On this date in 2016, a research plane discovered a mysterious radioactive particle in the atmosphere which should not have been there.
On August 3, 2016, seven kilometers above Alaska’s Aleutian Islands, a research plane captured something mysterious: an atmospheric aerosol particle enriched with the kind of uranium used in nuclear fuel and bombs.
It’s the first time scientists have detected such a particle just floating along in the atmosphere in 20 years of plane-based observations.
Uranium is the heaviest element to occur naturally on Earth’s surface in an appreciable amount. Normally it occurs as the slightly radioactive isotope uranium-238, but some amount of uranium-235, the kind humans make bombs and fuel out of, occurs in nature. Uranium-238 is already rare to find floating above the Earth in the atmosphere. But scientists have never before spotted enriched uranium, a sample uranium containing uranium-235, in millions of research plane-captured atmospheric particles.
“One of the main motivations of this paper is to see if someobody who knows more about uranium than any of us would understand the source of the particle,” scientist Dan Murphy from NOAA told me. 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.
Murphy has led flights around the world sampling the atmosphere for aerosols. These tiny particles can come from polution, dust, fires, and other sources, and can influence things like cloud formation and the weather. The researchers spotted the mystery particle on a flight over Alaska using their “Particle Analysis by Laser Mass Spectrometry” instrument. They considered that perhaps the signature came from something weird, but evidence seems to point directly at 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.
Wild guess: some country tested an atomic powered aircraft and one had a leak?