Radiation From Ancient Supernovae May Have Given Evolution an Astrophysical Push

High-energy cosmic rays mean an increase in cellular mutations, for good and bad.

by Michael Byrne
Jul 11 2016, 1:00pm

Remnant of supernova RCW 86. Image: NASA BlueShift

According to new research by astrophysicists at the University of Kansas, Earth's early biology was tested repeatedly by fierce gusts of cosmic wind originating from twin supernovae some 326 million light-years from Earth. For weeks, the night sky may have glowed with eerie blue light while Earth's animals received radiation doses equivalent to roughly one CT scan for every creature living on land or in shallower water. The cosmic rays would have been enough to ionize the planet's troposphere, possibly contributing to a minor mass extinction linked to global cooling.

The group's work is published in Monday's issue of the Astrophysical Journal Letters. As noted by the paper's lead author, physicist Adrian Melott, the findings were unexpected. "I was expecting there to be very little effect at all," he offered in a statement. "The supernovae were pretty far way—more than 300 light years—that's really not very close."

According to Melott, while the nighttime glow would have persisted for a weeks, the cosmic ray increase overall would have lasted for potentially hundreds or thousands of years and would have boosted ray intensity several hundred times over. "The high-energy cosmic rays are the ones that can penetrate the atmosphere," he said. "They tear up molecules, they can rip electrons off atoms, and that goes on right down to the ground level. Normally that happens only at high altitude."

Melott, who really isn't shy with the catchy claims, suggests that the radiation levels would have likely increased cancer rates in animals and may have even helped increase the rate of mutations friendly to evolution.

If this all sounds a bit familiar, Melott was part of a team that published a much-circulated study in April in Nature describing the discovery of the Earth-bound effects of the two supernovae in question. Their evidence came in the form of core samples extracted from beneath the Atlantic, Indian, and Pacific Oceans. Radionuclides of iron-60 delivered via interstellar dust particles were found to be spread globally, the result of two major fluxes occurring 1.5 to 3.2 million years ago and 6.5 to 8.7 million years ago.

"Could these supernovae have had substantial effects on Earth's climate and organisms—and perhaps even a role in human evolution?" Melott wondered at the time.

The answer to that question, it seems, has been upgraded to "probably." This is according to further work with computer simulations, finding the potential for "substantial effects on the terrestrial atmosphere and biota," according to the study.

The more recent blast of cosmic rays would have aligned well with the Pliocene–Pleistocene boundary marine extinction, which has been suspected of being tied to space weather since researchers first started turning up undersea iron-60 in the late-1990s and early 2000s. Of particular interest is the increase of muon radiation—muons being heavier versions of electrons, basically—on the ground.

"In a first cut at the most probable cases, combining photon and cosmic ray effects, we find that a supernova at 100 [parsecs] can have only a small effect on terrestrial organisms from visible light and that chemical changes such as ozone depletion are weak," the current paper explains. "However, tropospheric ionization right down to the ground due to the penetration of [high-energy] cosmic rays will increase by nearly an order of magnitude for thousands of years, and irradiation by muons on the ground and in the upper ocean will increase 20-fold, which will approximately triple the overall radiation load on terrestrial organisms."

The study concludes by noting that the effects of muon radiation on the ground should be quite different from what are more commonly considered astrophysical radiation events, which center around UV rays and ozone depletion. These effects may be detectable in fossil records, but that remains TBD.