Key Takeaways:

  • A supernova 150 light-years away exploded 2.6 million years ago.
  • The cosmic rays from the supernova might have reached Earth a few hundred years later.
  • This radiation increase could be linked to a mass extinction event in Earth’s oceans.
  • The extinction particularly affected large marine animals in shallow waters.
  • The Local Bubble, a giant cavity filled with hot gas, might have been created by past supernovae and intensified the cosmic ray effect.

The explosive final stages of massive stars are known as supernovae. One such supernova that was located roughly 150 light-years away brightened the sky over Earth 2.6 million years ago. After the new star had long since vanished from the sky, a few hundred years later, cosmic rays from the event finally made it to Earth and collided with it. According to a team of researchers at the University of Kansas led by Adrian Melott, this cosmic onslaught is now thought to be connected to the mass extinction of marine animals, such as the Megalodon, that were then present in Earth’s waters. Astrobiology published their work.

According to a press release from Melott, “supernovae should have affected Earth at some time or another.” It has proven challenging to determine the precise timing or impact of such events in the past, though. However, the group’s paper claims that “a newly documented marine megafaunal extinction” aligns with the arrival of a potentially fatal radiation influx, suggesting that they may be able to link a specific supernova to a specific event.

This composite image shows supernova remnant 1E 0102.2-7219, which lies 190,000 light-years away. The supernova that may have caused a mass extinction on Earth was much closer, only about 150 light-years distant. X-ray (NASA/CXC/MIT/D.Dewey et al. & NASA/CXC/SAO/J.DePasquale); Optical (NASA/STScI)

Nearly 36% of the genera in coastal waters, where radiation penetration would have been higher in shallower water, went extinct as a result of that event, which happened at the Pliocene-Pleistocene boundary. We possess proof of nearby [supernova] events during a particular period. We can actually calculate how that would have affected Earth and compare it to what we know about what happened at that time because we know how far away they were, according to Melott.

Incoming!

The killer radiation came in the form of cosmic rays made up of fast-moving muons, which are a few hundred times the mass of an electron, according to Melott. “They’re very penetrating. Even normally, there are lots of them passing through us. Nearly all of them pass through harmlessly, yet about one-fifth of our radiation dose comes by muons,” he said.

But what about under abnormal conditions, such as the wave of material from a supernova? Increase the number of muons by a few hundred when this cosmic ray wave attacks. Melott said that although very few of them will interact in any way, the high energy and vast number of them leads to an increase in mutations and cancer. The team calculated that the cancer rate would rise by roughly 50% in human-sized animals based on the rates of muons that Earth would receive from the stellar explosion. But in larger animals, that effect would have also been larger. “For an elephant or a whale, the radiation dose goes way up,” he said. And because high-energy muons can penetrate hundreds of yards into water, they could have peppered the coastal waters where the extinctions occurred, essentially targeting the animals that lived there for death.

Our Local Bubble is of a bubble of hot, diffuse gas that was likely generated by one or more supernovae. NASA; modified from original version by Wikipedia User Geni

Finding the Source

Identifying the potential cause of the radiation wave was the remaining puzzle piece. With a half-life of roughly 2.6 million years, iron-60 is a radioactive isotope of iron that has long since vanished from the Earth. Therefore, iron-60 could only have survived to this day if it had come from space, perhaps by “raining down” in the form of a supernova wave. Additionally, a massive iron-60 spike that was deposited roughly 2.6 million years ago suggests that material from a supernova event reached Earth at that time.

As for where that supernova came from, our Sun sits inside what astronomers call the Local Bubble. The region of the interstellar medium (ISM) that lies between stars is largely empty. A 300 light-year-wide area of hot, diffuse gas surrounded by cold, dense gas from the “regular” ISM is known as the Local Bubble. Astronomers believe that the numerous bubbles in our galaxy, including our own, were created by supernovae, whose energy has the ability to heat anything that is left behind after they sweep away matter.

A series of supernovae, one of which detonated very near to Earth 2.6 million years ago, may have been the cause of the Local Bubble, leaving behind that layer of radioactive material. And the Local Bubble itself could have exacerbated the amount of cosmic rays Earth received, increasing the deadliness of such events. Melott thinks that the bubble’s boundaries may have reflected cosmic rays back when they struck them, resulting in a 10,000–100,000 year “cosmic-ray bath” for each supernova. According to him, cosmic rays could bounce around the Local Bubble for millions of years if a series of supernovae explode relatively close to one another over time.

All of this comes down to a tantalizing link between an unexplained major extinction event and the supernovae that clearly changed our local region of the galaxy. “There really hasn’t been any good explanation for the marine megafaunal extinction,” Melott concluded. “This could be one.”

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