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Ancient minerals, formed six million years ago during the drying of the Mediterranean Sea and known as "evaporites," may hold valuable traces of interactions with cosmic rays and reveal important information about the history of our universe. This is the finding of a study published on December 17 in the scientific journal Physical Review D  by a group of researchers from INFN, the University of Milan Statale, the French National Center for Scientific Research (CNRS), and Sapienza University of Rome.

According to the study, analyzing the evaporites could provide an estimate of the cosmic ray flux that impacted the Earth between 5.5 and 6 million years ago. Specifically, by examining the damage caused by cosmic rays in these minerals, evidence of a cataclysmic event, such as a supernova explosion, occurring during that period and at a relatively close distance to Earth—within about a hundred light-years—could be found.

In fact, according to the most widely accepted geological models, about six million years ago, tectonic movements caused the closure of the Strait of Gibraltar, leading to the partial evaporation of the Mediterranean Sea and the formation of rocks (evaporites), mainly gypsum and halite, the latter being crystals of common table salt. These minerals were exposed to air or covered by a thin layer of water until the strait reopened approximately half a million years later, rapidly flooding the basin. During this dry period, the crystals were continuously bombarded by cosmic rays, which may have caused damage to the crystals.

"Our study is the first to propose using natural minerals to observe traces of cosmic rays, and it goes against the current trend in physics research," explains Lorenzo Caccianiga, INFN researcher and lead author of the study. "In recent years, researchers have proposed using natural minerals to search for rare events, such as those potentially produced by dark matter or neutrinos, by extracting minerals from deep underground to avoid cosmic ray exposure. However, it had not yet been suggested to use minerals exposed to cosmic rays to understand how the flux of these particles has changed over time."

This study thus opens a new avenue for investigating major astrophysical events that may have occurred in the past and is of great interest to biology and paleontology as well.

"It would now be interesting to carry out these measurements by collecting samples of these minerals from the depths of the Mediterranean. Moreover, analyzing minerals exposed to cosmic rays during mass extinction events in the same way could help us test some theories that attribute these extinctions to cataclysmic events such as supernova explosions near our planet," concludes Caccianiga. "It may even emerge that a high flux of cosmic rays played a crucial role in these extinctions."