A space rock recovered within just a few short hours of entering Earth's atmosphere has revealed there may not be such a thing as a "pristine" meteorite.

The Winchcombe meteorite fell to Earth in February 2021 after exploding mid-air over Gloucestershire in England, and in the days that followed, multiple fragments were retrieved from the surrounding fields and properties, the first in a driveway just 12 hours after the fireball was seen.

Still, that short amount of time was enough for changes to occur in the rock's chemistry. An analysis showed significant and extensive contamination from the terrestrial atmosphere and ground, with salts and minerals that had to have developed within the sample after its arrival at our home planet.

Specifically, researchers found halite, and calcite and calcium sulfate minerals, that formed after the meteorite broke apart in Earth's atmosphere. This contamination is something that scientists will need to take into consideration when studying meteorite fragments in the future.

On the other hand, the findings could also help efforts to protect newly fallen meteorites against terrestrial alteration, as well as geological samples brought home from space – like the samples of asteroid Ryugu delivered to Earth in 2020, or the planned delivery of samples from the surface of Mars.

"The Winchcombe meteorite is often described as a 'pristine' example of a CM chondrite meteorite, and it's already yielded remarkable insights," says Earth scientist Laura Jenkins of the University of Glasgow in Scotland.

"However, what we've shown with this study is that there's really no such thing as a pristine meteorite – terrestrial alteration begins the moment it encounters Earth's atmosphere, and we can see it in these samples which we analyzed just a couple of months after the meteorite landed."

Analysis revealed Winchcombe, as the 4.6-billion-year-old rock is known, to be a rare type of meteorite known as carbonaceous chondrite, made up primarily of carbon and silicon. There's a lot we can learn from such an ancient piece of space rock, but only if we are correctly interpreting what we are looking at.

Jenkins and her colleagues conducted scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy on several samples. These samples included one from the initial driveway find, and several from sheep fields, where they had lain for several days before recovery.

When an asteroid enters Earth's atmosphere, it doesn't just fall down. As it falls, the air in front of it is compressed and heats up, causing the exterior of the meteor to melt and be cast off. Then the next layer melts and sloughs, until it slows down enough that the air is no longer hot enough to melt the rock, leaving the last layer to cool and harden as a thin crust.

A sample of the sheep field meteorite, showing terrestrial alteration. (University of Glasgow)

This is known as a fusion crust, and is the main diagnostic feature to visually determine the difference between a meteorite and just a plain old Earth rock.

Jenkins and her team conducted a thorough study, and found calcite and calcium sulfates – gypsum, bassanite, and anhydrite – had formed on the fusion crust of the meteorite samples found in the sheep field. As this piece of meteorite had lain there for six days, they concluded that the minerals had likely precipitated from the damp environment.

On the driveway sample, they found halite, but only on a section of the sample that was polished after retrieval, in areas relatively rich in sodium. This, they found, was likely due to an interaction between the rock and the humid laboratory environment in which it had been stored for several months.

This finding, the researchers said, suggests that meteorites should be carefully stored in inert conditions, to try to minimize terrestrial contamination.

"It shows just how reactive meteorites are to our atmosphere, and how careful we need to be about ensuring that we take this kind of terrestrial alteration into account when we analyze meteorites," Jenkins explains.

"Understanding which phases are extra-terrestrial and which are terrestrial in meteorites like Winchcombe will not only help our understanding of their formation but will also aid in relating meteorites that have landed on Earth to samples returned by sample return missions. A more complete picture of the asteroids in our Solar System and their role in Earth's development can be built."

The research has been published in Meteoritics & Planetary Science.