Meteorites are Contaminated Quickly When They Reach Earth

On Earth, geologists study rocks to help better understand the history of our planet. In contrast, planetary geologists study meteorites to help better understand the history of our solar system. While these space rocks put on quite the spectacle when they enter our atmosphere at high speeds, they also offer insights into both the formation and evolution of the solar system and the planetary bodies that encompass it. But what happens as a meteorite traverses our thick atmosphere and lands on the Earth? Does it stay in its pristine condition for scientists to study? How quickly should we contain the meteorite before the many geological processes that make up our planet contaminate the specimen? How does this contamination affect how the meteorite is studied?

These are questions that a team of researchers at the University of Glasgow in the United Kingdom (UK) hope to answer in a recent study as they investigate the Winchcombe meteorite, which made landfall in the UK in early 2021 with a total known weight of 602 grams (21.2 oz). What makes the Winchcombe meteorite unique is that some of the fragments were collected and sealed within hours after blazing through our atmosphere, which helped preserve the meteorite for scientific study. However, even this quick containment might not be able to prevent contamination, as the researchers attempted to find out.

“Analysis of meteorites can provide insights into the asteroids they come from and how they have formed,” said Laura Jenkins, who is a PhD student in Glasgow’s School of Geographical and Earth Sciences, and lead author of the study. “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.”

For the study, the researchers used Raman spectroscopy, transmission electron microscopy, and scanning electron microscopy to analyze two polished samples of the Winchcombe meteorite for what are known as “terrestrial phases”, or the development of minerals and salts when the meteorite fragments interacted with Earth’s damp environment upon landing. One sample was collected on March 2, 2021, from a domestic garden lawn, and the other sample was collected on March 6, 2021, from a sheep-grazing field.

The researchers identified halite on the March 2^nd, driveway sample and a combination of calcite and calcium sulfates on the March 6^th, sheep-grazing sample. The calcite veins crosscut the meteorite’s fusion crust, the melted material that results from the heated entry into the Earth’s atmosphere, which indicates calcite veins postdate the crust, meaning it happened after the meteorite landed.

The halite was found on the surface of the polished section, and the calcium sulfates were found on the edges of the sample and contained anhydrite, gypsum, and bassanite. The team hypothesizes that the halite was formed from the laboratory’s humidity over a period of months, while the calcite and calcium sulfates likely formed from the sheep field’s damp environment.

“The Winchcombe meteorite is often described as a ‘pristine’ example of a CM chondrite meteorite, and it’s already yielded remarkable insights,” said Jenkins. “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 analyzed 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 analyze meteorites,” Jenkins continued. “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.” 

The researchers stress the importance of storing quickly recovered samples in an inert atmosphere, or an atmosphere that doesn’t contain oxygen or carbon dioxide, which are often responsible for creating the “terrestrial phases” found in meteorites. They used the Hayabusa2 and OSIRIS-REx missions as examples of pristine samples being returned to Earth and that sample return missions are important for scientists to better understand our solar system. The researchers concluded by noting that better understanding meteorite weathering will help improve sample storage protocols, along with comparing such samples to Earth-altered samples.

History of the Winchcombe Meteorite

Just before 10:00 pm on the evening of February 28, 2021, a small meteorite broke apart over the English county of Gloucestershire traveling at approximately 14 kilometers per second (8.7 miles per hour), briefly appearing as a green fireball witnessed by over 1000 people. The following morning, the Wilcock family residing in the small English town of Winchcombe found a pile of powder and dark stones littered in their driveway and a small dent in the driveway’s tarmac. While they heard the sonic boom the night before, they chose not to investigate it.

After collecting the fragments from their driveway, the family contacted the UK Meteor Observation Network (UKMON) and spent the following two days collecting powder and stones from their driveway and front lawn. In the following weeks, hundreds of fragments were recovered spanning an area from the neighbor’s driveway to several towns over and the surrounding countryside. When all fragments were collected, the total known weight came in at 602 grams (21.2 oz), and this meteorite was henceforth named the Winchcombe meteorite after the town it landed in.

What new discoveries will we make about meteorites and how Earth alters their composition in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!