Winchcombe meteorite: space rock may reveal how water came to Earth

A fragment of meteorite which fell to earth in a quiet village in Gloucestershire may hold the answer to a question scientists have been puzzling over for decades: where did our planet's water come from?

The Winchcombe meteorite "streaked from space into the atmosphere as a spectacular green fireball" in 2021, said The Times. After search teams managed to recover a substantial amount of the space rock – around 300g – it was named after the village on which it fell.

Now a new analysis of the meteorite, published in the Meteoritics & Planetary Science journal, has revealed how it was heavily altered by water as well as smashed apart and reformed multiple times before it fell to Earth. 

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What's so important about this meteorite?

Search teams managed to recover the meteorite from several sites around Winchcombe – including a sample which landed on someone's drive – within days. It meant that the rocks weren't exposed to the Earth's atmosphere for an extended period, unlike most recovered meteorites. 

"The speed which the fragments of Winchcombe were recovered left us with some pristine samples for analysis, from the centimetre scale all the way down to individual atoms within the rocks," said Martin Suttle from the Open University, who took part in the analysis, speaking to the BBC Sky at Night magazine.

Its excellent preservation meant that scientists were able to carry out a level of analysis "virtually unprecedented for materials that weren't directly returned to Earth from space missions, like moon rocks from the Apollo program or samples from the Ryugu asteroid collected by the Hayabusa 2 probe," said Dr Leon Hicks from the University of Leicester and the co-author of the new study.

What did the analysis reveal?

Initial analysis of the Winchcombe meteorite found that it was a very rare type of space rock called carbonaceous chondrite, which formed in the earliest periods of our solar system. While there are about 65,000 known meteorites on Earth, "only about 1,200 have been seen while falling and, of these, only 51 are carbonaceous chondrites", said The Times.

Closer analysis revealed that the meteorite was formed from eight types of CM chondrite, the most common variation of carbonaceous chondrite. The findings suggest the asteroid from which the meteorite originated had been smashed apart and reformed multiple times, possibly following collisions with other asteroids early in the solar system's history.

"If you imagine the Winchcombe meteorite as a jigsaw, what we saw in the analysis was as if each of the jigsaw pieces themselves had also been cut into smaller pieces and then jumbled in a bag filled with fragments of seven other jigsaws," said Luke Daly of the University of Glasgow, who led the research.

But what does it tell us about Earth's water?

The new analysis of the meteorite also gave scientists "insight into the very fine detail of how the rock was altered by water in space", said Daly.

One leading theory of how water came to Earth is that some of it was carried here in carbonaceous asteroids, much like the kind the Winchcombe meteorite originated from. 

The Winchcombe sample appears to support this idea, because the isotopic signatures of the hydrogen and oxygen (the atomic ingredients for water) found in the meteorite is a close match for that seen in the ocean, suggesting a shared source.

But carbonaceous chondrites probably aren't the only sources of the Earth's water. One leading theory published in Nature suggests that much of it has been here since the Earth first formed. 

However many planetary geologists believe a significant portion came from beyond our atmosphere. "There's so much evidence," said Anat Shahar, a geochemist at the Carnegie Institution for Science, speaking to Scientific American. "We can't argue against it."