These diamonds are tiny, flawed, and may come from a long-lost planet

In 2008, a rock laced with tiny diamonds hurtled through miles of thickening nitrogen, oxygen, and carbon dioxide, its exterior heating up as it raced through the thick air. A telescope tracked its progress, watching as the asteroid-turned-meteor exploded. The violent burst 23 miles above the ground sent fragments speeding toward their resting place, dark against the sands of the Nubian desert in Sudan.

The explosion and crash were just the latest of eons of indignities, from a high pressure beginning in a promising planetary start up, to a cataclysmic failure, to billions of years of aimless wandering around the solar system.

A new study published in Nature Communications today offers a dramatic origin story for the meteorite. Based on materials found inside the diamonds nestled within, researchers think this may be the remnant of a long-lost planet or planetary embryo; one that was still in its infancy when the chaos of the early solar system obliterated it.

Diamonds are a geologist’s best friend
In this case, the diamonds aren't the most important part of this story. They’re just the heavy-duty packaging for much more precious cargo held inside. While a jeweler might see a bit of rock trapped inside a diamond as a flaw, to a geologist it is precious. Because of their strong crystal structure, diamonds can preserve minuscule bits of material that would otherwise disappear under the relentless changeability of the universe over time.

Researcher Farhang Nabiei—of the École Polytechnique Fédérale de Lausanne in Switzerland—was looking at the relationship between the diamonds and the layers of graphite surrounding them when he started wondering about the small pockets of substances trapped within.

Upon closer inspection, he found that the material inside the diamonds could only have been formed at incredibly high pressures—much higher than anything the meteorite would have been subject to as it crashed toward Earth. These diamonds must have held the weight of an entire world—literally. At 20 gigapascals, the pressure necessary to form these substances is likely to occur deep within a planet—one between the size of Mercury and Mars.

This visitor was not from Mercury or Mars. The meteorite was classed as a ureilite, a group of meteorites with a mysterious origin, pieces of some planetary body or asteroid that don’t quite match any of the rocky bodies that humans have recorded today. Researchers already knew that whatever it was, it had probably met its end in the demolition derby of the early solar system, but the scale of the object (or objects) was still unknown until the inclusions were described. The size of the diamonds is another clue to their deep origins.

“100 microns doesn’t seem very large—it’s the size of a human hair—but these are much larger diamonds than you'd get during the transformation of graphite to diamond in a shock,” says Thomas Sharp, a geologist at Arizona State University who wasn't involved in the new research, but studies meteorites using similar electron microscopy tools.

"An important piece of the puzzle is that the diamonds are large and zoned, which strongly supports the idea that they formed deep in a body's interior (and not in an impact, for example)," Rebecca Fischer, a planetary scientist at Harvard University who was not involved in the study, said in an email. Fischer points out that the bulk composition of the material (an iron-sulfur compound with nickel (Ni) and phosphorous (P)) inside the diamond is likely to only form at high pressures. "Fe3S is a well studied phase only stable above 21 GPa. Adding in Ni and P can change the pressure at which it is stable, but the authors argue that this wouldn't be a major effect given the bulk composition that they see. This could be verified in future experimental studies. It will also be interesting moving forward to keep looking at diamonds from ureilites to see if they have captured any other high pressure phases in inclusions, which would offer strong support for the authors' interpretation." Fischer says.
https://www.popsci.com/diamonds-meteorites-long-lost-planet