What we learnt from a meteorite that landed in the Cotswolds

The meteorite landed in Winchcombe in the Cotswolds last February with fragments recovered from a domestic driveway hours after it entered the Earth’s atmosphere.

More pieces were found in a sheep field six days later.

Analysis of the meteorite show the fragments quickly developed salts and minerals from the interaction of their surfaces with the damp environment in which they landed.

The findings could help protect new meteorites after they are found.

Lead author, Laura Jenkins, a PhD student at the University of Glasgow said once meteorites are exposed to terrestrial contaminants they change.

She said: “Analysis of meteorites can provide insights into the asteroids they come from and how they have formed.

“Winchcombe and other meteorites like it contain extra-terrestrial water and organics, and the asteroids they come with may be responsible for delivering water to Earth, giving it enough water to form its distinctive oceans.

“However, when a meteorite is exposed to terrestrial contaminants, especially moisture and oxygen, it undergoes changes, affecting the information it provides.”

Lead author Laura Jenkins (Image: SWNS)

Researchers examined two small pieces of Winchcombe for signs of terrestrial modifications.

They used scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to scrutinise the surfaces of the samples.

One sample was taken from the driveway fragments and the other from those found in the sheep field.

They found that two forms of salt - sulfates of calcium and calcite - had formed on the fusion crust of samples recovered from the sheep field.

And they found halite, also known as table salt, on the sample taken from the driveway.

The fusion crust is the distinctive material formed when meteorites melt during their fiery entry to the Earth’s atmosphere.

Researchers said since the sulfates appeared on the outside of the fusion crust, it is likely they appeared after it landed, as a result of exposure to damp conditions in the sheep field.

The halite appeared only on the surface of polished sections of the driveway fragment.

Since the polishing was done after the meteorite was recovered, it is likely that it was formed by the interaction of the rock slice with the humid laboratory air.

Ms Jenkins added: “The Winchcombe meteorite is often described as a ‘pristine’ example of a CM chondrite meteorite, and it’s already yielded remarkable insights.

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

“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 analyse meteorites.

“To minimize terrestrial alteration, meteorites should be stored in inert conditions if possible.

“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.

Co-author Dr Luke Daly is a lecturer at the University of Glasgow and part of the UK Fireball Alliance which spotted the Winchcombe meteorite and estimated where it landed.

Dr Daly led the search party that recovered the largest chunk of the Winchcombe meteorite from a sheep field, where it was spotted by volunteer Mira Ihasz.

He said: “We’ve always known that exposure to Earth’s atmosphere affects the surface of meteorites but this is the first time we’ve been able to see just how quickly the process can begin and advance.

“We were very lucky to be able to recover the Winchcombe meteorite so quickly through the monitoring of the UKFall network and the efforts of the volunteers who helped us recover the largest pieces from the field.

“This research shows just how vital it is that we keep watching the skies and assemble search parties as quickly as possible after meteorites are spotted.”

The paper, titled ‘Winchcombe: An Example of Rapid Terrestrial Alteration of a CM Chondrite ‘, is published in early view in Meteoritics & Planetary Science at https://onlinelibrary.wiley.com/doi/full/10.1111/maps.13949