Digging up Our Galaxy’s History Under the Sea

Title: Sedimentary rocks from Mediterranean drought in the Messinian age as a probe of the past cosmic ray flux

Authors: Lorenzo Caccianiga, Lorenzo Apollonio, Federico Maria Mariani, Paolo Magnani, Claudio Galelli, and Alessandro Veutro

First Author’s Institution: INFN – Sezione di Milano Via Celoria 16, 20133 Milan, Italy

Status: published in Phys. Rev. D [closed access]

Digging up Our Galaxy’s History Under the Sea

Rocks on earth are millions of years old. All this time, they’ve been recording traces of the environment around them, allowing scientists today to uncover the Earth’s history. The authors behind today’s paper show that these rocks also have the potential to elucidate the history of our Galaxy and Solar System by acting as record-keeping cosmic ray detectors. Cosmic rays are charged particles (mostly protons and nuclei) that are emitted from a number of astrophysical processes. Cosmic rays that reach the Earth interact with the upper atmosphere, producing muons that continue traveling to the Earth’s surface and can collide with rocks. These collisions leave permanent lines in the rocks, known as tracks, that can range in scale from a few nanometers to hundreds of micrometers and can be detected using a variety of microscopy techniques. In this way, rocks can keep a record of all the cosmic rays they have encountered.

Cosmic rays can be produced by extreme astrophysical events, like supernovae. Rocks displaying evidence of cosmic rays could indicate the presence of supernovae explosions near our Solar System millions of years ago. But how can we tell when the cosmic rays occurred? Typically, deep water shields rocks on the seafloor from many of the muons produced by cosmic rays, but about 6 million years ago there was an event on Earth known as the Messinian salinity crisis. This led to the desiccation of the Mediterranean Sea for about half a million years, forming minerals like halite (NaCl). The halite was exposed to air (and thus, to cosmic rays) during this time before being submerged in water (and thus, largely shielded from cosmic rays) when the Mediterranean Sea refilled. This suggests that by examining samples of halite from the Mediterranean Sea, scientists can search for tracks left by cosmic rays occurring during the desiccation.    

To demonstrate this, the authors simulate the number of tracks expected in a halite sample produced during the Mediterranean desiccation in three scenarios. Scenario A assumes no supernova exploded near the earth while the halite was exposed to cosmic rays, so only the regular background of cosmic rays is considered (given by current measurements of this background). Scenario B assumes that in addition to this background, a supernova explosion occurred 20 pc from Earth, while Scenario C assumes the supernova explosion occurred 100 pc from Earth. The flux of cosmic ray muons in each scenario is incident on a cylinder of halite with a radius of 10 m and a height of 10 m. The simulations calculate the differential nuclear recoil rates that arise from the interaction of the muons with the nuclei in the halite as a function of the recoil energy, which is then converted to track rates as a function of track length from the stopping powers of each nucleus. The simulations also model the background effects of cosmic ray muons that end up hitting the halite while it’s underwater, along with the effects of other astroparticles and radioactive decay within the halite or in its surroundings. Alpha particles from radioactive decay can interact with neutrons in the halite, which also produce tracks. Thankfully, this signal is smaller than the cosmic ray signal by at least a factor of ten. The only background that is expected to be of the same strength as the cosmic ray signal is the one coming from the spontaneous fission of ²³⁸U, but this background is only present for tracks of very specific lengths, making it easily distinguishable.   

Figure 1 shows two versions of this simulation: one where the Mediterranean Sea refills at a rate of 30 m/kyr, and one where it refills at a rate of 2.5 m/kyr. The plots show the number of tracks (N) expected for different track lengths (x) for each previously described scenario and the background signals. The cosmic ray signal in all three scenarios dominates the backgrounds from cosmic ray muons reaching the halite while underwater and neutrons interacting with alpha particles. The background due to spontaneous ²³⁸U fission is only present for track lengths between about 1.6×10⁴ and 2.6×10⁴ nm, and for other track lengths, the three scenarios are able to be distinguished from one another.

In this paper, the authors present simulations suggesting that halite produced during the Mediterranean desiccation could store a record of tracks from cosmic ray muons incident on Earth during that time, giving us insight into what was happening around our Solar System at that time. In future work, the authors plan to extend this measurement technique to minerals produced or projected during volcanic eruptions, exposed to air, then submerged by deposits from a later eruption.

Astrobite edited by Caroline von Raesfeld

Featured image credit: Adapted from Wikimedia Commons