Exploding stars may have caused mass extinction on Earth, study shows


Date:
August 18, 2020
Source:
University of Illinois at Urbana-Champaign, News Bureau
Summary:
Killer cosmic rays from nearby supernovae could be the culprit
behind at least one mass extinction event, researchers said, and
finding certain radioactive isotopes in Earth's rock record could
confirm this scenario.



FULL STORY ========================================================================== [Devonian landscape | Credit: (c) nicolasprimola / stock.adobe.com]
Devonian landscape concept illustration (stock image).

Credit: (c) nicolasprimola / stock.adobe.com [Devonian landscape |
Credit: (c) nicolasprimola / stock.adobe.com] Devonian landscape concept illustration (stock image).

Credit: (c) nicolasprimola / stock.adobe.com Close Imagine reading by
the light of an exploded star, brighter than a full moon - - it might be
fun to think about, but this scene is the prelude to a disaster when the radiation devastates life as we know it. Killer cosmic rays from nearby supernovae could be the culprit behind at least one mass extinction
event, researchers said, and finding certain radioactive isotopes in
Earth's rock record could confirm this scenario.


==========================================================================
A new study led by University of Illinois, Urbana-Champaign astronomy and physics professor Brian Fields explores the possibility that astronomical events were responsible for an extinction event 359 million years ago,
at the boundary between the Devonian and Carboniferous periods.

The paper is published in the Proceedings of the National Academy of
Sciences.

The team concentrated on the Devonian-Carboniferous boundary because those rocks contain hundreds of thousands of generations of plant spores that
appear to be sunburnt by ultraviolet light -- evidence of a long-lasting
ozone- depletion event.

"Earth-based catastrophes such as large-scale volcanism and global warming
can destroy the ozone layer, too, but evidence for those is inconclusive
for the time interval in question," Fields said. "Instead, we propose
that one or more supernova explosions, about 65 light-years away from
Earth, could have been responsible for the protracted loss of ozone."
"To put this into perspective, one of the closest supernova threats
today is from the star Betelgeuse, which is over 600 light-years away
and well outside of the kill distance of 25 light-years," said graduate
student and study co- author Adrienne Ertel.



==========================================================================
The team explored other astrophysical causes for ozone depletion,
such as meteorite impacts, solar eruptions and gamma-ray bursts. "But
these events end quickly and are unlikely to cause the long-lasting
ozone depletion that happened at the end of the Devonian period," said
graduate student and study co-author Jesse Miller.

A supernova, on the other hand, delivers a one-two punch, the researchers
said.

The explosion immediately bathes Earth with damaging UV, X-rays and
gamma rays.

Later, the blast of supernova debris slams into the solar system,
subjecting the planet to long-lived irradiation from cosmic rays
accelerated by the supernova. The damage to Earth and its ozone layer
can last for up to 100,000 years.

However, fossil evidence indicates a 300,000-year decline in biodiversity leading up to the Devonian-Carboniferous mass extinction, suggesting the possibility of multiple catastrophes, maybe even multiple supernovae explosions. "This is entirely possible," Miller said. "Massive stars
usually occur in clusters with other massive stars, and other supernovae
are likely to occur soon after the first explosion." The team said the
key to proving that a supernova occurred would be to find the radioactive isotopes plutonium-244 and samarium-146 in the rocks and fossils deposited
at the time of extinction. "Neither of these isotopes occurs naturally on
Earth today, and the only way they can get here is via cosmic explosions,"
said undergraduate student and co-author Zhenghai Liu.

The radioactive species born in the supernova are like green bananas,
Fields said. "When you see green bananas in Illinois, you know they are
fresh, and you know they did not grow here. Like bananas, Pu-244 and
Sm-146 decay over time.

So if we find these radioisotopes on Earth today, we know they are
fresh and not from here -- the green bananas of the isotope world -- and
thus the smoking guns of a nearby supernova." Researchers have yet to
search for Pu-244 or Sm-146 in rocks from the Devonian- Carboniferous
boundary. Fields' team said its study aims to define the patterns
of evidence in the geological record that would point to supernova
explosions.

"The overarching message of our study is that life on Earth does not exist
in isolation," Fields said. "We are citizens of a larger cosmos, and the
cosmos intervenes in our lives -- often imperceptibly, but sometimes ferociously." Also participating in the study were scientists from
the University of Kansas; Kings College, UK; the European Organization
for Nuclear Research, Switzerland; the National Institute of Chemical
Physics and Biophysics, Estonia; the United States Air Force Academy;
and Washburn University.

The Science and Technology Facilities Council and the Estonian Research
Council supported this study.


========================================================================== Story Source: Materials provided by University_of_Illinois_at_Urbana-Champaign,_News_Bureau.

Original written by Lois Yoksoulian. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Brian D. Fields, Adrian L. Melott, John Ellis, Adrienne F. Ertel,
Brian
J. Fry, Bruce S. Lieberman, Zhenghai Liu, Jesse A. Miller, Brian C.

Thomas. Supernova triggers for end-Devonian extinctions. Proceedings
of the National Academy of Sciences, 2020; 202013774 DOI: 10.1073/
pnas.2013774117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200818142104.htm

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