Thanks to ranjani for the link.

Reposted from:
http://www.firstscience.com/home/articles/space/gamma-ray-wipe-out_31878.html

A massive blast of radiation from space once almost sterilized the Earth

Gamma-ray bursts: the biggest, most powerful explosions in the Universe since the Big Bang. They are a million trillion times brighter than the Sun. Each day, we detect a burst of gamma radiation coming from a random direction in the sky. Lasting anywhere from a fraction of a second to several minutes, these high-energy astronomical enigmas have a deadly power.


The monitoring of nuclear-weapon tests led researchers to the biggest bangs in the Universe

Until very recently, we really didn't know very much about them. Gamma-ray bursts were only discovered in the late 1960s, quite by accident. During the Cold War, the US military sent up satellites to detect signs of illegal nuclear testing by the Soviet Union. These Vela satellites were fitted with gamma-ray detectors, since nuclear explosions release copious amounts of this extremely energetic radiation. The American military was surprised that their satellites instead detected great explosions of gamma-ray photons coming from space.

So what are these gamma-ray bursts? How are they formed, and where do they come from? Over the past 15 or so years, satellites and ground-based observations have provided a mass of data that has helped scientists to answer these questions.

Scientists have identified two kinds of gamma-ray burst (GRB): short duration (average 0.3 second) and long duration (average 30 seconds). Experts suspect that these form in totally different ways. Although they are fairly confident they understand the origin of long-duration GRBs, short duration GRBs remain a puzzle.

Long-duration bursts

Long-durations GRBs come from a particular kind of star known as a "Wolf-Rayet". Wolf-Rayet stars are huge and feisty, very hot and very heavy. They spit out a huge amount of stellar material. There are Wolf-Rayet stars in our Galaxy, but the culprits that produce GRBs sit at the most distant edges of the Universe, so far away that we can't see them twinkle, even using the most powerful telescopes.


Hypernova: the origin of the most dangerous gamma-ray bursts?

We can, however, see them when they die. When a Wolf-Rayet star reaches the end of its life, it explodes as a massive supernova (a 'hypernova') before collapsing into a black hole. In the explosion, a super-heated blast of energy from the core rips through the body of the star at almost the speed of light. Gamma rays form when the blast wave smashes into stellar material. They rush outward until they explode from the star's surface. The blast wave follows closely behind, colliding with gas and dust to produce photons of all wavelengths, from X-ray through visible to radio. This is the dramatic supernova 'afterglow' we see from Earth.

Gamma-ray bursts take billions of years to reach Earth, even though they travel at the speed of light - 186,000 miles per second (300,000 km/s). The GRBs we observe today may have originated from a star that exploded when the Earth was half its present age.

What would happen if a star within the Milky Way erupted as a gamma-ray burster? Astrobiologist Adrian Melott from the University of Texas believes that this did happen; and the consequences were devastating.

Working with paleontologist Bruce Lieberman from the University of Kansas and others, Melott believes he has evidence that the second largest mass extinction of all time was a result of a gamma-ray burst.

How a gamma-ray burst could exterminate life on Earth

The Ordovician extinction occurred around 440 million years ago, when Earth was a water-world teeming with life. Lieberman describes the planet in those times: "If an alien from outer space was visiting they would probably characterise life on Earth as dominated by trilobites." Trilobites were a highly diverse family of crab-like creatures. In the earliest mass extinction event, they died out - along with two-thirds of all other species.


Trilobites once ruled the Earth: then, 440 million years ago, they were totally wiped out

For years, scientists thought that this wipe-out was caused by a global ice age. However, there were problems with this hypothesis: there was seemingly no trigger to this sudden big freeze. Lieberman was unconvinced that an ice age alone could have killed so many species: "We have had other glaciations in the geological past that do not produce mass extinctions on scale that we see at the end of the Ordovician. So that's why we can't treat it as a smoking gun."

Working together, Melott and Lieberman pieced together an alternative theory, trying to work out what might have happened. They believe that, around 440 million years ago, a supernova within 10,000 light years of Earth exploded in a gamma-ray burst. The radiation destroyed the composition of the Earth's atmosphere, ripping apart the nitrogen and oxygen molecules. The planet became enveloped in brown smog - made up of lethal nitrogen dioxide - which blocked out the Sun's heat. Though the gamma ray burst lasted just a few seconds, the resulting smog triggered an ice age.

The nitrogen dioxide cloud blocked the Sun's infrared rays, but it didn't obstruct the shorter wavelengths of the ultraviolet. This was doubly bad news for the Earth, because the gamma-rays had also produced nitrogen oxide which destroyed the ozone layer. As a result, our planet's surface had no protection from a 300% increase in UV radiation. The Sun's deadly ultraviolet radiation ripped through the DNA of every living creature. Only those living deep underwater stood a chance of surviving.

Melott believes that the Earth has probably been fried by gamma ray bursts several times in its 4.5 billion year history. Could it happen again? Melott thinks so: "It could happen tomorrow or it could be in millions of years." Is it worth worrying about? No. Gamma-ray bursts hit without warning. We can't predict when or where they come from. And we can't do a thing to stop them.

For more information

Hooper Virtual Paleontological Museum - mass extinctions
http://park.org/Canada/Museum/extinction/tablecont.html

NASA - the low-down on gamma-ray bursts
http://imagine.gsfc.nasa.gov/docs/science/know_l1/bursts.html