Millions of years ago, multiple stars located some 325 light years from the Earth burst into spectacular supernovae. Though our planet was well beyond the 30-light-year kill zone of these explosions, stellar shrapnel was blown out with such force that it dusted the Earth with radioactive iron-60 isotopes centuries later.
Now, in new research published Wednesday in Nature, two international teams of scientists report that this isotopic signature has been discovered in 120 core samples extracted from the Pacific, Atlantic, and Indian Oceans. The find offers concrete proof that the fallout of ancient supernovae has been deposited on Earth, and may have even influenced our planet's recent geological and biological history.
"It is now possible to ask key questions with some precision," said Adrian Melott, an astrophysicist at the University of Kansas, in a Nature statement on the new studies. "For example, could these supernovae have had substantial effects on Earth's climate and organisms—and perhaps even a role in human evolution?"
Indeed, according to the timelines laid out by the teams, there could be some interesting correlations between these prehistoric supernovae and events on Earth. The core samples suggest that there were at least two explosions, one that occurred about 6.5 to 8.7 million years ago and a more recent detonation that went down about 1.7 to 3.2 million years ago.
The latter event lines up with some significant shifts in evolutionary history. The giant ocean predator Megalodon was dying off in the seas, while the first anatomically modern humans were dominating ecosystems on land. It was also a time of cooling temperatures that would eventually usher in the most recent Ice Age.
"This climatic variation may be one of the conditions that led to human evolution," Melott said, "although we do not know if there is a link between supernova activity and colder temperature."
"Ionization of the atmosphere by supernovae may also lead to an increase in lightning and possibly other climatic effects," he continued. "The new studies will open up unexplored avenues of modelling and detailed investigation, providing deeper insight into what might have happened on Earth over the past 10 million years as a result of nearby stellar fireworks."
It's mind-boggling that the ashes of distant dead stars can rain down on our planet, scrawling epitaphs into the geological record in isotopic ink. It doesn't get more vintage Carl Sagan than that.