NOAA plane
An NOAA research plane Lt. Kevin Doremus / NOAA

Atmospheric scientists have been left completely stumped after a research flight in August, 2016, led to a very strange discovery.

A team from the US National Oceanic and Atmospheric Administration (NOAA) unexpectedly detected a mysterious aerosol particle, seven kilometres up in the atmosphere, which was confirmed to be enriched uranium (known as uranium-235) – of the kind that is used by humans in nuclear fuel and atomic bombs.

This is significant because it is the first time that such a particle has been detected simply floating around in the atmosphere. The finding is described in a new paper published in the Journal of Environmental Radioactivity.

Uranium is a metal and the heaviest element which is naturally found on the Earth's surface in significant amounts. Usually, uranium takes the form of a mildly radioactive isotope – variants of a particular element which differ in the number of neutrons – known as uranium-238.

However, it also occurs in nature as uranium-235, albeit rarely - and never in 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," Dan Murphy from the NOAA told Gizmodo UK.

"Aerosol particles containing uranium enriched in uranium-235 are definitely not from a natural source," he wrote in the report.

Murphy and his colleagues were not searching for the uranium when they made their discovery, but were instead researching concentrations of trace gases, and other particles, in the atmosphere over the Pacific and Atlantic Oceans.

But a piece of equipment on their plane - known as the Particle Analysis by Laser Mass Spectrometry - detected the strange particle as they were flying over Alaska's Aleutian Islands.

Despite their efforts, the scientists have not been able to explain where the mystery molecule originated. Their initial analysis showed that the source was not a nuclear disaster, such as Fukushima or Chernobyl, because it was not reactor-grade uranium.

In addition, they tried to investigate whether the source was actually burnt fuel contaminated with uranium, but after examining the direction of the wind, they concluded that even if this was the case, the location of origin could only be narrowed down to the entire continent of Asia.

The researchers have reached out to other members of the scientific community for help in trying to solve the riddle of the rogue uranium particle, but the mystery is unlikely to be solved any time soon.