Geologists and atmospheric scientists have just unearthed the oldest meteorites ever found on our planet. They're 2.7 billion years old and paint a striking picture of Earth's atmospheric history. Oddly, they reveal hints that Earth had an oxygen-rich atmosphere at a time many scientists still agree the planet was still nearly devoid of oxygen.

The 60 meteorites were fossilized in limestone in Western Australia's Pilbara region and unearthed in 2014 and 2015. Each are dust-sized grains, individually smaller than a crosscut of your hair. They were discovered and analyzed by a research team led by Andrew Tomkins, a geologist at Monash University in Melbourne, Australia. As Tomkins reports in a paper published today in the journal Nature, the chemical composition of these meteorites provide the first direct evidence of any kind of the makeup of ancient Earth's upper atmosphere.

"As geologists, we're taught that the Earth had no oxygen in its atmosphere before 2.3 to 2.4 billion years ago," says Tomkins. Nevertheless these much meteorites "traveled through an upper atmosphere with almost [modern day Earth-like] amounts of oxygen."

One of 60 micrometeorites extracted from 2.7 billion year old limestone, from the Pilbara region in Western Australia. Andrew Tomkins

A Thought Experiment

Tomkins's team found the Earth's oldest meteorites after an interesting (and frankly ingenious) thought experiment. Tomkins says he got an idea while reading an article in a scientific journal about the geology of tiny so-called micrometeorites like the ones found today. "[They] are actually quite common," he says. "There are reports of people just fishing micrometeorites out of the gutters on their roofs."

It got him to thinking: As shooting stars, these minute grains of rock and metal must have roiled through Earth's upper atmosphere. The chemical makeup of the atmosphere would have affected how the meteors melted and what molecules the cooling meteorites formed. So could you walk through the process backwards and use these meteorites to deduce what that atmosphere looked like?

With this question in mind, Tomkins's research team looked for someplace nearby in Australia where they might find such ancient micrometeorites. They wanted supremely old rocks, where sediment had accumulated slowly enough that incoming micrometeorites might collect in abundance. "And ideally limestone," says Tomkins. Limestone would be dense enough to protect these micrometeorites from water and weather for billions of years.

Tomkins's team scoured geological maps, found a location in Western Australia's Pilbara region that fit the bill, and started digging. The team took samples of limestone, dissolved the soft rock in a mild acid, and found dozens of tiny meteorites left over. "I guess it is that simple: We were the first to find meteorites this old simply because we were the first to look for them," Tomkins says.

Monash University scientists Andrew Tomkins and Sasha Wilson. Steven Morton

The Ancient Atmosphere

Micrometeorites like these melt and quench-cool in about two seconds as they scream through the Earth's atmosphere. Tomkins discovered that his 2.7-million-year-old meteorites had traveled though an atmosphere that was roughly 20 percent oxygen, deduced by the exact composition of meteorites' metal oxides (such as "a thin rim of iron oxide called wüstite," he says.)

This is strange, because scientists are in wide agreement that Earth before 2.5 billion years ago had an atmospheric oxygen level less than 0.001 percent that of today's. Back then, Earth hadn't yet been flooded with the respiration of primordial organisms producing the first puffs of photosynthesis. This atmospheric revolution happened around 2.5 to 2.4 billion years ago, and is referred to as the Great Oxidation Event.

How could this be? Tomkins's team believes that while the lower atmosphere was almost certainly devoid of oxygen, the far reaches of the upper atmosphere could have had a healthy dose of the element. It would have been formed by UV light from the sun breaking apart water vapor and carbon dioxide. Tomkins hopes to uncover even more micrometeorites from around the world, "perhaps in even older rocks," he says, to paint a richer picture of the history of Earth's upper atmosphere. And he doesn't see why this endeavor should be limited to just our planet.

"Potentially, the Curiosity rover on the surface of Mars might be able to find micrometeorites like this, and then look back into the history of Mars' atmosphere with them." he says.

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