Scientists have released an analysis of a meteorite fragment gathered after a 2008 asteroid near-collision with Earth. They show the parent asteroid was huge, and the results suggest that special, water-holding type of asteroids can be larger and have different mineral compositions than previously thought.
The study’s findings were published this week in the journal Nature Astronomy and look at the chemical composition of a sliver of those meteorite fragments.
The story of the fragments begins in October 2008, when scientists became aware of an asteroid on a collision course with Earth. They knew most of the rock would burn up on entry into the Earth’s atmosphere, and that the remnants, if any, would fall in the windswept sands of the Nubian Desert. It provided a unique opportunity for an international team of researchers, NASA scientists among them, to anticipate the rocks’ arrival and then comb the sands for any surviving fragments.
Though the asteroid was relatively small — only about nine tons — its detritus was minuscule; less than 8.8 pounds (4 kilograms) of meteorite were collected from the desert. They collectively were dubbed Almahata Sitta, after a nearby train station. It was the first time an asteroid was spotted and then its meteorite remnants were collected.
Since its recovery, different pieces from Almahata Sitta have been analysed, revealing information about the origins and chemical compositions of different parts of the asteroid. The meteorite sample the team studied — dubbed AhS 202 — was so small you could fit 10 copies of it on a nailhead, but it came from a gargantuan space rock, an origin point that precedes the fragment’s joining with the rocky mass of Almahata Sitta. The team used infrared and x-ray light to study the sample. They found the fragment was a carbonaceous chondrite, a type of meteorite that formed during the early days of the solar system, and which may have brought water to Earth, giving rise to…all of this. Carbonaceous chondrites generally weren’t previously thought to be able to come from parent bodies (origin asteroids) larger than about 62 miles (100 kilometres) in diameter.
But the researchers found tremolite in their itsy-bitsy fragment, a mineral that requires an immense amount of pressure to form. Tremolite’s existence in the sample suggests the origin asteroid’s diameter is in the range of 398 to more than 1,119 miles (640 to more than 1,800 kilometres), putting it in the wheelhouse of Ceres, the largest object — a dwarf planet, in fact — in the asteroid belt.
“This is a piece of evidence for a very large parent body that we didn’t know existed previously,” Vicky Hamilton, a staff scientist at the Southwest Research Institute and lead author of the recent paper, said while noting this the first known presence of tremolite in a carbonaceous chondrite. “The fact that we don’t have other evidence of it in our meteorite collections helps confirm what we already suspected, which is that the meteorites we manage to find on Earth are a biased sample.”
As asteroids hurtle through space, they’re bound to make contact with other bodies. These conglomerations of metals and minerals come together and break apart as their trajectory continues. When a meteorite is actually found on Earth, it’s a globbed-together compendium of stories from space, and the only way to read it is to do a whole bunch of analyses.
“You can have one group of scientists looking at one piece of meteorite and another group looking at another piece of that same meteorite, and you’ll see two different parts of the history of the solar system,” Hamilton said.
That’s how Hamilton’s sliver could speak to some origins in a massive asteroid, while another piece of Almahata Sitta could hint at the onetime-existence of a proto-planet. The electroscopy work the team recently did is a sort of reverse engineering, to go from what looks like a typical space rock to its specific story, in this case being its reference to a massive parent asteroid. It’s like finding a crumb on your kitchen counter — it could be from anywhere — but looking at it chemically may tell you the temperature and pressure conditions that gave rise to it, and whether that crumb really came from this morning’s toast or last week’s birthday cake.
Though much rarer than other types of asteroids, new information about carbonaceous chondrites could fall from the sky at any time. It’s just a matter of whether meteoriticists are vigilant — or lucky — enough to spot them.