Billions of years of cosmic history lie hidden on the Moon’s surface, churned up by countless tiny impacts. A new mathematical model is helping scientists decipher this data, and their findings could reveal whether Earth was recently showered with debris from a dying star.
When a star explodes as a supernova, it ejects a wave of matter into space—including rare radioactive atoms capable of traveling many light-years through interstellar space to settle on planets and moons. Scientists have already detected traces of one such atom—a radioactive isotope known as iron-60—in deep-sea sediments on Earth and in soil samples brought back by Apollo astronauts.
These traces point to at least two episodes where debris from nearby supernovae reached Earth: a strong pulse roughly 2.3 million years ago and a weaker one about 7.3 million years ago. However, interpreting this data is challenging because the lunar surface has been constantly churned by meteorite impacts—both large and small—over billions of years.
A team of researchers has developed a model that accounts for this mixing process—known as “impact gardening”—more comprehensively than previous efforts. The study, published in the journal Physical Review Letters, predicts where radioactive atoms from supernova debris should be hidden within the lunar soil today.
Unlike Earth, the Moon lacks weather, plate tectonics, or oceans to erode and recycle its surface, making it an extraordinary archive of everything that has ever passed through this corner of the galaxy. Meteorite impacts—ranging from microscopic dust grains to building-sized rocks—constantly churn the upper layers of lunar soil, burying some material deeper while bringing other material to the surface.
Previous models treated this process as simple random mixing, but detailed measurements of Apollo core samples revealed distinct boundaries and layered structures that could not be explained by a simple mixing model alone.
Their new model views the process of “impact gardening” as a competition between two forces: the slow, steady downward movement of surface material caused by repeated impacts, and the more chaotic, random mixing caused by smaller impacts. When tested against actual data from core samples collected during the Apollo 15, 16, and 17 missions—spanning over 400 million years of lunar history—the model’s predictions matched the observations precisely.
After validating the gardening model, the team applied it to one of the most striking discoveries in planetary science: the detection of iron-60 in Apollo soil samples at concentrations far exceeding what could be explained by ordinary cosmic processes. This strongly suggests that supernova debris fell onto the lunar surface around the same time it was deposited in Earth’s oceans.
One question was whether iron-60 levels depended on the amount of ordinary iron already present in the local rock. Although iron content varies significantly among Apollo landing sites, core samples from the Apollo 15 and 16 missions show similar iron-60 abundances, despite this difference. The article describes this as evidence that “supernova dust deposition is independent of native iron content,” which aligns with the relatively uniform distribution of dust across the latitudes where the Apollo missions landed.
This uniformity sheds light on how the debris entered the Solar System. Had it arrived from a single direction, the quantity would likely have varied across different lunar latitudes. Instead, the consistent level of debris at various locations suggests it was evenly distributed—possibly scattered by interstellar magnetic fields before even reaching the Solar System.
Iron-60 is not the only atom of interest. Researchers also detected a radioactive form of plutonium—plutonium-244—in deep-sea ocean sediments on Earth; its much longer half-life allows it to serve as a record of stellar events that occurred in the more distant past.
Plutonium-244 is of particular interest because it is produced via the r-process—an extreme form of element formation believed to occur during exceptional supernovae or neutron star collisions known as kilonovae. It remains an open question whether it shares a common origin with the iron-60 pulses or resulted from a separate, and perhaps more exotic, event.
Using their “gardening” model, the researchers simu They modeled three scenarios for the arrival of plutonium-244: the same two pulses associated with the detection of iron-60, a slow influx over the last 10 million years, or accumulation over 80 million years. Discrete pulses would leave the signal concentrated in the top 10 centimeters of soil, whereas 80 million years of accumulation could shift it to a depth of about 100 centimeters; this means a sufficiently deep core sample could reveal which scenario actually occurred.
The researchers also modeled several other radioactive atoms bearing the same cosmic signature. Detecting them at the appropriate depth could help determine whether the source was a supernova explosion, a collision between two neutron stars, or a combination of both.
NASA’s Artemis program plans to return astronauts to the Moon and collect new samples, including from the previously unvisited South Pole. The collection sites from the Apollo program are located too close together to determine the direction of the material’s arrival. A sample from the South Pole—located much further away—could help verify whether the debris came from a single direction, although interstellar magnetic fields might have distorted the signal long before it reached the Moon.
Future missions should prioritize collecting core samples to a depth of at least 100 centimeters, as shallow sampling would result in the loss of crucial data, and contamination could skew measurements of these extremely rare atoms.
Stellar radioactive fallout has rested undisturbed in the lunar soil for billions of years. Scientists finally have a plan to detect it.
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