Gemstones of a special kind: Researchers have discovered the largest known extraterrestrial diamonds in ureilite meteorites and, above all, clarified how they could have been formed. Their results contradict the previous assumption that the carbon crystals slowly formed inside a lost, at least Mercury-sized protoplanet in our solar system. Instead, the scientists found evidence that the exotic diamonds were created in the early days of our solar system as a result of the pressure when minor planets collided with each other or with large asteroids.
Millions of lumps of different sizes float through our solar system. These asteroids are leftovers from the youth of our cosmic homeland, when the planets formed from a gigantic cloud of gas and dust that surrounded our sun. If the chunks reach the earth’s surface without burning up, they can therefore form interesting sources of information: Investigations of the material allow conclusions to be drawn about processes in the mysterious early phase of the solar system. A special and rare category of meteorites are the so-called Ureilites. It is assumed that these are fragments of a larger celestial body, which was completely shattered in the context of the cosmic game of billiards by violent collisions with other minor planets or large asteroids.
How did the “noble” inclusions come about?
It is already known that ureilites often contain large amounts of carbon, including in the form of graphite or tiny diamonds. Until now it has been assumed that these special stones were created similarly to their earthly counterparts. On our planet, they were formed in the depths from carbon through long-lasting pressure from the rock above and later reached surface areas. A corresponding formation of the diamonds from the Ureilites would mean that they have also formed inside a comparatively massive celestial body. In order to convey the necessary pressure, it would have to have been about the size of Mars or Mercury. Thus, this educational scenario would be an interesting indication of the existence of such protoplanets in the early solar system, which disappeared again in the course of further development.
An international team of researchers has now devoted a study to the Ureilites and their “precious” inclusions. To do this, they took samples from three ureilite meteorites and examined them using electron microscopy, spectroscopy and other analytical methods. In doing so, they came across the extraterrestrial gemstones they were looking for, the largest known to date: they could hardly adorn a ring, but the record holders are over 0.1 millimeters in size. In addition to these specimens, the scientists also found numerous nests of nanometer-sized diamonds and nano-graphite in the material.
As they report, closer analysis of the material did not reveal any features that would require formation under the high static pressures or the long growing times inside a planet. Instead, the researchers came across properties that indicate the formation of diamonds as a result of the rapid shock transformation of graphite. Ultimately, the diamonds were apparently created in a process similar to that used in the industrial production of synthetic diamonds.
Sudden emergence
Specifically, the researchers found so-called Lonsdaleit layers in the nano-diamonds, a modification only created by sudden, very strong pressure. In addition, the silicate minerals of the ureilite rocks examined also showed typical signs of such a shock, the scientists report. “Our extensive new investigations show that these unusual extraterrestrial diamonds formed on the surface of the ureilite mother body as a result of the immense shock pressure when a large asteroid or even a minor planet hit. It is quite possible that it was precisely this gigantic impact that ultimately led to the complete destruction of the minor planet, ”says co-author Frank Brenker from the Goethe University in Frankfurt.
The study also clearly shows that the formation of the diamonds in the Ureilites did not require a Mars-sized mother body – a finding that is of great importance for models of the development of our solar system. “Even the larger ureilite diamonds are not an indication of the existence of Mars or Mercury-sized protoplanets in the early phase of our solar system, but nevertheless for the immense, destructive forces that prevailed at that time,” says Brenker in conclusion.
Source: Goethe University Frankfurt, specialist article: PNAS, doi: 10.1073 / pnas.1919067117