Traces of exploding stars litter the Earth and Moon
The stardust cloud may have chilled our planet,
Scraps of exploding stars rained down on the Solar System twice in the past 10 million years, a trio of studies published in April show, possibly chilling the Earth’s climate at the time.
The first two studies, published in Nature, uncovered smatterings of stardust in the Earth’s crust. One layer rained down about 1.7-3.2 million years ago and another eight million years ago. Both coincide with periods of global cooling.
Supporting the evidence that our solar system was showered by stardust, a report in Physical Review Letters found the same signature in moon rocks collected by Apollo missions.
When a massive star explodes at the end of its life, the force of the blast crushes atoms into heavy elements and sprays them into space. One of the signatures of this stardust is iron-60, a rare atom that decays into lighter elements over millions of years. German scientists identified bits of iron-60 in Earth’s crust in the 1990s, but the technology to accurately measure those miniscule amounts was only recently developed.
The ocean floor would be a good place to find iron-60, figured Aaron Wallner, a nuclear physicist at the Australian National University. His team pulled 120 samples from the bottom of the Atlantic, Pacific and Indian oceans.
They separated traces of iron-60 from their samples. Then, using radioactive dating, they pegged the interstellar fallout to two periods: eight million years ago and 1.7-3.2 million years ago.
The next step was to figure out where the supernovae exploded. To answer that question, Dieter Breitschwerdt from the Berlin Institute of Technology and colleagues modelled how iron-60 is transported across space following a stellar explosion. They found that the amounts of iron-60 in the samples corresponded to two supernovae that exploded about 300 light years from Earth within a region of our galaxy called the “Local Bubble” – a one-time hotbed of exploding stars just a cosmological stone’s throw away. The two stars were each about nine times the size of our Sun.
While the researchers found traces of only two star blasts, they suspect there could have been as many as 16, based on the time span covered by the iron-60 samples. “We were very surprised that there was debris clearly spread across 1.5 million years,” from 1.7 to 3.2 million years ago, says Wallner, who is lead author of one of the Nature papers. “It suggests there were a series of supernovae, one after another.”
STARDUST ON THE MOON —
If Earth was bombarded with iron-60, the Moon must have been pelted too, since it’s so close, researchers thought. To find out, Leticia Fimiani of the Technical University of Munich in Germany and colleagues decided to look for iron-60 in lunar samples from the Apollo missions in the late 1960s and early 1970s.
Sure enough, they found a spike that coincides with the iron-60 in ferromanganese crusts on Earth laid down between 1.7 and 2.6 million years ago.
And it turns out that the Moon preserves iron-60 better than Earth. That’s because Earth has an atmosphere, wind and water, all of which may have dispersed and diluted any iron-60 flung the planet’s way. The relatively atmosphere-free Moon “may retain a more concentrated signal”, Fimiani and her team write.
SUPERNOVAE COOLING? —
So how might these exploding stars have affected life on Earth? According to one theory, all the atomic debris floating through Earth’s atmosphere may have boosted cloud formation, ushering in a cool period in the planet’s climate history. “It’s an interesting coincidence” that a supernova event occurred during a period of glaciation — and the rise of modern humans, says Wallner.
Could an even closer supernova cause a mass extinction? German palaeontologist Otto Schindewolf thought so. In the 1950s, he suggested a nearby supernova caused the end-permian mass extinction that wiped out more than 90% of species 250 million years ago. Evidence of such an old blast would probably be too decayed to detect with current technology.
But it’s just a matter of time before a new shower of supernovae debris falls to Earth. Researchers have calculated that a supernova within what’s known as the “kill zone” (about 26 light years’ radius from Earth) should happen once every 800 million years or so. Hopefully we should be right for a while yet.