar system are the only survivors of a brutal series of collisions. Several protoplanets piled up, met, fragmented, joined, and repeated the process to eventually form the rocky planets, moons, and asteroids we see today.
It is believed that the meteorite is a fragment of a large protoplanet between the sizes of Mercury and Mars. Previously, the scientists thought that the protoplanets were not much larger than the dwarf planet Ceres or about one-sixth of the diameter of Mercury. The space rock in question is a unique type of stone meteorite called ureilite, due to the area of Russia where it was first found, Urey. Ureilites have already been discovered, but the reported rocks are the first evidence that they could have originated in large protoplanets.
"Either we [other meteorites] do not come from a big planet, or we do not know exactly what it was. It was a kind of planet," said Farhang Nabiei of the Ecole Polytechnique Fédérale de Lausanne, who co-authored the study. "This is the first time we know that a sample comes from such a large body."
The meteorite probably formed at pressures of around 20 gigapascals, which corresponds to the core-mantle boundary on Mars. This indicates that the rock is located deep within an object the size of a current planet of the inner solar system. The evidence is in the diamonds that were formed in the rock that have been contaminated with elements and joints that generally form under these strong pressures. Graphite samples from the rock also show that they were probably once diamonds that melted and solidified on impact with graphite.
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The meteorite also contains iron-sulfur compounds, but no silicates, which are typically found in higher planets. Some of these iron-sulfur compounds are leached into the nanodiamonds. The diamonds themselves may have been larger until the protoplanet was destroyed.
Ureilites probably all came from this womb, though at different depths. "There is much evidence that they are related," says Nabiei. But there is no evidence that it came from the giant protoplanet that collided with Earth to form the Moon.
When the meteorite formed, this confirms that some early protoplanets of the solar system were quite large. The protoplanet in question may have come from the outer solar system and has migrated inward into the inner solar system. The study also shows that the protoplanet, where it once was, has long since been destroyed, leaving behind only a number of ueilites, some of which crash for scientists on Earth.
The stone is a fossil from a lost world – and it could be a clue to decipher the secrets of the turbulent early solar system.
* This article has been updated to include a commentary by Farhang Nabiei of the École Polytechnique Fédérale de Lausanne.