A team of scientists from the Federal Polytechnic School of Lausanne (Switzerland) has discovered that the small diamonds found inside a meteorite that fell to Earth in 2008 show signs of origin on a lost planet, similar in size to Mercury or Mars; a planet dating back to the early days of our solar system. This surprising finding could help us answer one of the most long-standing questions in astronomy, that of planet formation.
The meteorite had a diameter of four meters and after its explosion in the atmosphere, it projected many fragments on the surface of the desert, finally recovering about 50 of a size between 1 centimeter and 10 centimeters. According to the researchers, nanodiamonds from the asteroid Almahata Sitta, which fell in the Nubian desert in October 2008, formed at high pressures consistent with the first protoplanets in the solar system.
The beggining
It all starts with grains of dust in the protoplanetary disk around a young star. Thanks to electrostatic forces, these grains begin to stick to each other. If enough dust particles gradually accumulate to form larger clumps, and enough of those clusters collide and merge, the entire body will gravitationally collapse into a more solid body between 1 and 10 kilometers in size, a planetesimal.
Most planetesimals maintain that size. But some, with the help of their magnetic field and eccentric orbits, collide and coalesce, often at high speed, forming larger objects between the size of the Moon and the size of Mars, with a molten core.
They are what we call protoplanets, the “embryos” of planets, and the early solar system had hundreds of them.
Over time, they repeatedly collided with each other, merging into larger and larger masses until they formed the planets we know today.
According to astronomers, the asteroids that still float around the solar system are the remains of those days , of the repeated collisions that threw material into space.
The case of Almahata Sitta is particularly curious: it was the first time that material had been recovered from an asteroid that had been tracked from space and during its collision with Earth. It also turned out to be a rare type of calcium-poor meteorite that contains clusters of nano-sized diamonds – ureilites.
Using a transmission electron microscope, which uses a beam of electrons to visualize a three-dimensional object, the experts examined the largest diamonds, with a diameter of 100 micrometers (which could not have formed in an impact event due to the brief duration of this event) and discovered that the asteroid from which they detached came from the embryo of a planet of a size situated between those of Mercury and Mars.
As diamonds form, they often trap the minerals present in their formation environment. With this same method they discovered that diamonds had chromite, phosphate and iron and nickel sulfide with compositions and morphologies that could only have occurred under a pressure greater than 20 gigapascals , almost 200,000 times more than atmospheric pressure at sea level (in the Earth diamonds form at 4.5 gigapascals). This is the first time they have been observed in an extraterrestrial body, the authors state in the journal Nature.
“Although this is the first convincing evidence for a body so large that it has since disappeared, its existence in the early solar system has been predicted by models of planetary formation. Mars-sized bodies were common, and they grew larger to form planets. larger, either collided with the Sun or were ejected from the solar system. This study provides compelling evidence that the parental body of ureilite was a large ‘lost’ planet before it was destroyed by collisions, “the scientists clarify.
Referencia: A large planetary body inferred from diamond inclusions in a ureilite meteorite. Farhang Nabiei et alia. Nature Communications volume 9, Article number: 1327 (2018). DOI:10.1038/s41467-018-03808-6
Image credit: NASA
