The team was on a routine pollution-sampling flight from Anchorage to Hawaii when they discovered it by chance, floating alone in the evening Alaskan sky.

At an altitude of about 7  kilometres (4.3 miles) above Alaska's Aleutian Islands, it was unlike anything the researchers had seen in two decades of air sampling: a single radioactive aerosol particle, containing a very small amount of enriched uranium.

Despite the radioactivity, the discovery – made back in August 2016 – wasn't a cause for concern in itself, owing to the fact the windswept particle was incredibly tiny (at just 580 nanometres in width) and given it seemed to be floating in isolation in the troposphere.

But just what the heck was it doing up there?

"It's not a significant amount of radioactive debris by itself," one of the researchers, Daniel Murphy from the National Oceanic and Atmospheric Administration (NOAA) told Gizmodo.

"But it's the implication that there's some very small source of uranium that we don't understand."

The ultimate significance of that implication remains as yet unknown, but the particle's existence is a puzzling mystery, given the kinds of uranium it was made up of: uranium-238 and, even more bizarrely, uranium-235.

Uranium 238 is common in nature – if not the atmosphere – but the particle's richness in the rarer uranium 235, as the researchers explain, meant the sample detected was "definitely not from a natural source".

"During 20 years of aircraft sampling of millions of particles in the global atmosphere, we have rarely encountered a particle with a similarly high content of 238U and never a particle with enriched 235U," the authors write in their paper.

Uranium-235 is the fissile isotope capable of sustaining a fission chain reaction, meaning it's the kind of uranium used in nuclear weapons and nuclear fuel.

While uranium-235 does occur in nature, the sample contained about three times as much uranium-235 content as would ever naturally occur in a sample of the metal at this size, meaning the particle qualifies as enriched uranium for the purposes of nuclear power generation or military weapons.

In light of that, the fact this particle was astray on the breeze over Alaska doesn't make an awful lot of sense to the researchers, who say the size of the uranium particle is too small to have escaped the industrial production of nuclear fuel.

Hypothetically, the researchers suggest the particle could have originated in a nuclear accident like Chernobyl or Fukushima – except it wouldn't have floated in the air all this time, and no significant events like forest fires (which could have lifted the particle back into the air) occurred around the time of the sampling.

It's more likely, the team thinks, that the particle was carried on air currents over the Pacific Ocean to the Aleutian Islands, with the researchers' analysis of wind trajectories and particle dispersion suggesting somewhere in Asia, such as Japan, China, and North or South Korea.

"My best guess is that the source is North Korea," nuclear energy expert Arnie Gundersen, who was not part of the research, suggested to EnviroNews.

"North Korea has a small reactor and does have gas centrifuges for slightly enriching uranium 235… It is possible in either creating new fuel or in extracting plutonium from fuel that has already been in their reactor, some enriched uranium escaped and went airborne."

There's no proof of this, however, and given the extreme scarcity of the sample we're working with – one tiny microscopic particle floating in the sky above Alaska – the researchers are hopeful, now that they've published their research, other scientists more highly versed in radioactive elements might be able to offer some new leads.

"One of the main motivations of this paper," Murphy acknowledged to Gizmodo, "is to see if somebody who knows more about uranium than any of us would understand the source of the particle."

The findings are reported in the Journal of Environmental Radioactivity.