Supernovas are amazingly bright explosions of massive stars at the end of their lives. During the gravitational collapse, the outer layers of the star are pushed away, and chemical elements formed inside the star are released into space. This cosmic dust rains down onto the Earth continuously, including exotic elements formed inside the dying star.
Research published in the journal Physical Review Letters used the concentration of two such exotic elements preserved in ocean sediments to hypothesize that a supernova exploded near Earth just 2.5 million years ago.
The authors, led by Dr. Gunther Korschinek from the Technical University of Munich, focused their study on ferromanganese crusts collected in the Pacific Ocean. Ferromanganese crusts form on the bottom of the ocean by layers of iron- and manganese-oxides precipitating out of seawater. The studied samples started to grow some 20 million years ago at depths ranging from 5,200 feet to 3.18 miles (approximately 1.600 to 5.120 meters). The researchers measured the concentrations of iron-60 and manganese-53 isotopes in the hardened crust. They differ from Earth’s most common form of the elements by their varying number of neutrons in the atomic nucleus. Both isotopes are synthesized in large stars shortly before supernova explosions and are unstable, decaying completely after 4 to 15 million years. Their presence in sediment samples is evidence for Earth passing through a cloud of cosmic dust generated by an exploding star in – geologically speaking – recent times.
Research published in August 2020 already noted a peak of iron-60 in sediments dating back 33,000 years, about 2.6 million years, and possibly another at around 6 million years ago. The new research also found elevated concentrations of manganese-53 in a 2.5 million-year-old layer, supporting the idea of a nearby supernova explosion as the source of both isotopes.
Based on the measured concentrations, the researchers calculated that the exploding star was around 11 to 25 times the size of our sun. The researchers also note that the age of the presumed supernova coincides with the end of the Pliocene epoch on Earth, some 2.58 million years ago. The end of the Pliocene is marked by a general cooling trend leading into the ice-age, but also a mass extinction event involving many large land mammals. In theory, a supernova close enough to the Earth (roughly less than 30 to 1000 light-years) could irradiate Earth with a dangerous dose of Gamma rays. The energetic rays could damage and alter the chemical composition of Earth’s atmosphere, exposing the surface to harmful solar and cosmic radiation and triggering a runaway climate change. For now, this scenario is only a hypothesis, even if an intriguing one.