Cosmic Surprise: Iron Behaves Differently ✎ express
In the stream of charged particles flying toward us from the depths of the Galaxy, iron turned out to be the odd one out. The ATIC-2 instrument, lifted on a balloon over Antarctica, compared how the fraction of nuclei from sulfur to chromium relative to iron changes with energy. Up to an energy of about 80 GeV per nucleon, everything went by the book: the higher the energy, the fewer heavy nuclei compared to iron. But above 100 GeV per nucleon, the curves sharply turned upward, and this was true for all elements from sulfur to chromium.
The oddity was not limited to heavy nuclei. For light even nuclei (carbon, oxygen, neon, magnesium, silicon) they also found a rise in the ratio to iron, but weaker. It turns out that the spectrum of iron differs from the spectra of other common even nuclei. Iron seems to come to us from a different place or is born under different conditions.
For now, one thing is clear: cosmic accelerators that boost particles to such energies do not work the same for all elements. Perhaps supernova explosions or other cataclysms eject iron and lighter nuclei in different ways. Or something happens to iron on its way to us — for example, it collides more often with interstellar gas and loses energy. Future measurements will provide the answer, but already it is clear: iron holds some secret of its own, and solving it could change our understanding of how the Galaxy is structured.
An energy of 100 GeV per nucleon is like accelerating each proton in the nucleus to a speed at which its mass increases a hundredfold. Such particles reach us from sources thousands of light-years away.
In Stanislaw Lem's novel "Eden," the heroes encounter a civilization that has learned to control particle streams in space. Perhaps someday we too will learn to read the history of the Galaxy from the composition of cosmic rays, like from a book.