In the past year, multiple studies have found evidence pointing to a nearby supernova explosion occurring around 2.5 million years ago–around the same as a minor extinction event on Earth. This explosion may have caused an increase in radioactive particles, or cosmic rays, to hit the young Earth, leading scientists to question the linkages between supernovas, the Earth’s climate, and biota. One of those scientists, University of Kansas cosmology professor, Adrian Melott, is drawing upon research across a number of fields to find the answer. Melott presented his results in April at the Astrobiology Science Conference in Tucson, Arizona.
Lightning From Space, and Red Sprites at Night. Credit: NASA
A LIGHTNING TRIGGER
Supernovas produce cosmic rays, which are charged particles. These particles bombard Earth on a daily basis at different speeds. Scientists think that the supernova linked to the species extinctions was located a mere 150 light years from Earth; thus, the fastest particles would have arrived at Earth’s lower atmosphere within 150 years. Slower particles would have reached our planet gradually, over a handful of years. The effect could have been electrifying.
“Cloud-to-ground lightning is thought to be a side effect of cosmic rays,” Melott said to the crowd in Tucson.
This drawing illustrates air showers from very high energy cosmic rays. Credit: NASA
As cosmic rays raced into the atmosphere, they may have set the sky ablaze with lightning. Lightning is a difficult beast to track, but over the last few decades, several studies have confirmed the link between cosmic rays and lightning.
“I think that [cosmic rays] are by now the prevailing theory for lightning inception,” Ute Ebert said by email. Ebert is the leader of the Multiscale Dynamics research group at Centrum Wiskunde & Informatica (CWI) and a professor of Applied Physics at Eindhoven University of Technology, both in the Netherlands. She and her colleagues published a paper in the journal Physical Review Letters studying the link between cosmic rays and lightning.
According to Melott, cosmic rays from a supernova could have provided the kick required for an increase in lightning on a global scale; however, Ebert’s coauthor, Casper Rutjes, a Ph.D. student at CWI, cautioned that cosmic rays aren’t the only requirement for lightning. Other conditions have to be met, including clouds at the right stage of development. That means cosmic rays wouldn’t trigger lightning from a clear blue sky.
STARTING THE FIRE
Today, lightning is the leading cause of naturally caused wildfires, and Melott said that would have been equally true in the past. According to Mark Flanner, an associate professor of Climate and Space Sciences and Engineering at the University of Michigan, fires can affect climate through a variety of methods, each occurring over different timescales. “Some mechanisms warm, some cool,” he said by email. Thus, fires could have caused climate change that led to the species drop off. So to recap: if cosmic rays ejected from a supernova around two million years ago did, in fact, spark lightning storms here on Earth that in turn, led to forest fires which changed the climate—it’s possible that a supernova explosion could have indirectly caused species extinctions.
This is a mosaic image, one of the largest ever taken by NASA’s Hubble Space Telescope of the Crab Nebula, a six-light-year-wide expanding remnant of a star’s supernova explosion. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
“The thing that’s interesting about the timing of the supernova is that there was an extinction event at that transition, complicated by a climate change to an icehouse climate,” Melott said in his presentation. “This may potentially be a result of a supernova.”
To confirm this hypothesis, more work needs to be done to see if there was an increase in wildfires during the same timeframe that cosmic rays collided with the Earth. Melott and another researcher have unearthed signs of charcoal data during that period that he said during his presentation “provides a good correlation” of wildfires and supernova. Furthermore, around 2.6 million years ago Africa experienced a loss of tree cover and an increase in grassland, which may have also been caused by lightning-driven wildfires.
“We think it’s possible that the cosmic rays may have had something to do with that,” he said.
The researchers are presently working to publish their research.