A nearby supernova 2.5 million years ago

[John Clark]

In the July 16 2021 issue of the journal Science evidence is reported that 2.5 million years ago a supernova occurred between 160 and 320 light years from the Earth, and another rather more distant one occurred 6.3 million years ago. By way of comparison Betelgeuse is 642 light years away. Although it would've been much brighter than the full moon, that ancient supernova was not close enough to cause a mass extinction, but it would have had a noticeable effect on the environment. The  x-rays and gamma rays probably wouldn't have had a major effect but the charged particles emitted by the supernova that would've arrived about 500 years after the light may have. The ozone layer would be depleted by at least 7%, and the amount of ionization in the upper atmosphere would've increased by a factor of 10 and increase the production of nitrogen oxide compounds by 30%. 

This increase in the production of nitrogen compounds  would continue for at least 5000 years, and these compounds would combine with water and fall to the earth as rain and act as an excellent fertilizer that would increase plant growth and so reduce the amount of carbon dioxide in the air and decrease greenhouse warming. It may have been the cause of the Pleistocene cooling known to have occurred 2.5 million years ago. Also, when charged particles from the supernova hit the upper atmosphere they would produce a heavier version of the electron called a "muon" which would be able to reach the earth surface and even into the ocean to a depth of one kilometer which would triple the normal radiation level and possibly increase the cancer and mutation rate.

The isotope iron-60 has been found in sediments dredged up from the northern Pacific, it can only be manufactured in the cores of very massive stars and it has a half-life of only 2.6 million years and decays into nickel-60. By finding the ratio of these two elements in sediments dredged up from the northern Pacific scientists determined that the iron-60 must've arrived on the earth 2.5 million years ago. Iron-60 has even been found in dust from the moon brought back by Apollo missions. They also found Plutonium-244 in those ocean sediments and it  has a half-life of 80 million years and decays into uranium 236. The plutonium uranium ratio is consistent with the 2.6 million year age obtained by the iron nickel ratio.

By using the incredibly precise data about the position and velocity of nearby stars obtained by the Hipparcos satellite, it was determined that 2.5 million years ago there was a rather dense clump of large stars called the "Scorpius-Centaurus OB Association"  less than 300 light years from the earth.  Today that clump contains 79 stars that are large enough to someday end their life as a core collapse supernova, based on that it is statistically estimated that in the last 13 million years about a dozen former members of that clump did indeed end their lives in a supernova. Today that clump of stars is moving away from us.

Statistically speaking, a few supernovas must've exploded less than 30 light years from Earth sometime during its 4.5 billion year history. It has been hypothesized that a major mass extinction at the end of the Devonian period 359 million years ago was caused by a supernova that was 60 light years away or less.

A nearby supernova 2.5 million years ago

John K Clark

[Lawrence Crowell]

It is thought that supernova generate cosmic rays, and some of these have really high gamma values. These can be up to  γ = 10^9. So potentially cosmic rays should have started arriving very quickly after the first light appeared. The bulk of them were low gamma  γ ~ 1.2 or v = .6c or so and arrive later, but there should be some rising flux of cosmic ray particles with decreasing energy.

LC