When a star reaches its final stages of life, it ends in a bright and powerful explosion called a supernova and scatters material across the universe. A new study led by a University of Hawaiʻi at Mānoa researcher discovered that the Moon's mixed-up soil can be read as a cosmic time capsule for exploding stars.
Emily Costello, research scientist at the Hawaiʻi Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), and co-authors developed a mathematical model to decode the history recorded in the lunar surface.
"Deep-sea deposits on Earth preserve interstellar debris, but only back about 10 million years," Costello said. "The lunar regolith [soil], however, acts as a long-term cosmic archive that can preserve history spanning 80 to 100 million years or more. Understanding the physics of regolith mixing ensures that when future astronauts return deeper cores, we can properly read the scrambled layers to reconstruct the history of our Solar System's journey through the galaxy."
Finding order in chaos
Costello and co-authors developed a specialized mathematical model that unscrambles "impact gardening," the continuous process where crater-forming impacts flip, mix and redistribute the Moon's surface soil over time. This is a highly random process driven by meteorites ranging in size from microscopic grains of dust to giant asteroids.
"Our mathematical model treats lunar impact gardening as a competition between forces burying the soil and impacts digging it back up," Costello said. "It also accounts for radioactive decay of the star remnants while mapping exactly when and where new stardust was delivered by episodic supernovas."
Validating the model
Based on radioactive isotopes in deep-sea sediments on Earth and in lunar soil samples returned by Apollo, it is known that supernova explosions hundreds of light-years away scattered radioisotopes across the Earth and Moon. But once the radioisotopes arrived, they started getting mixed into the lunar surface by impact gardening.
The team discovered that the model can accurately predict the depth-concentration profiles of iron found in Apollo regolith samples. They then extended the model to predict how other heavy elements, such as plutonium, iodine, hafnium and curium, are buried over time.
