About 2.7 billion years ago, tiny meteorites the size of sand grains rained down on a primeval landscape, sampling our planet’s ancient atmosphere in the process.

The not-so-great balls of fire—now the oldest micrometeorites ever found—started out as hunks of mostly iron and nickel. But as they careened through Earth’s upper atmosphere, roughly 47 miles (75 kilometers) above the planet’s surface, they felt the burn and crystallized, forming droplets mostly made of iron oxides.

These oxides suggest that the upper atmosphere of the time was about 20 percent oxygen, in line with modern levels. That’s a surprise for the Archean eon, a time when Earth’s atmosphere closer to the surface seems to have been almost oxygen free.

The finding, described on May 11 in Nature, is the latest and perhaps most unorthodox attempt to sniff Earth’s ancient atmosphere, a hot topic in the study of life’s early history.

“It started as ‘Hey, wouldn’t this be cool?’ kind of science—it’s nice that it worked out like this,” says study leader Andy Tomkins of Monash University in Melbourne, Australia.

“I never would have guessed you could do it,” says Jim Kasting of Penn State University, an expert on Earth’s early atmosphere who wasn’t involved with the study.

High-Speed Sniffing

Based on the composition of ancient minerals, researchers have known for decades that up until about 2.3 billion years ago, any oxygen that photosynthetic life produced on the surface failed to accumulate in the atmosphere’s lower layers.

But chemical models had suggested that the upper atmosphere, between 31 and 62 miles (50 and 100 kilometers), was practically a different world, since solar radiation could break up plentiful carbon dioxide molecules, forming ample amounts of oxygen and carbon monoxide.

The trouble was that scientists didn’t have any way of sampling this portion of the ancient atmosphere, until a team led by Tomkins decided to hunt for Earth’s oldest micrometeorites in ancient limestone deposits in Western Australia.

The team took extra care to ensure that their samples weren’t contaminated with today’s space schmutz, or that they would dissolve away when the scientists separated the micrometeorites from the limestone with an acid bath.

The micrometeorites they found are almost a billion years older than the next oldest samples. And despite their age, the 60 micrometeorites the team recovered appear to be in good shape, retaining the telltale signs of extraterrestrial origin and a high-oxygen burn.

Meenakshi Wadhwa, the director of Arizona State University’s Center for Meteorite Studies, applauds the team for their attention to detail. Micrometeorites are a pervasive source of extraterrestrial contamination, making up most of the 100 tons of space debris that rain on Earth daily.

“I was initially quite skeptical that such particles could be preserved in 2.7-billion-year-old rocks, but the evidence looks convincing,” she wrote in an email.

Fooling ET

The pioneering study, however, leaves more questions than answers. The atmospheric oxygen levels suggested by the micrometeorites are about 10 times more than what current models predict should have existed. What’s more, if so much oxygen did exist in the atmosphere’s upper reaches, why didn’t it mix in with the surface layers?

But Kasting, a world expert in Archean atmospheric models, is confident that the new method is sound. He has even signed on to join the team in potential follow-up studies.

In addition, the study’s implications could extend far beyond Earth, or even the solar system itself.

Astrobiologist Timothy Lyons of the University of California, Riverside, points out that the team’s findings may color future searches for extraterrestrials. One of the most promising future strategies for hunting ET calls for telescopes to sniff exoplanet atmospheres for compounds that only life could produce in abundance.

But if Earth’s upper atmosphere contained massive amounts of oxygen—which is often associated with life—then other planets’ atmospheres could appear more oxygen-rich than they actually are, potentially creating a false positive in the search for aliens.

“These guys have possibly shown us that there might be a lot of oxygen that has nothing to do with what we’re looking for—the presence of life producing that oxygen,” he says.