© NASAA NASA WB-57 plane, like the one that located the mystery particle.
On 3 August 2016, 7km 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 somebody 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 such as 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.
Comment: A few days ago spectacular polar stratospheric clouds were captured over Peru. It is likely that atmospheric dust loading from increased comet and volcanic activity is contributing to the 'strange skies' we are witnessing, the cooling effect of which causes ice crystals to form. See also: