There's a whiff of something radioactive in the air.

A research plane flying over the Aleutian Islands on Aug. 3, 2016 detected a single speck of enriched uranium floating about 4.3 miles (7 kilometers) above Alaska's far-western island chain, according to a new research paper that will be published in April in The Journal of Environmental Radioactivity.

The uranium sample was tiny and harmless, a small chunk of a mote of dust just 580 nanometers wide (about half the size of a red blood cell). And it was completely alone; no other radioactive material turned up in that stretch of sky. But, the researchers wrote, it was "definitely not from a natural source."

And the scientists can't explain how it got there.

The plane's onboard mass spectrometer, brought along to analyze standard-issue pollution, detected just a single uranium particle, mixed with traces of chemicals from burning oil, the authors reported. And on its own, that find wouldn't be too remarkable — uranium is the heaviest element commonly found on Earth, after all.

"Particulate matter containing uranium can originate from sources such as combustion of coals with trace uranium, windblown crustal material, and mining and processing of ores, whether it be for the uranium itself or other minerals such as rare earths [a group of chemically similar elements that aren't actually that rare, but are difficult to mine] and phosphate," the researchers wrote. [The 10 Greatest Explosions Ever]

What makes this particle unusual is that it was rich in an isotope called uranium-235, or U-235, which made up about 2.6 percent to 3.6 percent of its mass, according to the paper. Naturally occurring uranium typically contains just 0.7 percent U-235, with the rest given over to the much more common uranium-238.

That's a big deal.

As Richard Rhodes described in his book "The Making of the Atomic Bomb," published by Simon and Schuster in 1987, uranium-235, an atom made up of 92 protons and 143 neutrons, is special because it can easily sustain a nuclear chain reaction. That's the process of one atom splitting, flinging neutrons out into space, those neutrons smashing into the atom's neighbors and causing them to split, and so on. Uranium-238, with its extra three neutrons, just doesn't give itself over to sustained chain reactions of the kind needed for nuclear power, or nuclear weapons.

Rhodes wrote that refining uranium-235 out of large natural samples of mostly uranium-238 was one of the most important challenges during the race to build the first atomic bomb in the 20th century. And that process remains a challenge today.

The Aleutian Islands sample, with its relatively high percentage of uranium-235 content, is already refined enough to serve in a nuclear reactor, the researchers wrote. (A bomb requires something closer to 90 percent uranium-235 content.)

Finding a sample of refined uranium in the open air is bizarre and remarkable, but it's not dangerous on its own, experts said.

"It's not a significant amount of radioactive debris by itself," Dan Murphy, a National Oceanic and Atmospheric Administration scientist and one of the paper's authors, told Gizmodo, which originally reported on the paper. “But it’s the implication [of this finding is] that there's some very small source of uranium that we don’t don't understand."

For one thing, as the paper stated, the particle is much smaller than the particles of uranium dust that emerge from typical nuclear facilities. It's possible, the authors suggested, that a forest fire or something similar kicked up old particles from an event like the Chernobyl meltdown — but there haven't been any recent incidents that would be obvious culprits for that kind of thing.

Beyond the uranium particle itself, the air sample the plane collected wasn't unusual, with its only notable feature being some diluted trace burnt-oil pollution, the researchers wrote. Based on prevailing air currents, it's likely the particle came to Alaska from somewhere within a broad swath of Asia, including China, Japan and the Korean Peninsula.

But, the researchers cautioned, the science of figuring out just how individual particles might have arrived in a given patch of air is too inexact to determine the mysterious uranium's origin with any certainty.

Originally published on Live Science.