Did Supernovae Affect the Earth?

Telltale debris suggests effects on atmosphere, evolution of fauna

New studies from international teams of researchers based on chemical isotopes in ocean bed sediments with ages up to 11 million years have shown that the Earth could have been affected by a series of nearby supernovae millions of years ago.

According to the findings of two studies based on sediment cores taken from Pacific, Atlantic and Indian oceans, conducted by researchers from universities at Australia, Austria, Germany, Israel, Japan and U.S. and published in Nature, the Earth came under the bombardment of cosmic rays emanating from a series of supernovae explosions taking place 325 light years away between 3.2 and 1.7 million years ago, and an older event 8 million years ago. Cosmic rays are generally energetic protons and nuclei of heavy elements hurled into space by supernovae.

A close call

It is assumed that cosmic rays from a supernova explosion within 30 light years of Earth would stamp out life in our planet.   Although the supernovae mentioned in the new studies were too far away to cause mass extinctions, they, nevertheless, were close enough to affect the Earth in other ways.

A point to which the researchers draw attention is the fact that the newer of the explosions coincided with a period of cooling which triggered the transition from Pliocene epoch to Pleistocene in the geological history of the Earth. This suggests that cosmic rays from these events could have affected the cloud cover. According to Adrian Melott, professor of physics and astronomy at the University of Kansas, these effects, in turn, could have had repercussions on evolution of humans.

The researchers also point out that the earlier explosion (eight million years ago) has coincided with changes observed in species of fauna across the globe in the Late Miocene epoch.

Recipe for the dough

Supernovae are gigantic explosions which mark the end of stars with masses at least eight times that of the Sun when they exhaust their fuels in short astronomical timescales measured in millions of years.  When the cores of these stars are filled with iron at the end of consecutive series of thermonuclear reactions through which lighter elements are fused into heavier ones under tremendous pressure and temperatures, reactions stop and the core, no longer able to produce the energy that counterbalances the weight of the outer layers, collapses onto itself to form a black hole. And a shockwave triggered by the collapse tears apart the outer layers, or the envelope, and hurls it to space. The dense flux of neutrons from the collapsing core drives the rapid synthesis of elements heavier than iron.  These elements, spreading into space “enrich” the interstellar clouds of dust and gas to end up in the “dough” from which new generations of stars are formed. The Sun is one such second generation star containing heavy elements forged inside the exploded stars.

One of the heavy nuclei formed in the process of supernova explosion is the unstable (radioactive) iron-60 (60Fe)isotope which contains four extra neutrons compared to the ordinary (stable) iron (56Fe)  formed of 30 neutrons and 26 protons. And it is this isotope which the researchers identified in the cores collected from the ocean floor sediments that provides the evidence that the Earth was affected by the suspect supernovae. The half-life of radioactive iron-60 isotope is 2.6 million years. That is, the number of iron-60 isotopes in a sample is halved after 2.6 million years due to the process of decay into lighter stable elements and the process goes on. Hence, the iron-60 content contained in the solar nebula which gave birth to the Sun and its planets about 4. 5 billion years ago should have disappeared by now.

Then, the iron-60 that the researchers  detected in the  ocean  bed sediments in ratios one-quadrillionth that of the ordinary iron abundances, should have been produced in later supernovae.

Cosmic CSI

Painstakingly analysing the evidence with novel methods and supersensitive equipment like sleuths in popular TV series CSI, researchers determined the ages of ocean bed sediments bearing iron-60 isotopes by looking at decay rates of other radioactive isotopes berilium-10 and aluminum-26. The findings show that heavy iron isotopes which came to Earth in cosmic ray fluxes arrived in two distinct epochs; one between 3.2 and 1.7 million years ago, and the other, about 8 million years ago.

According to researchers, the likely sources of heavy iron isotopes are two supernovae going off some 325 light years from the Earth.  The explosions are believed to have taken place in a stellar cluster which has moved further away from the Earth since then.  Absence of massive stars in the cluster now is seen as evidence pointing to the fact that they had gone supernova long ago and had spread their debris into space.