Scientists have found the first hard evidence of a large and ancient protoplanet embedded in extraterrestrial diamonds that fell to Earth about 10 years ago.
To be clear, the diamonds did not fall to Earth on their own. Instead, they were discovered inside a small asteroid that slammed into the atmosphere over the Nubian Desert in northeastern Sudan in October 2008.
And in case you are imagining large twinkling space gems, let's also be clear that these diamonds are extremely tiny — the biggest are about the width of a human hair.
And yet, in a paper published Tuesday in Nature Communications, researchers say that within these small diamonds are chemical clues that suggest they could only have formed deep within a Mercury- or Mars-sized almost-planet that formed in the chaotic early days of the solar system.
"What makes this study so exciting is that it is direct evidence from an actual rock that there was a large protoplanetary body that is no longer around," said Meenakshi Wadhwa, who studies meteorites at Arizona State University and who was not involved in the new work.
"That's pretty exciting," she added.
Their first line of evidence that the space rock was a chunk of what was originally a large planetesimal came from the size of the diamonds themselves.
Diamonds can form in a few ways, including in the high-temperature and high-pressure environments similar to those found in the interior of our planet. They also can be formed by a shock wave impact that creates a temporary high-pressure and high-temperature environment.
However, these processes result in different size diamonds.
"Shock waves produce very tiny diamonds," said Paul Asimow, a geochemist at Caltech who was not involved in the new work. "The diamonds they found are too big to have been produced that way."
That means that the diamonds had to be formed toward the center of an object large enough to produce the requisite amount of pressure and heat.
Next, the researchers turned their attention to what are known as chemical inclusions inside the diamonds. These are small bits of the chemical environment that are captured inside the diamond at the time of its formation.
"Diamonds make good containers for inclusions because they are so rigid and nonreactive," Asimow said. "For most diamonds on Earth, we can't tell where they came from based on the diamond itself, so instead we look at inclusions."
This holds true for diamonds formed in space as well. When the study authors pointed their microscopes on the inclusions in the diamonds, they discovered that each inclusion contained several different minerals, but always in the same relative proportion to those in the other inclusions.
The researchers took this as evidence that the diamonds actually captured these minerals when they were bound together in a higher-pressure phase, and they have subsequently decomposed into lower-pressure phases within the inclusions.
Furthermore, the authors explain that the parent material that eventually decomposed can form only under an extremely high-pressure environment that could be found only in a planetary body between the size of Mercury and Mars.
Asimow said this all seems plausible, based not only on the work outlined in the paper but also considering what scientists believe occurred at the dawn of the solar system.
"A commonplace theory for the origins of planets is that there were a lot of small objects that ran into each other to make larger objects," he said. "It is not surprising at all that there were Mars-sized objects that are now gone."
For example, many planetary and lunar scientists believe that it was a collision between Earth and a Mars-sized planet that led to the formation of our moon.
However, he added, this is the first time that a meteor has revealed any evidence that it formed inside one of these large objects.
And that, he said, makes this a very cool discovery.
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