Up 2.5% of the sand on beaches near Hiroshima may in fact be fallout debris from the World War II atomic bomb that devastated the Japanese city.

US researchers have made a detailed study of numerous small glassy spheres found in nearby coastal areas and concluded that there can be only one possible explanation for their origin.

The details of their search and findings are published in the journal Anthropocene.

It all began simply enough back in 2015. Mario Wannier, a retired geologist, was sorting sand samples collected by a colleague to see if they could help him gauge the health of local and regional marine ecosystems on the Moto Ujina Peninsula, at the south end of Hiroshima Port, when he noticed “these extraneous particles” up to a millimetre across.

Some appeared to be fused together, while others had “tail-like features”, and while some resembled those associated with meteorite impacts, others were much less familiar.

Among them were particles with a rubber-like composition or featuring a variety of materials coated in a layer or multiple layers of glass or silica.

Intrigued, Wannier visited the area himself to collect more samples and found that each kilogram of sand contained between 12.6 and 23.3 grams of the spheroids. They thus accounted for between 0.6 to 2.5% of all of the grains examined.

Such consistently high concentrations on beaches six to 12 kilometres from Hiroshima city raised his suspicions that they were related to the atomic bomb blast on 6 August 1945 that destroyed or damaged 90% of structures in the city and eventually killed at least 145,000 people.

“This was the worst manmade event ever, by far,” he says. “In the surprise of finding these particles, the big question for me was ‘you have a city, and a minute later you have no city’.

“There was the question of ‘where is the city – where is the material?’ It is a trove to have discovered these particles. It is an incredible story.”

To find out more, Wannier called in Rudy Wenk, a professor of mineralogy at University of California Berkeley, who first studied the samples using an electron microscope.

He observed a wide variety in their chemical composition, including concentrations of aluminum, silicon and calcium; microscopic globules of chromium rich iron; and microscopic branching of crystalline structures. Some were composed mostly of carbon and oxygen.

“Some of these look similar to what we have from meteorite impacts, but the composition is quite different,” Wenk says. “There were quite unusual shapes. There was some pure iron and steel. Some of these had the composition of building materials.”

The next stage was to take selected samples to Berkeley’s Advanced Light Source laboratory (ALS).

Experiments and related analyses determined that the particles had formed in extreme conditions, with temperatures exceeding 1800 degrees Celsius, as evidenced by the assemblage of anorthite and mullite crystals that the researchers identified.

ALS staff scientist Nobu Tamura notes that the unique microstructure of the studied particles and the sheer volume of melt debris present also provide strong evidence for how they were formed.

“The atomic explosion hypothesis is the only logical explanation for their origin,” he says.

Many of the sphere-shaped particles and other bits likely formed at a high elevation around the rising fireball of the blast. The materials swept up from the ground bubbled and mixed in this turbulent environment before cooling and condensing and then raining down.

Wannier explains the processes that likely formed the materials in an atomic cloud: “The ground material is volatised and moved into the cloud, where the high temperature changes the physical condition,” he says. “There are a lot of interactions between particles. There are lots of little spheres that collide, and you get this agglomeration.”

The researchers also found that the composition of the debris particles corresponds closely with materials that were common in Hiroshima at the time of the bombing, such as concrete, marble, stainless steel, and rubber.

Other studies have analysed melt debris from the Trinity test site in New Mexico – where the first nuclear explosion was triggered – and from underground nuclear test sites in Nevada. But those samples have a distinctly different composition that is associated with their local geological environment.

The Trinity debris is dubbed trinitite, and researchers in the latest study have dubbed the melt particles they studied as Hiroshimaite to highlight their distinct characteristics and their likely origin in the Hiroshima bombing.

“Hiroshimaite particles are much more complex and diverse than trinitites,” Tamura says, owing to their likely genesis in Hiroshima’s urban centre.

Wannier hopes to explore whether the melt debris exhibits similarities to materials associated with volcanic eruptions, and suggests someone could look around Nagasaki, the other Japanese city destroyed by the only two atomic bombs dropping during war.