Meteorite which hit Earth ‘came from unknown object up to 1,100 miles wide’

Rob Waugh
·2-min read
The meteorite came from a large object, and formed in the presence of water (SWRI)
The meteorite came from a large object, and formed in the presence of water. (SWRI)

A meteorite which hit Earth in 2008 came from a much larger, unknown object roughly the size of Ceres, a 500-mile-wide object in the main asteroid belt.

The Almahata Sitta (AhS) meteorite was formed in the presence of water, researchers from the Southwest Research Institute (SwRI) now believe.

In 2008, an eight-tonne, 13-foot diameter asteroid entered Earth’s atmosphere, exploding into some 600 meteorites over Sudan.

It was the first time scientists had predicted an asteroid impact prior to its arrival, and allowed the recovery of 23 pounds of samples.

The research was published in Nature Astronomy.

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SwRI Staff scientist Dr Vicky Hamilton said: “We were allocated a 50-milligram sample of AhS to study.

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

“Spectral analysis 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 at least 400, and up to 1,100, miles in diameter.”

Asteroids – and the meteors and meteorites that sometimes come from them – are leftovers from the formation of our solar system 4.6 billion years ago.

Ceres, illustration. Ceres is a dwarf planet, the only one that orbits in the asteroid belt. It is also an asteroid, the largest one known, and the first to be discovered, in 1801. The surface of Ceres is heavily cratered due to impacts from meteorites. It orbits the Sun at an average distance of 2.76 astronomical units (AU), and has a mean radius of 473 km - large enough for it to be spherical. It has been studied by the spacecraft Dawn, which entered into orbit around the mini-world in March 2015.
Ceres is a dwarf planet, the only one that orbits in the asteroid belt, and was studied by the Dawn spacecraft. (NASA)

Most reside in the main asteroid belt between the orbits of Mars and Jupiter.

Collisions and other events have broken them up and ejected remnants into the inner solar system.

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The fragments can offer insights into the earliest days of the solar system, according to researchers.

Hamilton said: “Carbonaceous chondrite (CC) meteorites record the geological activity during the earliest stages of the solar system and provide insight into their parent bodies’ histories.”

“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.”

The researchers say that analysis of samples from asteroids Ryugu and Bennu visited by Japan’s Hayabusa2 and NASA’s OSIRIS-REx spacecraft this year will offer further insight.

Hamilton said: “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.

“However, we think that there are more carbonaceous chondrite materials in the solar system than are represented by our collections of meteorites.”

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