Radiation simulation of ⁶⁰Fe in lunar regolith decay

Deep in the history of our solar system, there are moments of violent transformation that leave behind invisible fingerprints. When a massive star reaches the end of its life and explodes as a supernova, it acts as a cosmic forge, creating heavy elements and flinging them across the galaxy. One of these elements is iron-60, a rare and radioactive version of iron that does not exist naturally on Earth in significant quantities. Because this isotope decays at a steady, known rate over millions of years, it serves as a precise clock for astronomers. If a supernova occurred relatively close to our solar system within the last few million years, the debris from that explosion would have rained down on everything nearby, including the Moon. Unlike Earth, where wind, water, and shifting tectonic plates constantly recycle the surface and erase such ancient records, the Moon has been geologically quiet for billions of years. Its dusty surface, known as regolith, acts as a pristine archive, preserving a layer of this extraterrestrial iron that fell from the sky.

A team of researchers from Xizang University and the Xizang Museum of Natural Science recently set out to understand exactly how much of a signal this buried iron sends out. When iron-60 decays, it releases energy in the form of gamma rays, which are high-energy particles of light. The scientists wanted to know if these rays could escape the Moon's dusty surface and travel all the way to Earth, where our telescopes might catch them. To find the answer, they built a detailed computer model that simulates the journey of these gamma rays as they try to punch through the lunar soil. They accounted for the thickness of the dust layer, the density of the soil, and the specific energy of the rays, calculating how many would be absorbed by the ground versus how many would successfully break free into space.

The simulation revealed a stark difference between the low-energy rays and the high-energy ones. The iron-60 decay produces three main types of gamma rays, but the lowest energy type, at 58.6 kiloelectron volts, faces a difficult journey. The lunar soil is very effective at stopping these weaker rays; they are absorbed almost immediately by the dust particles they encounter. The model showed that only the iron atoms sitting in the very top layer of the soil, within about one and a half centimeters of the surface, could contribute to a signal that might escape. Even then, the amount of energy reaching Earth is incredibly faint, measured at roughly 6.55 times 10 to the negative 24 ergs per square centimeter per second. This signal is so weak that it is essentially invisible against the background noise of the universe.

In contrast, the higher-energy rays, specifically those at 1173 and 1332 kiloelectron volts, behave very differently. Because they carry much more energy, they can penetrate deeper into the lunar soil, traveling through up to ten centimeters of dust before being stopped. This ability to travel through the ground allows a much larger volume of the buried iron to contribute to the signal. As a result, the flux, or the amount of energy arriving at Earth, for these high-energy rays is about ten thousand times stronger than the low-energy signal. While this is a significant improvement, the researchers found that the total amount of radiation reaching us is still far too faint for our current instruments to detect. The best space telescopes we have today simply lack the sensitivity required to pick up this specific signal from the Moon.

The study concludes that while the Moon is indeed a perfect archive for preserving the history of nearby supernova explosions, listening to its story through gamma rays is currently beyond our reach. The signal is there, buried in the dust, but it is too quiet for our ears to hear with today's technology. This finding does not mean the search is over; rather, it provides a clear target for future missions. The researchers suggest that if we build new, more sensitive detectors specifically tuned to these high-energy rays, or if we place instruments directly on the lunar surface to avoid the distance and interference of space, we might finally be able to hear the echo of those ancient stellar explosions. Until then, the iron-60 remains a silent witness in the lunar dust, waiting for a future generation of instruments to unlock its secrets.