About two million years ago, a giant star in Earth’s neighborhood exploded. If australopithecines and other early hominins were prone to skywatching, they surely would have noticed the sudden appearance of a star blazing brighter than the full moon—an eerie bluish beacon that may have even been visible during the day.
At about 300 light-years away, the explosion wasn’t close enough to harm life on Earth. But that doesn’t mean the planet escaped without a scratch: When that star exploded, it spattered our world and the moon with a form of unstable, radioactive iron.
Now, astronomers studying the iron’s decaying fingerprints in seafloor sediments have deduced that multiple supernovas probably exploded between 1.5 and 2.3 million years ago, and they think they know where in the sky the explosions could have occurred.
Two papers published today in Nature describe the events and explain how they may have helped sculpt the Local Bubble, a large, cosmic cavern that envelops the solar system and neighboring stars.
Inside dying stars, churning nuclear furnaces forge the bulk of the elements in the periodic table. The suite of elements blasted outward by a supernova offers a clue about the kind of star that died, which in turn can help astronomers pinpoint when and where the cosmic death occurred.
Trouble is, first they have to find that dust.
“The total amount, if it all fell on your head at once, it would hurt,” says Brian Fields of the University of Illinois. “But spread over the entire Earth? It’s imperceptible.”
In 1999, scientists studying Earth’s deep-sea crusts realized they were seeing minuscule amounts of an unstable form of heavy iron called iron-60, which may have come from inside the belly of a long-dead giant star.
Five years later, studies of the amount and age of iron-60 suggested that the star must have exploded relatively nearby sometime around two million years ago.
New Killer Star
Now, another team has taken a closer look at the iron-60 tucked into deep-sea crusts collected from the Indian, Pacific and Atlantic oceans, as well as related materials called nodules and seafloor sediments. The story those atoms tell is considerably more complex.
For starters, this deep-sea astronomy, as study author Anton Wallner of Australian National University calls it, reveals an older iron signature that may be the result of a roughly eight-million-year-old stellar explosion.
“They’re the first ones to report evidence that there’s more than one supernova,” Fields says. “The evidence there is not as strong [as the two-million-year-old signal], but nonetheless, they see tantalizing evidence for one in the distant past.”
More perplexing is the rate at which iron-60 has accumulated in seafloor sediments over time. As expected, that iron fingerprint is around two millions years old, but it is somewhat smeared, suggesting the iron was deposited over more than a million years.
“Such a signal is much broader than a signal of only one supernova,” says Technical University of Berlin's Jenny Feige. Could that smeared signal be the work of multiple supernovas, or the expanding shell of debris created by eons of explosions? Part of that answer depends on how iron accumulates in sediments, which are stirred up by currents and marine organisms.
“We’re using sea sediments as a telescope,” Fields explains. “You gotta understand your telescope, and this telescope has beasties that live in it. That’s a new one for astronomers.”
Ashes to Ashes
If it’s real, that smeared fingerprint could coincide nicely with the results from the second report in Nature. Rather than considering when iron-60 arrived on Earth, this team of scientists wanted to know where it came from.
The space in between stars is filled with a diffuse curtain of gas and dust. But the solar system and its neighbors sit in a large, hot void known somewhat uncreatively as the Local Bubble. Astronomers think it was carved sometime between 10 and 20 million years ago, when nearby giant stars grew up, exploded, and blasted gobs of billion-degree gas into the cosmos.
Scientists suspect these long-gone stars lived in a nearby stellar cluster known as the Scorpius-Centaurus association, which could also have been responsible for spitting the iron-60 on Earth.
Based on the observed motions of stars, Dieter Breitschwerdt of Technical University of Berlin and his team rewound the clock and figured out where in space the Scorpius-Centaurus cluster would have been several million years ago. Then they populated the cluster based on existing theories and ran simulations that let the stars explode.
They found that young stars in the association could have deposited the iron-60 atoms observed in Earth’s crust. The timing of interstellar transport, as well as the stellar sizes and distances, all fit.
“It’s like putting together various parts of a puzzle and finding that you get a coherent picture,” Breitschwerdt says.
Specifically, the team says the deep-sea fingerprints likely came from multiple supernovas that would have been visible in the constellations Libra and Lupus. The two nearest, responsible for most of the iron signal, include an explosion 2.3 million years ago—when australopithecines and the first members of the genus Homo were still sharing Africa—and one 1.5 million years ago, when the australopithecines had disappeared and Homo erectus was beginning to populate the globe.
However, the team notes that it’s also possible the iron-60 didn’t come directly from discrete explosions. The population of exploding stars could have already carved a bubble in space, with debris from all those explosions accumulating in the bubble’s shell. The iron might have been deposited on Earth as the shell swept over and enveloped the solar system several million years ago. That would make the iron-60 signal from eight million years ago even more mysterious.
“If the Local Bubble shell has really reached us only two to three million years ago, than the older signal has nothing to do with the Local Bubble evolution, and we must find another explanation,” Feige says.
Supernovas going off in the nearby neighborhood might sound a bit alarming. But for a supernova to ruin your day, it needs to explode within roughly 30 light-years of Earth. These recent explosions are “outside of the kill range,” says Fields. They “sort of have to be, because they didn’t kill us.”
Instead, some scientists hypothesize that the explosions’ effects may be more subtle: Highly energetic particles blasted into Earth’s atmosphere could play a role in seeding cloud formation, which in turn could be responsible for large-scale shifts in climate.
That portion of this ancient tale hasn’t quite been worked out. But by using the seafloor as a giant telescope, teams of astronomers are continuing to study the stars and piece together the ways the rest of the galaxy can influence Earth.