The Solar System Used To Have Another Massive Asteroid Like Ceres

False-color image of a part of the 50-milligram sample of the Almahata Sitta meteorite. The amphibole crystals can be seen in orange. NASA/USRA/Lunar and Planetary Institute

A serendipitous discovery in a recently found meteorite has provided astronomers with a unique window into the very distant past of the solar system and how different it was back then. Today, Ceres is the largest body in the asteroid belt, and it is classified as a dwarf planet. But new research suggests that there was another equally large asteroid in the past.

As reported in Nature Astronomy, the findings come from the unusual composition of a piece of the Almahata Sitta meteorite, where the team found a mineral known as amphibole. This can only be created at a certain range of temperatures and pressures, and with a prolonged and significant presence of water. This is the first time that such a mineral has been identified in a meteorite.

These conditions cannot be found on small solar system bodies. The presence of the mineral suggests that it must have formed on a very large body that has since been destroyed. They estimated it had a diameter between 640–1,800 kilometers (400 to 1,100 miles), comparable to the 950-kilometer (580-mile) Ceres.

"Carbonaceous chondrite (CC) meteorites record the geological activity during the earliest stages of the Solar System and provide insight into their parent bodies' histories," lead author Dr Vicky Hamilton, from the Southwest Research Institute, said in a statement. "Some of these meteorites are dominated by minerals providing evidence for exposure to water at low temperatures and pressures. The composition of other meteorites points to heating in the absence of water. Evidence for metamorphism in the presence of water at intermediate conditions has been virtually absent, until now."

Almahata Sitta's composition is certainly unique, but it might not be truly an exception. Asteroid Ryugu visited by the Japanese probe Hayabusa2, and Asteroid Bennu visited by NASA’s OSIRIS-Rex both have features suggesting a prolonged exposure to water. Both spacecraft have collected samples (Hayabusa2’s one landed early this month) and researchers are excited to see if what has been seen from orbit is confirmed in the samples.

"If the compositions of the Hayabusa2 and OSIRIS-REx samples differ from what we have in our collections of meteorites, it could mean that their physical properties cause them to fail to survive the processes of ejection, transit and entry through Earth's atmosphere, at least in their original geologic context," explained Hamilton, who also serves on the OSIRIS-REx science team. "However, we think that there are more carbonaceous chondrite materials in the Solar System than are represented by our collections of meteorites."

The finding of this meteorite was a truly special discovery. Almahata Sitta is one of 600 fragments from a 9-ton 4-meter (13-foot) asteroid that entered our atmosphere in 2008. About 10 kilograms (23 pounds) of material were recovered in Sudan. This event marked the first time scientists were able to predict an asteroid impact before entry.

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