Winchcombe is a carbonaceous chondrite (CM chondrite), sunset on February 28, 2021 in Gloucestershire, England. While some fragments were found within 12 hours of the March 1 fall, others took weeks to find, with the last fragment collected on April 1.
Analysis of the Winchcombe meteorite shows how quickly Earth’s atmosphere affects meteorites that fall to Earth. The study, led by Laura Jenkins, a doctoral student in the School of Geography and Earth Sciences at the University of Glasgow in Scotland, UK, shows that meteorite fragments interact with Earth’s atmosphere to immediately form salts and minerals.
Specifically, the researchers found rock salt, calcite, and calcium sulfate minerals that formed after the meteorite ruptured the Earth’s atmosphere. This contamination is something scientists will need to consider when studying meteorite fragments in the future. On the other hand, these discoveries also help protect newly fallen meteorites from changes on the ground, as well as rock samples brought back from space, such as the Ryugu asteroid sample returned to Earth in 2020, or samples brought back from Mars.
When a meteoroid enters Earth’s atmosphere, the air ahead of it compresses and heats up during the fall, causing the meteor’s shell to melt and fall off. Then the next layer begins to melt and fall off, until it slows down and the air is no longer hot enough to melt the rock, and the last layer cools and hardens, forming a thin crust. Fusion crust features are the main way to judge the difference between meteorites and ordinary earth rocks visually.
The research team collected two samples (one from a sheep farm and the other from a driveway) and examined them with scanning electron microscopy, Raman spectroscopy and transmission electron microscopy. The results show that the fusion crust of the meteorite samples found at Sheep Farm has formed calcite and calcium sulfate (gypsum) and anhydrite etc. The meteorite was exposed to the ground for six days, and the minerals were likely formed due to the humidity of the environment.
▲ A sample of the Sheep Farm meteorite, where the arrow points shows minerals produced by the Earth’s atmosphere. (Source: University of Glasgow)
Samples were found in the driveway that had been stored in the laboratory for several months, and rock salt was found on the polished surface of the meteorite, likely formed by the interaction of rock slices with humid laboratory air.
The discovery means meteorites should be carefully stored in inert conditions to minimize contamination of Earth, the researchers said. The research was published in the journal Meteoritics & Planetary Science.
(This article is reproduced with the authorization of Taipei Planetarium; the first picture is a schematic diagram; source: Pixabay)
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