Tiny diamonds from a rare type of meteorite are evidence for a lost planet between the size of Mercury and Mars, according to new research (Nat. Commun. 2018, DOI: 10.1038/s41467-018-03808-6). The findings support a proposed picture of a crowded early solar system where collisions between planetlike bodies were common.
In 2008, a car-sized asteroid exploded over Sudan, scattering meteorites known as Almahata Sitta across the Nubian desert. Many of the meteorites belong to an unusual class called ureilites thought to come from a single, long-since-destroyed body in our solar system referred to as the ureilite planetary body (UPB). Ureilites contain a mix of iron and magnesium silicate minerals along with graphite and small diamonds. Scientists previously hypothesized three ways diamonds in ureilites could form: The shock that demolished the UPB could have transformed graphite into diamond; vapor deposition of hydrocarbon gases could have produced the minerals; or they could have formed as they do in Earth’s interior through a high-temperature, high-pressure growth process.
Farhang Nabiei, a post doc at the Swiss Federal Institute of Technology, Lausanne (EPFL), and colleagues argue that the size of the diamonds found in the Nubian meteorites—as much as 100 µm across—rules out the first two mechanisms, which would have produced diamonds just nanometers wide. With transmission electron microscopy, the researchers observed lines of minerals called intrusions within the diamond fragments that seem to be interrupted by seams of graphite. Nabiei says that observation indicates the graphite formed from the diamond during the UPB’s destruction—not the other way around—and split larger diamond crystals apart.
To narrow down the conditions under which these diamonds formed, the group turned to energy dispersive X-ray spectroscopy and electron crystallography to characterize the minerals in those intrusions. Nabiei says diamonds formed inside planetary bodies incorporate other minerals that hint at their origin. Most of the inclusions are iron-rich sulfide minerals with some phosphorus and nickel, which could only form at pressures above 20 gigapascals, Nabiei says.
Those data suggest the UPB was fairly large. If the diamonds formed in its center, the body was probably about the size of Mercury. If they formed at the core-mantle boundary, the UPB would have been about the size of Mars.
The researchers’ conclusions add to a growing consensus in planetary science that the early solar system had many planetary bodies, most of which were either destroyed or subsumed by the eight that remain. For example, the Moon is thought to have formed as a result of a collision between the early Earth and one of these planetary embryos. “This [study] represents the first real physical evidence of a large body” from the early solar system, says Thomas Sharp, a geologist at Arizona State University who studies mineral transformations during meteorite impacts.
Hundreds of other ureilites exist on Earth, and Nabiei and his colleagues are already starting to study those to try to learn more about the UPB. He says it’s exciting to think he can make discoveries about the solar system without a telescope. “Here we’re talking about a planet maybe 6,000 km in diameter and we’re talking about an electron microscope,” he says.