Distance at which supernova would spark mass extinctions on EarthStaff Writer | May 12, 2017
In 2016, researchers published "slam dunk" evidence, based on iron-60 isotopes in ancient seabed, that supernovae buffeted the Earth - one of them about 2.6 million years ago.
Space Like 40 or 50 light years
Melott has followed up since those findings with an examination of the effects of the supernovae on Earth's biology.
In new research to appear in Astrophysical Journal, the KU researcher and colleagues argue the estimated distance of the supernova thought to have occurred roughly 2.6 million years ago should be cut in half.
"There's even more evidence of that supernova now," he said. "The timing estimates are still not exact, but the thing that changed to cause us to write this paper is the distance.
"We did this computation because other people did work that made a revised distance estimate, which cut the distance in half. But now, our distance estimate is more like 150 light years."
A supernova exploding at such a range probably wouldn't touch off mass extinctions on Earth, Melott said.
"People estimated the 'kill zone' for a supernova in a paper in 2003, and they came up with about 25 light years from Earth," he said. "Now we think maybe it's a bit greater than that.
"They left some effects out or didn't have good numbers, so now we think it may be a bit larger distance. 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. So, an event at 150 light years should have some effects here but not set off a mass extinction."
In addition to its distance, interstellar conditions at the time of a supernova would influence its lethality to biology on Earth.
"Cosmic rays like to travel along magnetic field lines," Melott said. "They don't like to cut across magnetic field lines as they experience forces to stop them from doing that.
"If there's a magnetic field, we don't know its orientation, so it can either create a superhighway for cosmic ray, or it could block them
"The main interesting case did not assume the superhighway. It assumed that much of the magnetic field was blasted out by a series of supernovae, which made the Local Bubble -- and we and the most recent supernovae were inside. This is a weak, disordered magnetic field. The best analogy I can think of is more like off-road driving."
In such a case, the authors think cosmic rays from the supernova at 150 light years would have penetrated to Earth's lower atmosphere.
"This is a much stronger thing," he said. "The cosmic rays from the supernova would be getting down into the lower atmosphere - having an effect on the troposphere.
"All kinds of elementary particles are penetrating from altitudes of 45-10 miles, and many muons get to the ground. The effect of the muons is greater - it's not overwhelming, but imagine every organism on Earth gets the equivalent of several CT scans per year.
"CT scans have some danger associated with them. Your doctor wouldn't recommend a CT scan unless you really needed it." ■