Winchcombe: 4.6 billion year old meteorite wasn't so alien after all

The 4.6 billion-year-old Winchcombe meteorite, which landed in Gloucestershire in February last year, has undergone a new examination that is the first to demonstrate how space rocks that fall to Earth can be quickly contaminated by our atmosphere. 

Winchcombe, renowned for its "pristine" condition, was the first of its kind to be discovered in the UK in 30 years. It provided fascinating insights into how meteorites could have transported significant molecules to Earth, such as the delivery of water to create the planet's distinctive oceans. 

However, the new study calls into question any past explanations of meteorites that failed to consider how their entrance into Earth introduced a variety of impurities with solely terrestrial origins.

In particular, the researchers observed a range of salts, including halite, which is our common table salt. 

What do meteorites reveal about our solar system?

"Understanding which phases are extra-terrestrial and which are terrestrial in meteorites like Winchcombe will not only help our understanding of their formation," said Ph.D. student Laura Jenkins in a press release by the University of Glasgow, where she studies and led the new study.

"[It] 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," she added. 

The research team studied two small chunks of Winchcombe to look for evidence of terrestrial alterations. 

The group examined the samples' surfaces using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Samples were obtained from fragments retrieved from a domestic driveway, as well as a sheep field.

They discovered that the 'fusion crust' of samples taken from the sheep field had developed two types of salt - sulfates of calcium and calcite - on it. Meanwhile, they noticed that the sample obtained from the driveway included halite.

What is a fusion crust, and how does it form?

When meteorites melt during their ferocious entry into the Earth's atmosphere, a peculiar substance called the fusion crust is created. 

The scientists concluded that since the sulfates were found on the fusion crust's exterior, they most likely formed after the object crash-landed and was exposed to the sheep field's moist conditions.

Additionally, halite, found only on the driveway’s polished samples, was most likely introduced by the damp laboratory air.

"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," said Dr. Luke Daly, a co-author on the paper.

Jenkins emphasized that terrestrial alteration begins when a meteorite encounters Earth's atmosphere. The fact that her team's research observed alteration in samples only a couple of months after the meteorite landed supports this.

"To minimize terrestrial alteration, meteorites should be stored in inert conditions if possible," advised Jenkins. 

The full study was published in Meteoritics & Planetary Science on February 9, 2023.

Study abstract:

Winchcombe is a CM chondrite that fell in England on February 28, 2021. Its rapid retrieval was well characterized. Within two polished sections of Winchcombe, terrestrial phases were observed. Calcite and calcium sulfates were found in a sample recovered from a field on March 6, 2021, and halite was observed on a sample months after its recovery from a driveway on March 2, 2021. These terrestrial phases were characterized by scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Calcite veins crosscut the fusion crust and therefore postdate it. The calcite likely precipitated in the damp environment (sheep field) where the meteorite lay for six days prior to its retrieval. The sulfates occur on the edges of the sample and were identified as three minerals: gypsum, bassanite, and anhydrite. Given that the sulfates occur only on the sample's edges, including on top of the fusion crust, they formed after Winchcombe fell. Sulfate precipitation is attributed to the damp fall environment, likely resulted from sulfide-derived H2S reacting with calcite within the meteorite. Halite occurs as euhedral crystals only on the surface of a polished section and exclusively in areas relatively enriched in sodium. It was likely produced by the interaction of the polished rock slice with the humid laboratory air over a period of months. The sulfates, fusion crust calcite, and halite all post-date Winchcombe's entry into the Earth's atmosphere and showcase how rapidly meteorite falls can be terrestrially altered.