Scientists found a "highly unusual" particle containing enriched uranium-235 during a routine sampling of the air above the Aleutian Islands in August 2016. The source of the material, typically used in nuclear fuel and bombs, remains unclear - while the particle itself is unique in that it's the first of its kind to be detected in 20 years of plane-based observations.

While uranium normally occurs in the ground as the moderately radioactive isotope U-238 - and typically not floating in the air, it must be refined using various methods - typically centrifuges, in order to produce U-235. Particles containing 3-4% U-235 are considered "low enriched" for civilian reactors, while anything north of 90% is considered "weapons grade."

“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 [uranium-238] and never a particle with enriched 235U [uranium-235],” reads an abstract from the study.

The mystery particle - the bulk of which consisted of "material consistent with combustion of heavy fuel oil" and a "very small amount of enriched uranium" was discovered at an altitude of 7km (4.3 miles) - lower than Mount Everest.

Researchers involved in the joint project between NOAA ESRL Chemical Sciences Division, the Norwegian Institute for Air Research, and UC Irvine say they were making no special attempts to sample radioactive material during the routine flight to sample the atmosphere.

“One of the main motivations of this paper is to see if somebody who knows more about uranium than any of us would understand the source of the particle,” scientist Dan Murphy from NOAA told Gizmodo reporter Ryan F. Mandelbaum. After all, “aerosol particles containing uranium enriched in uranium-235 are definitely not from a natural source,” he writes in the paper, published recently in the Journal of Environmental Radioactivity.

They were not intending to look for radioactive elements. “The purpose of the field campaign was to obtain some of the first global cross-sections of the concentration of trace gases and of dust, smoke, and other particles in the remote troposphere over the Pacific and Atlantic Oceans,” according to the paper. -Gizmodo

The precise origin of the radioactive particle remains a mystery, however the abstract suggests it could have originated "from a variety of areas across Asia," and researchers are noting its existence "in case it indicates a novel source where enriched uranium was dispersed."

“It’s not a significant amount of radioactive debris by itself,” said Dan Murphy of NOAA. “But it’s the implication that there’s some very small source of uranium that we don’t understand.”

But where the particle came from is a mystery. It’s pretty clear it came from recently made reactor-grade uranium, the authors write (aka, not from Fukushima or Chernobyl). Perhaps from burnt fuel contaminated with uranium, they thought. They tried to trace it to a source using the direction of the wind—but their best estimate pointed vaguely to Asia. Higher probability areas include some parts of China, including its border with North Korea, and parts of Japan. -Gizmodo

The NOAA scientists are hoping that other experts in the field will chime in with an answer. “We’re hoping that someone in a field that’s not intimately associated with atmospheric chemistry can say ‘a-ha!’ and give us a call.”