The parent body of the Almahata Sitta meteorites — space rocks that rained down on the Nubian Desert in Sudan in 2008 — is a 640 to 1,800 km-wide water-rich asteroid that is as yet unknown, according to new research.

Hamilton et al. studied the composition of Almahata Sitta 202 to determine that it likely originated from a previously unknown parent asteroid. Image credit: Hamilton et al., doi: 10.1038/s41550-020-01274-z.

Hamilton et al. studied the composition of Almahata Sitta 202 to determine that it likely originated from a previously unknown parent asteroid. Image credit: Hamilton et al., doi: 10.1038/s41550-020-01274-z.

In October 2008, a 4.1-m-diameter fragment of a carbonaceous chondrite asteroid entered Earth’s atmosphere and exploded 37 km above the Nubian Desert.

About 600 fragments of the object, ranging in size from 1 to 10 cm, were eventually gathered.

“Carbonaceous chondrite meteorites record the geological activity during the earliest stages of the Solar System and provide insight into their parent bodies’ histories,” said Dr. Vicky Hamilton, a staff scientist in the Department of Space Studies at the Southwest Research Institute.

“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.”

Using infrared microspectroscopy, electron microprobe analysis and transmission electron microscopy, the researchers characterized the mineralogy of Almahata Sitta 202 (AhS 202), one of the Almahata Sitta fragments.

“We were allocated a 50-mg sample of Almahata Sitta to study,” Dr. Hamilton said.

“We mounted and polished the tiny shard and used an infrared microscope to examine its composition.”

The scientists identified a range of hydrated minerals, in particular amphibole, which points to intermediate temperatures and pressures and a prolonged period of aqueous alteration on a parent asteroid between 640 and 1,800 km in diameter.

“Amphiboles are rare in carbonaceous chondrite meteorites, having only been identified previously as a trace component in the Allende meteorite,” Dr. Hamilton said.

“Almahata Sitta meteorites are a serendipitous source of information about early Solar System materials that are not represented by carbonaceous chondrite meteorites in our collections.”

The study was published in the journal Nature Astronomy.

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V.E. Hamilton et al. Meteoritic evidence for a Ceres-sized water-rich carbonaceous chondrite parent asteroid. Nat Astron, published online December 21, 2020; doi: 10.1038/s41550-020-01274-z