The more results it delivers, the more surprises it reveals. That pretty much sums up the outcome so far of the AMS experiment – a space-based detector that was assembled at CERN and has been detecting electrically charged particles from outer space, known as cosmic rays, since 2011. And, surprise, surprise, the latest result from the experiment, described in a paper published in Physical Review Letters, is no exception. The new result shows that the properties of iron nuclei – the most abundant primary cosmic rays beyond silicon nuclei and the heaviest cosmic rays measured by AMS until now – are surprisingly different from those of other heavy primary cosmic rays.

Historically, cosmic rays are classified into two classes, primaries and secondaries. Primary cosmic rays are produced in supernovae explosions in the Milky Way and beyond, whereas secondary cosmic rays are produced by interactions between the primary cosmic rays and the interstellar medium. But an AMS study from last year revealed that, contrary to expectations, primary cosmic rays have at least two distinct classes, one made of light nuclei and another made of heavy nuclei. And now the new AMS study shows that iron nuclei, which are much heavier than any other nuclei measured by AMS so far, belong unexpectedly not to the same class as the other heavy nuclei but instead to the class of light nuclei.

The AMS team arrived at this conclusion using AMS data on the number, or more accurately the flux, of iron nuclei and how this flux varies with rigidity – a measure of a charged particle’s momentum in a magnetic field. Analysing the data in the rigidity range from 2.65 GV to 3.0 TV, the team found that, above a rigidity of 80.5 GV, the rigidity dependence of the flux of iron cosmic rays is identical to the rigidity dependence of the fluxes of the light primary helium, carbon and oxygen cosmic rays, which is different from the rigidity dependence of the fluxes of the heavy primary neon, magnesium and silicon cosmic rays.

“Our results are mind-bending, defying again conventional models of cosmic-ray origin and propagation in the interstellar medium,” says AMS-experiment spokesperson Samuel Ting. “It will no doubt be interesting to see what theorists and modellers make of them.”