A Supernova Exploded Dangerously Close to Earth 2.5 Million Years Ago

In its 4.5 billion year history, Earth has had to run the gauntlet. Numerous catastrophes have imperilled the planet, from massive impacts, to volcanic conflagrations, to frigid episodes of snowball Earth. Yet life persists.

Among all of the hazards that threaten a planet, the most potentially calamitous might be a nearby star exploding as a supernova.

Whan a massive enough star reaches the end of its life, it explodes as a supernova. The hyper-energetic explosion can light up the sky for months, turning night into day for any planets close enough. If a planet is too close, it will be sterilized, even destroyed. As the star goes through its death throes, it produces certain chemical elements which are spread out into space.

For years, researchers have puzzled over evidence that a supernova exploded somewhere in Earth’s vicinity a couple million years ago. The evidence is a concentration of ⁶⁰Fe, an isotope of iron produced by supernovae, found around the Earth.

Now, a new study presents additional evidence of a supernova explosion near Earth 2.5 million years ago. This time, its a concentration of ⁵³Mn, another radioisotope produced by supernovae.

The study presenting the findings is titled “Supernova-Produced ⁵³Mn on Earth.” The lead author of the paper is Dr. Gunther Korschinek from the Technical University of Munich. The research is published in the journal Physical Review Letters.

The study is centered on what are called Ferromanganese crusts. They’re made of rock, but look more like chocolate cake. They’re deposits of marine sediments that grow over time, as iron and manganese oxides precipitate out of the seawater. They keep a record of the chemicals in the source water as they form over time. Besides being a potential source of valuable minerals, they’re also valuable evidence for scientists. The team of researchers behind this study examined samples of these ferromanganese crusts and found not only ⁶⁰Fe, but also ⁵³Mn.

The ⁶⁰Fe found on the Earth is potential evidence of a supernova explosion in Earth’s rough vicinity. ⁶⁰Fe is known as an extinct radionuclide. Because its half life is 2.6 million years, any ⁶⁰Fe on Earth should have decayed into Nickel long ago. Finding it now means it was produced in more recent times, in astronomical terms.

But supernovae aren’t the only thing that can synthesize ⁶⁰Fe. It can also be produced by asymptomatic giant branch (AGB) stars. All stars in the low to intermediate mass range (0.6 to 10 solar masses) go through this stage of evolution near the end of their lives. So it’s possible that the ⁶⁰Fe found on Earth came from one of these within the last few million years, rather than a supernova. How can the question of the source of the ⁶⁰Fe be answered?

In their paper the researchers write that “An unambiguous, only SN formed radionuclide, such as 53Mn, detected in the same samples as the 60Fe, can solve this open question.”

Now scientists have found it in the same ferromanganese crusts as the ⁶⁰Fe. Unlike ⁶⁰Fe, ⁵³Mn can’t be produced by AGB stars. It can only be produced by supernovae.

Chocolate Cake? Or Smoking Gun?

“The increased concentrations of manganese-53 can be taken as the “smoking gun” – the ultimate proof that this supernova really did take place,” says first author Dr. Gunther Korschinek, in a press release.

The ferromanganese crusts at the center of this discovery look like some of the moistest, richest chocolate cakes you can imagine. But of course, they’re hard as rock. They are rock.

To find the ⁵³Mn in these curious-looking crusts, the researchers had to go hunting for individual atoms. They employed a method called accelerator mass spectrometry. “A feasible way to detect 53Mn in Earth’s reservoirs is, as in the case of the finding of 60Fe, direct atom counting by accelerator mass spectrometry (AMS),” the authors write in their paper.

That method of mass spectrometry is especially effective at separating a rare isotope from its more common neighbours. In this case, the researchers were looking for ⁵³Mn in the midst of its more common brethren, ⁵⁵Mn. ⁵⁵Mn is the only “naturally occurring” isotope of manganese, and is stable.

It’s not just the discovery of ⁵³Mn that’s significant; it’s the concentration. Some ⁵³Mn is expected to drift down to Earth as cosmic dust. But it’s unusual to find more of it, like the researchers behind this study did. And its presence means there definitely was a supernova explosion in Earth’s vicinity, about 2.5 million years ago.

“This is investigative ultra-trace analysis,” said Korschinek. “We are talking about merely a few atoms here.” Remarkably, the measurements not only detect the presence of ⁵³Mn, but can also help understand the size of the star it came from. “But accelerator mass spectrometry is so sensitive that it even allows us to calculate from our measurements that the star that exploded must have had around 11 to 25 times the size of the sun,” Korschinek added.

That points to a large and extremely energetic explosion. Other than the presence of the supernova-born ⁵³Mn, and the ⁶⁰Fe, what effect did it have?

It was too far away to cause a mass extinction, but it likely did shower the Earth with cosmic rays. That likely affected the climate.

“However, this can lead to increased cloud formation,” says co-author Dr. Thomas Faestermann. “Perhaps there is a link to the Pleistocene epoch, the period of the Ice Ages, which began 2.6 million years ago.”

So while it may not have been ultra-calamitous for Earth, it did have an effect.

Some researchers think that the supernova explosion at that time did trigger at least a partial extinction, called the Pliocene marine megafauna extinction. They point to not only the presence of elevated levels of ⁶⁰Fe, but also to a feature out in space called the Local Bubble. It’s a gigantic, cavernous hole in the interstellar medium caused by one or more supernovae explosions. Now, the discovery of ⁵³Mn just strengthens that hypothesis.

In their work, the researchers looked at samples of ferromanganese crusts of hydrogenetic origin, from four different locations in the Pacific Ocean. Two were from the Midway Atoll, one was from the Donizetti Ridge, and the fourth was from the Central Pacific. The samples ranged in depth from 1589 meters (5213 ft) down to 5120 meters (3.17 miles). Each sampling location yielded 15 samples of increasing depth, for a total of 60 samples.

They compared the concentrations of the supernova-born ⁵³Mn with the more common ⁵⁵Mn. When they graphed the ratios, it was clear that the amount of ⁵³Mn laid down in the ferromanganese crusts was much higher around 2.5 million years ago.

“Since the excess of 53Mn is detected in the same samples and time range in which 60Fe has been identified,” the authors write in their paper, “it confirms the SN origin of that 60Fe.”

So not only do we now know that there definitely was a SN explosion in Earth’s vicinity about 2.5 million years ago, we also have the very first detection of the unstable ⁵⁵Mn isotope.

“Thus, 53Mn is the second radioisotope from the same SN where 60Fe has been detected, and it is for the first time that 53Mn, formed by
nucleosynthesis during a SN, has been found.”

As for the effect that the SN had on Earth, that is the source of great conjecture, study, and debate. Some say it triggered a partial extinction in the aformentioned Pliocene marine megafauna extinction. Others say that it might have helped trigger humanity’s conversion to bipedalism.

While this study doesn’t venture into the effects the SN may have had on life, it does present ever stronger evidence in favor of the supernova explosion.

And while this SN was mostly just a close call for life on Earth, it’s also a good reminder that Earth will be in the suspected kill-zone of a supernova once every 800 million years.

The cosmic clock is ticking.

More:

• Press Release: Stellar explosion in Earth’s proximity
• Published Research: Supernova-Produced 53 Mn on Earth
• Universe Today: A Supernova 2.6 Million Years Ago Could Have Wiped Out the Ocean’s Large Animals