Terrestrial Contamination: Winchcombe Meteorite Reveals Challenges of Studying Pristine Celestial Rocks [STUDY]

The discovery of the Winchcombe meteorite, retrieved only hours after entering the Earth's atmosphere, suggests that there may not be any meteorites that remain entirely "pristine." The meteorite fell to the ground in February 2021, exploding in the sky over Gloucestershire, England. Within the following days, several fragments were collected from nearby fields and properties, with the first found in a driveway just 12 hours after the fireball was sighted.

Despite the brief period that the Winchcombe meteorite spent on Earth, it underwent significant changes to its chemical composition. An analysis revealed that the rock had experienced substantial contamination from the planet's atmosphere and surface, resulting in the formation of salts and minerals that developed after its arrival. In particular, the researchers identified halite, calcite, and calcium sulfate minerals that originated from the fragmentation of the meteorite in Earth's atmosphere. Scientists studying meteorite fragments in the future will need to account for this type of contamination.

The discovery of contamination in the Winchcombe meteorite could have implications on the protection of newly fallen meteorites and geological samples brought back from space. By understanding how meteorites are altered by their interactions with Earth's atmosphere and surface, researchers may be able to develop methods to prevent these changes and preserve the integrity of these samples, as reported by Science Alert.

Winchcombe Meteorite Analysis

The findings may also have implications for the analysis of samples collected from space, such as the recent delivery of asteroid Ryugu samples and the upcoming planned delivery of samples from Mars. According to Laura Jenkins, an Earth scientist from the University of Glasgow, the Winchcombe meteorite, despite its contamination, has already provided significant insights and is often considered a "pristine" example of a CM chondrite meteorite.

The study of the Winchcombe meteorite shows that no meteorite can remain entirely pristine after encountering Earth's atmosphere. Terrestrial alteration begins to affect the meteorite immediately upon impact. The analysis of Winchcombe, which is a carbonaceous chondrite meteorite composed mainly of carbon and silicon, revealed that it had changed within just a few months of falling to Earth. Despite its age of 4.6 billion years, the meteorite can provide valuable information, but only if researchers accurately interpret their findings.

To study the Winchcombe meteorite, Jenkins and her colleagues used various techniques such as scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The team analyzed several samples, including one from the initial discovery on a driveway and several from nearby sheep fields where the meteorite fragments had remained for several days. As an asteroid enters Earth's atmosphere, the air in front of it compresses and heats up, causing the outer layer of the meteorite to melt and be shed. This process continues layer by layer until the meteorite slows down enough for the air to no longer be hot enough to melt the rock, leaving a thin outer crust to cool and harden.

(Photo : (University of Glasgow))
An image of a sample of the meteorite showing terrestrial alterations in red and purple.

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Fusion Crust from Winchcombe Samples

The thin outer crust that forms on a meteorite as it enters Earth's atmosphere is known as a fusion crust and is a key feature used to distinguish meteorites from ordinary rocks. Jenkins and her team focused on the fusion crust of the Winchcombe meteorite samples and found that minerals such as calcite, gypsum, bassanite, and anhydrite had formed on them. The researchers determined that these minerals had likely precipitated from the damp environment in the sheep field where the meteorite fragments had been lying for six days. The presence of these minerals indicates that even a short exposure to Earth's environment can significantly alter the composition of a meteorite.

When analyzing the Winchcombe meteorite sample from the driveway, the researchers found halite only in the areas of the sample that were polished after retrieval, and which contained relatively high levels of sodium. The team determined that this was likely due to an interaction between the rock and the humid laboratory environment where it had been stored for several months. The researchers concluded that meteorites need to be stored carefully in inert conditions to minimize terrestrial contamination. Jenkins emphasized that this finding highlights how sensitive meteorites are to our atmosphere, and how important it is to take terrestrial alteration into account when analyzing meteorites.

The research on the Winchcombe meteorite and its contamination from terrestrial material is important not only for understanding its formation but also for comparing it with samples returned by sample return missions. By distinguishing between extraterrestrial and terrestrial phases in meteorites like Winchcombe, scientists can gain a better understanding of the asteroids in our solar system and their role in Earth's development. The findings of the study have been published in the journal Meteoritics & Planetary Science.