Diamonds fell from lost planet, researchers say
Believed formed within body now disappeared
Specks of diamond inside a meteor that fell from space into the Nubian Desert of northern Sudan were formed inside a long-lost planet in the early years of the solar system, according to a new scientific report.
The surprisingly large size of these extraterrestrial gemstones is the first compelling and direct evidence for the mysterious planet that has since disappeared from the solar system, either by colliding and being absorbed into another planet, or being incinerated in the sun, or cast off into outer space.
At least some of it stuck around, however, in the form of a small asteroid, perhaps four metres across, that spent millions of years in some kind of local orbit until one special night in 2008, when astronomer Richard Kowalski noticed a blip on a screen at the Mount Lemmon telescope in Arizona.
The Almahata Sitta asteroid, as it has come to be known, is special for more than just its diamonds. It was the first such object ever to be tracked from space all the way to impact on Earth.
A few hours after Kowalski spotted it, the meteor entered the Earth’s atmosphere over the Nubian desert in northern Sudan, exploding in a massive fireball that was visible to a passing passenger jet whose pilots had been warned to watch for it. Then the search began, helpfully guided by NASA’s tracking expertise. Because it landed in a desert, this was comparatively easy, and about 10 kg of it has been recovered, in hundreds of pieces.
Ever since, this meteorite has been a metaphorical gold mine for scientists, and a literal treasure trove of tiny diamonds.
The latest report, published Tuesday in Nature Communications, describes the results of experiments — with an X-ray spectroscope and electron microscopes — to investigate the relationship between the diamonds and surrounding graphite.
One key conclusion is that the diamonds must have been formed by pressures greater than 20 Gigapascals, which is the minimum pressure for creating diamonds in the traditional way — underground, as carbon is compressed under the weight of a planet’s own gravity.
Although the diamonds in the Almahata Sitta remnants are small compared to the diamonds in jewelry, at only a fraction of a millimetre across, they are so large that they could only have been created by “static high-pressure growth,” the same process that creates diamonds on Earth, according to the study by European researchers, led by Farhang Nabiei of École Polytechnique Fédérale de Lausanne.
There are other natural ways to create diamonds, which the scientists claim to have eliminated as possibilities. One is in the intense but fleeting pressures of impact, but this method cannot make diamonds as large as those found in the Almahata Sitta fragments, the researchers conclude.
The other is by what is known as “chemical vapour deposition,” in which diamonds could have been created as the solar system first formed from a swirling cloud of hot, carbon-rich gas. But this would create perfectly pure diamonds, and the Almahata Sitta diamonds have internal impurities, which are more likely to originate inside a planet.
By process of elimination, the researchers conclude that the diamonds were first formed inside a large object, a proto-planet, that was roughly between Mercury and Mars in size, or a little less than half the size of Earth.
“Although this is the first compelling evidence for such a large body that has since disappeared, their existence in the early solar system has been predicted by planetary formation models,” they write.
This conclusion fits with the standard understanding of how the solar system developed, first as a swirling cloud of gas around the sun, with bits that eventually globbed on to each other to form larger objects, which smashed into each other over and over again before settling into the roughly stable orbits we see today.
Earth’s moon, for example, was created by the cataclysmic meeting of the protoEarth and a smaller, Marssized planet called Theia. Known as the Big Splash, this collision set the Earth spinning faster than ever, tilted its axis by 23 degrees, and created the moon from the debris, mostly from Theia, that was blasted into orbit.
THEIR EXISTENCE IN THE EARLY SOLAR SYSTEM HAS BEEN PREDICTED BY PLANETARY FORMATION MODELS.