"ScienceDaily Magazine"

Source: Johns Hopkins University 
Date Posted: Wednesday, January 09, 2002 
Web Address: http://www.sciencedaily.com/releases/2002/01/020109074626.htm

Ancient Supernova May Have Triggered Eco-Catastrophe 

An exploding star may have destroyed part of Earth's protective ozone layer 2
million years ago, devastating some forms of ancient marine life, according to a
new theory presented at this week's meeting of the American Astronomical
Society. 

The new theory brings together puzzling clues from several different fields of
research, including paleontology, geology and astronomy. 

Narciso Benitez, an associate research scientist in astronomy in the Krieger
School of Arts and Sciences at The Johns Hopkins University, says the "missing
smoking gun" that brought the clues together was the revelation that a stellar
cluster with many large, short-lived stars prone to producing supernovae had
passed near Earth's solar system several million years ago. 

That discovery, made by co-author and Space Telescope Science Institute
astronomer Jesus Maiz-Apellaniz, led Benitez to check the scientific record for
potential effects of nearby supernovae on the Earth. 

"Nobody had realized that this cluster of stars that Jesus had tracked, which is
known as the Scorpius-Centaurus OB association, could have been so close to
Earth during the past several million years," Benitez says. "And when I did a
search, one of the first things that popped out was a 1999 finding where a team
of German astronomers led by Klaus Knie detected the presence of a highly
unusual isotope of iron in samples of the Earth's crust drilled from the deep
ocean bottom." 

Knie had proposed that the iron isotope was debris from a recent supernova
explosion that took place close to Earth. But astronomers had no plausible
candidates for such a nearby explosion until Maiz-Apellaniz's work with the
Scorpius-Centaurus association, which is also being presented at this week's
meeting of the American Astronomical Society. 

Benitez compared data produced by Maiz-Apellaniz and Knie's results, and found
"very good agreement, both in the amount of iron deposited and in its time
distribution." 

Benitez consulted with his wife, Matilde Canelles, an immunologist at the
National Institutes of Health who had done her master's thesis on microscopic
algae, to learn if the paleontological record included an extinction that had
unusual characteristics suggestive of a potential link to a supernova. 

"Such an extinction would have had especially pronounced effects on the plankton
and the marine organisms," Benitez explains. 

Canelles pointed out that evidence existed for a widespread extinction of
plankton and other marine organisms about 2 million years ago, and noted that
scientists are still debating the possible causes of the event. 

"Based on the minimal distance we expected for a supernova in the
Scorpius-Centaurus association at that time, I then did some calculations to
explore the potential effects on Earth," says Benitez. 

He found that cosmic ray emissions from a supernova could have had a potentially
devastating effect on the Earth's ozone layer, an upper layer of the atmosphere
that absorbs harmful ultraviolet emissions from the sun and other sources. 

"This would have produced a significant reduction in phytoplankton abundance and
biomass, with devastating effects on other marine populations, such as
bivalves," Benitez says. 

Benitez emphasizes that the theory, while provocative, is consistent with the
paleontological evidence, and also with the pattern of movement of the
Scorpius-Centaurus group, which would have been at its closest to Earth at that
time. 

He concedes, though, that more evidence will be needed to firmly establish the
theory. In particular, more detailed searches for supernova-produced isotopes in
the geological record would show whether there was a tight temporal
correspondence between the supernova explosion and the extinction event. Isotope
searches could also offer crucial information about the physical processes
involved in supernova explosions. 

"People study supernovae using telescopes and supercomputer simulations. In the
future, some of the most relevant information in this field may be found in the
deep ocean floor," says Benitez. 

While the new theory may further heighten concern about human impacts on the
ozone layer today, Benitez and Maiz-Apellaniz say there's no need to worry about
another supernova in the Scorpius-Centaurus group affecting Earth in the near
future. The next star due to explode in the association, Antares, is now located
at a distance of almost 500 light-years, which is too far away to have a
significant effect on our planet. 

This research was funded by an Advanced Camera for Surveys grant from NASA, the
Johns Hopkins Center for Astrophysical Sciences, and a grant from the Space
Telescope Science Institute.

POWRÓT