The U.S. National Oceanic and Atmospheric Administration (NOAA) reported that the mysterious particles appeared in the upper troposphere more than four miles (seven kilometers) above Alaska. One of NASA's WB-57 high-altitude research aircraft made the discovery and took a sample.
Most of the particle is comprised of the typical by-products from burning heavy fuel oil. However, it also shows traces of enriched uranium oxide with uranium-235 and uranium-238.
Uranium is the densest element that occurs in nature. It usually appears as the isotope uranium-238, which is only slightly radioactive and is rare to find that high up in the atmosphere.
Uranium-235, on the other hand, is even rarer to find in nature. This far more radioactive isotope is more commonly manufactured as the fissile material for nuclear fuel and atomic warheads. (Related: Review of nuclear damage from Hiroshima, Fukushima focuses on dangers of nuclear radiation and the need to ban such weapons.)
What makes this discovery very unusual is the fact that NOAA airborne atmospheric research aircraft has never encountered atmospheric samples rich in both U-235 and U-238 during 20 years of operations. In fact, the team wasn't looking for radioactive particles in the first place.
"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," explained the researchers who authored the scientific study.
Study leader Dan Murphy is in charge of high-altitude scientific flights that ply the skies around the world for signs of aerosols formed by dust, fires, and pollution. He and his NOAA co-workers are atmospheric chemists who study how large amounts of tiny particulate matter in the air can affect the composition of clouds and the weather itself.
"It's not a significant amount of radioactive debris by itself," he said during an interview with Gizmodo. "But it's the implication that there's some very small source of uranium that we don't understand."
From what he and his team could determine, the U-235 and U-238 isotopes that formed the uranium oxide had to be man-made.
"Aerosol particles containing uranium enriched in uranium-235 are definitely not from a natural source," Mr. Murphy explained in the paper itself.
The NOAA study further theorizes that the uranium oxide came from recently-made reactor-grade fuel destined for a fission power plant. The newness and freshness of the material ruled out major nuclear disasters like Chernobyl, Fukushima, or Three Mile Island that took place years or decades ago.
They are looking at China, Japan, or the Korea peninsula as the source of the radioactive parts of the strange particle. All three regions have active nuclear power plants and the prevailing weather patterns could carry the particle to Alaska.
"Analysis of wind trajectories and particle dispersion model results show that the particle could have originated from a variety of areas across Asia," they wrote in their report.
The study was published in the Journal of Environmental Radioactivity. Mr. Murphy and the rest of the NOAA team are hoping to collaborate with a recognized authority on uranium so that they could determine how the particle was formed and where it could have come from.
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