NASA, ESA, J. Hester, A. Loll (ASU)
- Supernovas may have affected the Earth’s climate in the last 40,000 years.
- Tree-ring data suggests supernovas caused spikes in radiocarbon.
- Could the next nearby supernova cause a collapse of civilization?
Dendrochronology is a fancy word for tree-ring dating, where the age of a tree can be determined by the number of growth rings across its trunk. But there’s a lot more to learn from looking at a tree’s rings than simply its age.
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Like fingerprints, tree rings give scientists clues to what the world was like when a tree was alive. By studying tree rings, we can determine when the tree lived, the climatic conditions through which it lived, and possibly, what was happening in the universe at the time.
In a new study published in the International Journal of Astrobiology, geoscientist Robert Brakenridge, of the University of Colorado, suggests a number of supernovas may have left their mark on life on Earth over the last 40,000 years. By poring through countless tree ring records and matching them to known supernova events, Brakenridge discovered that of the eight recent supernovas he studied, each one seemed to leave their mark on trees.
The alarming part? Four of those supernovas may have significantly disrupted Earth’s climate, leading scientists to wonder what the next supernova event might mean for civilization.
Supernovas are brilliant explosions caused by the deaths of massive stars. They’re the most massive and energetic explosions known to science, sometimes shining brighter than the combined light of their galaxies. The overwhelming energy released in such an explosion has caused scientists to worry that a nearby supernova could wipe away life on Earth. But even distant supernovas could pose a risk by damaging Earth’s protective ozone layer, Brakenridge said in a press release.
“These are extreme events, and their potential effects seem to match tree ring records,” he said.
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Brakenridge’s research relies on the scientific art of radiocarbon dating. When carbon atoms in Earth’s atmosphere get struck by cosmic rays from space, they can form a radioactive isotope called carbon-14, or radiocarbon. Some of those carbon isotopes get taken in by plants and animals, leaving a lasting fingerprint that scientists use to date their remains.
When dendrochronologists look at the amount of radiocarbon in tree rings, they expect a steady decline in the isotope as they look at older rings. To their surprise, scientists have discovered a number of cases where the concentration of radiocarbon in tree rings spike. Without any earthly explanations, scientists have looked toward the cosmos for answers.
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Many scientists believe these spikes may be caused by solar activity. Violent solar flares can cause the ejection of plasmas and solar particles that bombard Earth’s upper atmosphere and may explain the spikes in radiocarbon. But a handful of scientists believe the answer lives farther out, beyond the comforts of our solar system.
“We’re seeing terrestrial events that are begging for an explanation,” Brakenridge said. “There are really only two possibilities: A solar flare or a supernova. I think the supernova hypothesis has been dismissed too quickly.”
To dig deeper, Brakenridge created a list of recorded supernovas that occurred near Earth over the last 40,000 years. When he compared these records to the carbon fingerprints left in tree rings, eight of the closest matching supernova events coincided with spikes in radiocarbon.
The last time scientists observed a supernova with the naked eye was in 1604, when Johannes Kepler described SN 1604 (Kepler’s Supernova) in De Stella Nova. And the earliest account of a supernova, some believe, is a stone carving in Burzaham, India. The artwork, estimated to be between 4,000 and 6,000 years old, depicts a hunting scene where two people hunt under a sky with two suggestively bright celestial objects. Some researchers think this may represent a supernova estimated to be as bright as the moon that occurred during the same time period.
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Astronomers are able to record these past explosions by observing the colorful nebulas they leave behind and estimate when they occurred. However, this method of dating supernovas isn’t an exact science, and estimates can be off by as much as 1,500 years.
This goes to show that when looking at some historical records, scientists rarely can be certain. We may not know the real danger a nearby supernova poses to civilization until it happens. Let’s just hope that doesn’t take place for a very, very long time.
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