Moon rocks collected by Apollo missions hint at a neighbouring star exploding two million years ago, showering the Solar System with radioactive particles.
Leticia Fimiani from the Technical University of Munich in Germany and colleagues analysed minute levels of iron in lunar samples. They found 10 times as much of a rare form of iron – a supernova signature – than would be expected from cosmic ray interactions and meteorites.
Some scientists believe such interstellar blasts may affect evolution and extinction on Earth by cooling the planet.
When stars explode as supernovae, they spray vast quantities of heavy elements, such as iron, into surrounding space. If iron, which usually contains 26 protons and 30 neutrons, ends up with four extra neutrons jammed in its nucleus during fission in the stellar explosion, it becomes the unstable iron-60.
Iron-60 decays with a 2.6-million-year half-life, and it's this Fimiani and her colleagues measured in the Moon rocks.
Why Moon rocks? Unlike our biggest satellite, Earth has an atmosphere, wind and water. These may have dispersed and diluted any iron-60 flung its way. The relatively atmosphere-free Moon "may retain a more concentrated signal", they write.
So they analysed the iron-60 levels in rock samples collected from the Moon's surface by Apollo 12, 15 and 16 missions in the late 1960s and early 1970s.
There is a catch, though. Cosmic rays, unrelated to supernovae, bombarding heavy isotopes of nickel on the Moon can turn them into iron-60. And meteorites landing on the Moon may carry iron-60 too.
To determine the proportion of iron-60 produced by cosmic rays, the researchers measured an isotope of manganese – manganese-53 – which is formed when an energetic ray hits an iron-56 atom. But they found very little manganese-53 compared to iron-60: the ratio was around 10,000 to one.
And when they examined meteoritic iron content, they concluded less than a tenth of iron-60 found on the Moon could have come from meteorites.
It's likely this spike in iron-60 "coincides with the iron-60 surplus in ferromanganese crusts [on Earth], with was collected between 1.7 and 2.6 million years ago", they write.
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