How an Ancient Supernova Turned the Sky on Earth Electric Blue

When it comes to getting the most bang for your buck in terms of doomsday scenarios—like a 2012-style runaway environmental catastrophe or a lethal airborne virus—not much beats the idea of Earth turning into a giant popcorn kernel when faced with the enormous destructive power of a supernova.

When some types of star reach the end of their lifecycles, they collapse in on themselves and explode, producing a burst of radiation that can briefly outshine an entire galaxy. Huge quantities of cosmic rays burst forth, creating a deadly wall of stellar material that screams through space at more than 99.9 per cent of the speed of light. Woe betide any object—like our delicate blue-green orb, for example—that stands in its way.

If a supernova happened to a star close to our home, the results would be catastrophic. We know this because new research shows how, throughout its history, Earth was buffeted by the remnants of supernovae, causing tumultuous changes in the chemistry of our planet and affecting the development of life.

A tour of supernova 1987A. Video: NASA/YouTube

In fact, the last such event was just 2.6 million years ago, a drop in the bucket on evolutionary timescales. (Earth is 4.5 billion years old.)

This most recent event is thought to be the result of an exploding star located a mere 150 light years from Earth. (Our Sun, by contrast, is 150 million kilometres away.) Fortunately for us, this was enough distance to give our planet a bit of a buffer, meaning a mass extinction event was unlikely when our planet was knocked around by those supernova winds. But Earth still took a bit of a licking.

"We think that there's some evidence for increased radiation effects on the ground—not super-severe, but equivalent to every organism having several CAT scans a year," said Adrian Melott, who led the research, in a phone interview with Motherboard from the University of Kansas, where he is a professor. "This is enough to increase cancer rates, but not enough to cause a mass extinction."

"People estimated the 'kill zone' for a supernova in a paper in 2003, and they came up with about 25 light years from Earth," said Melott. "Now we think maybe it's a bit greater than that. We don't know precisely, and of course it wouldn't be a hard cutoff distance. It would be a gradual change. But we think something more like 40 or 50 light years."

Even though this most recent supernova to batter Earth failed to wipe out all life, including the Smilodon, a prehistoric mammal that had just come onto the scene, the cosmic rays it blasted forth caused profound changes to the chemistry of the atmosphere and, potentially, to life. "The cosmic rays from the supernova would be getting down into the lower atmosphere—having an effect on the troposphere," said Melott.

To examine whether their theory held water, Melott and his team examined the fossil record in Africa during the period about 2.6 million years ago. They hypothesized that when the torrent of cosmic rays produced by the gargantuan explosion struck the atmosphere, they knocked electrons out of atoms, creating perfect conditions for lightning.

"Lightning is the number one cause of wildfires other than humans," said Melott. "So, we'd expect a whole lot more wildfires, and that could change the ecology of different regions, such as a loss of tree cover in northeast Africa, which could even have something to do with human evolution. The Great Plains has recently been largely kept grass-covered by a bunch of wildfires. A big increase in lightning would also mean a big increase in nitrate coming out of the rain, and that would act like fertilizer."

Indeed, there is evidence that 2.6 million years ago in northeast Africa, tree cover diminished substantially and grasslands swelled—helped along by extra atmospheric nitrates, giving a boost to the growing grass.

The flood of cosmic rays bathing Earth would have caused the sky to continuously glow blue at night for about a month. As anyone who's stared at the blue light from a smartphone in a dark room can attest, this type of radiation can profoundly affect biological systems, wreaking havoc with sleep patterns. "There'd be a lot of animals that wouldn't be able to sleep. There's a whole lot of research on the effects of blue light on animals," said Melott. "It interferes with their reproduction. There might be some lethality in the animals."

The really important question is not about the past, but the future. What is the likelihood that another supernova will calamitously erupt in our stellar neighborhood?

Alas, for all those of you who need an excuse to live in a buried schoolbus to wait out the apocalypse, the chance of a supernova-mediated catastrophe is vanishingly small. In about seven or eight billion years, our own Sun will, in rather undramatic fashion, begin to simply swell up into a red giant, first baking us and then incinerating us.

The closest potential candidate for a star that could go bang is Betelgeuse, located an eye-watering 600 light years away. If Betelgeuse went out of business, "it's close enough to be spectacular in the sense that it would be bright and you'd see it in daytime, but there'd be no harmful effects," said Melott.

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