In 2008, something unique fell from the sky over Sudan, exploding in fragments over the vast, arid expanse of the Nubian desert.
This rustling object from above became known as Almahata Sitta: a collection of about 600 meteorite fragments, carefully recovered by researchers and named – ‘Station Six’ – taken from a nearby train station.
The unique thing about Almahata Sitta is that it represented something unprecedented in astronomy: the first time an asteroid impact was successfully predicted by scientists in advance.
Since then, the splinters from that asteroid – dubbed 2008 TC3 – have been analyzed by researchers, looking for chemical clues to the origin of this mysterious, widespread visitor.
Now, a new study fleshes out that intriguing backstory.
The AhS 202 shard. (Muawia Shaddad)
By looking at the splinters, it can tell us about 2008 TC3, which in turn can tell us where 2008 TC3 itself came from – like an astronomical chain of nested Matryoshka dolls.
“Our surprising result suggests the existence of a large, water-rich mother body,” said lead author and planetary geologist Vicky Hamilton of the Southwest Research Institute in Boulder, Colorado.
In the new work, Hamilton and fellow researchers didn’t have much to work with, they just analyzed the tiniest bits of this remarkable space rock.
“We were assigned a 50 milligram sample of Almahata Sitta to study,” explains Hamilton. “We mounted and polished the small shard and used an infrared microscope to examine its composition.”
The spectral analysis revealed something the scientists didn’t expect to find. Inside the shard – a fragment called AhS 202 – an extremely rare form of hydrated crystals known as amphibole was found.
This type of mineral requires prolonged periods of extreme heat and pressure to form, of a kind not usually believed to be possible in carbonaceous chondrite (CC) meteorites.
This section shows amphibole crystals in orange. (NASA / USRA / Lunar and Planetary Institute)
The implications suggest that 2008 TC3 most likely once belonged to a much, much larger body – something so big, in fact, that it would pretty much fall in the same class as Ceres: the dwarf planet, which represents the largest known object in the solar system. main asteroid belt, between the orbits of Mars and Jupiter.
“Most CC parent bodies are believed to be less than 100 km in diameter, thus would not be large enough to produce the range of pressure and temperature conditions represented by the mineral assembly in AhS 202,” the authors explain in their paper.
“As such, it is our interpretation that the original parent body of AhS 202 was likely an unknown object, possibly the size of Ceres (~ 640 – 1,800 km in diameter under most likely circumstances).”
While this mysterious, gigantic asteroid is thought to no longer exist, the fact that it once inhabited our solar system suggests that more of its kind may have done the same, even though we have found no evidence of these large, water-rich bodies in recovered meteorite fragments before TC3 from 2008.
In the same way that asteroids Ryugu and Bennu reveal some surprises in composition that differ from most known meteorites, the multiple splinters of the 2008 TC3 prove that space rock is more than current hypotheses can fully explain.
“We are not arguing that AhS 202 is a spectral analog to Bennu or Ryugu; instead, AhS 202 is an accidental source of information about materials from the early Solar System that are not represented by entire meteorites in our collections,” the researchers conclude.
“The difference between its mineralogy and that of known CC meteorites suggests that unique samples such as AhS 202 (and xenolites in other, non-CC meteorites) may be critical missing links in our understanding of parent asteroid diversity.”
The findings are reported in Nature astronomy.