The persistence of life on Earth may depend on massive
explosions on the other side of the galaxy, according to a new theory
that suggests powerful bursts of space radiation could have played a
part in some of our planet's major extinction events.
The explosions — gamma-ray bursts thought
to occur when two stars collide — can release tons of high-energy
gamma-ray radiation into space. The researchers found that such blasts
could be contributing to the depletion of the Earth's ozone layer.
Disruption of the ozone layer lets ultraviolet light filter down to the
surface of the Earth, where it can change organisms by mutating their
genes. Now, researchers are beginning to connect the timing of these gamma-ray bursts to extinctions on Earth that can be dated through the fossil record.
"We find that a kind of gamma-ray burst — a short gamma-ray burst — is probably more significant than a longer gamma-ray burst," study researcher Brian Thomas of Washburn University, in Topeka, Kan., said in a statement. "The duration is not as important as the amount of radiation."
The research will be presented Sunday at the Geological Society of America's annual meeting in Minneapolis.
Bursting out Gamma-ray bursts come in two flavors: a longer, brighter burst and a "short-hard" burst, which lasts less than a second but seems to give off more radiation than a longer burst.
If such a burst were to happen inside the Milky Way, its effects on Earth would be much longer lasting. These bursts of radiation reach the Earth's atmosphere and cause free oxygen and nitrogen atoms to bang together, and some recombine into ozone-destroying compounds called nitrous oxides. Nitrous oxides in the atmosphere are long-lived; they keep destroying ozone until they fall out of the sky in rain drops.
The short bursts may be caused by fender-benders between stars, such as dense neutron stars or black holes colliding. The researchers were able to estimate that such stellar collisions probably happen about once every 100 million years in any given galaxy. At this rate, Earth would have been hit by several of these short-hard events over the course of its 4.5-billion-year history.
Life on Earth Destruction of the ozone layer can have many effects on life on our planet. Radiation blasts on the world's plants and animals could wreak havoc on Earth's food webs and possibly lead to planetwide extinction events
Improved and accumulated data collected by NASA's SWIFT satellite, which catches gamma-ray bursts in action in other galaxies, is providing a better case for the power and threat of the short bursts to life on Earth. Researchers are also looking for evidence of past bursts, including special elements that are created only during radiation events hitting Earth, such as a heavy version of iron.
Thomas is now working with paleontologists to correlate levels of this heavy iron with evidence of extinction events in the fossil records.
"I work with some paleontologists, and we try to look for correlations with extinctions, but they are skeptical," Thomas said. "So if you go and give a talk to paleontologists, they are not quite into it. But to astrophysicists, it seems pretty plausible."
According to their newest models, gamma radiation from a nearby burst would quickly deplete much of Earth's protective ozone layer, allowing increased ultraviolet radiation (UV) from the sun to reach the surface.
In the longer term, chemical reactions in the atmosphere would produce dark, nitrogen-based gases that would block the sun's heat and trigger global cooling, even as the gamma rays continued to deplete ozone and let in UV rays, the authors suggest.
Some of the pollution would fall as damaging acid rain, which can severely disrupt ecosystems.
The atmosphere might be able to recover within a decade, and a rise in DNA damage caused by increased UV exposure might pass after a few months or years, the researchers note.
But other biological impacts—such as reduced ocean productivity—could linger for an unknown length of time, Thomas said.
The Trouble With Trilobites
Bruce Lieberman, a paleontologist at the University of Kansas, helped develop the initial theory about the Ordovician die-off but did not co-author the recent papers.
The prevailing idea is that an ice age caused the extinction event, he said, but he questions the completeness of that hypothesis.
"At other times there have been ice ages without mass extinctions," he said.
Furthermore, the ice age during the Ordovician was comparatively short, lasting only about 500,000 years before the climate cycled back to a warm spell—almost as if something unusual set the icy period in motion.
So far Thomas and Melott have uncovered a pattern of higher UV radiation during the Ordovician extinction that would match cosmic bombardment over the South Pole.
And Lieberman believes the disappearance of trilobites, extinct arthropods related to horseshoe crabs, could be tied to the Ordovician event.
Although most trilobites are mud-scurrying bottom dwellers, the juveniles of some species have a life stage that sends them floating in the shallow water column, making them vulnerable to higher UV radiation.
But like NASA's Thompson, Lieberman adds that worry over a future gamma-ray burst is "not the thing that's keeping me up at night."
Instead he appreciates the new work for pointing out that Earth is a vulnerable part of the cosmos.
"It gives us a new perspective on things like natural selection and adaptation.
No comments:
Post a Comment