When a 10-kilometer-wide asteroid hit Earth about 66 million years ago, the ensuing blast annihilated the dinosaurs. When the United States dropped an atomic bomb on the city of Hiroshima, Japan, on 6 August 1945, the ensuing blast killed over 100,000 people.
When the asteroid struck, it turned the rock that it hit into molten glass. That glass, while whizzing through the air as ejecta, formed and solidified into little beads, or spherules, that rained down over parts of the planet around the impact site.
When the atomic bomb detonated over Hiroshima, according to new research published in the journal Anthropocene, it turned parts of the city itself into molten glass. That glass formed spherules that rained down around the city, and the scientists that discovered the beads have called them “Hiroshimaites.”
Mario Wannier, a retired geologist who led the new research, retrieved 17 samples of beach sand from six beaches around Motoujina Peninsula and Miyajima Island in Hiroshima. Wannier initially set out to find the shells of tiny marine organisms because he wanted to study the health of the local marine ecosystem. But when he looked under a microscope at the samples, he saw spherules sitting alongside the shells.
“The main discovery is the elephant in the room that we’ve never seen before,” said Wannier, who explained that he immediately recognized the spherules for what they are, as they look very much like the spherules made by the dinosaur-killing asteroid. Wannier thinks the beads survived for so long without being washed away because the waters around the beaches he sampled tend to be relatively calm.
To turn material into glass—that is, to vitrify it—temperatures need to soar to heights of about 1,800°C, Wannier explained. The atomic bomb, codenamed Little Boy, caused sand and rocks in Hiroshima Bay to reach such hellish temperatures.
The rocks that form Hiroshima Bay are Cretaceous-aged granite, and crystals from this granite, including quartz and feldspar, form the majority of the sand grains at the beaches that Wannier and his team sampled. But these are not the only materials that went into forming the beads.
The spherules also have pieces of Hiroshima itself. Some of the beads, Wannier explains, contain iron and steel—material that likely came from the city’s buildings.
One thing the spherules do not have and which Christian Koeberl, a geochemist at the University of Vienna who was not involved in the work, thinks they ought to have is some kind of radioactivity. Spherules collected from Alamogordo, N.M., the test site of the 1945 Trinity atomic explosion, are radioactive, so one would expect the Hiroshima beads to be similar, says Koeberl.
“The ultimate proof that this really is Hiroshima fallout isn’t there yet,” Koeberl said. “[Wannier and other researchers] make a reasonable interpretation of what they have. Further studies clearly are necessary.”
It could be, Koeberl mused, that the Hiroshima beads, collected several kilometers south and southwest from the hypocenter of the atomic blast, were cast too far afield to acquire much of a dose of radiation.
Although the Hiroshimaites appear to lack radioactivity, this absence does not negate the idea that they formed during the atomic blast. To be sure, researchers need to find and study more spherule samples.
Wannier thinks that there could be more such spherules in sediment either at different sites around Hiroshima or, perhaps, at the only other place where an atomic bomb was dropped on humans: Nagasaki.
—Lucas Joel, Freelance Journalist