August 2, 2005 5:15 PM PDT
Bacteria froze the Earth, researchers say
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Several graduate students, along with supervising professor Joe Kirschvink, have released a paper presenting their explanation of what caused "Snowball Earth," a periodic deep freeze of Earth's atmosphere that has been theorized for years. The Caltech team argues that 2.3 billion years ago, cyanobacteria, or blue-green algae, gained the ability to break down water, which in turn released a flood of oxygen into the atmosphere.
That oxygen reacted with the atmospheric methane, which insulated the Earth at the time, and broke it down. While the oxygen-methane reaction created the greenhouse gas carbon dioxide, the protective nature of the barrier cracked.
Temperatures plunged to minus 50 degrees Celsius, and ice at the equator grew to 1 mile thick. Although this process took several million years, substantial damage to the methane layer could have occurred in the first 100,000 years.
Life-forms only recovered after microorganisms, clinging then to thermal vents or living underground, evolved the ability to consume oxygen and turn it into carbon dioxide.
It was a close call to a planetary destruction," said professor Kirschvink, who oversaw the project, in a prepared statement. "If Earth had been a bit further from the sun, the temperature at the poles could have dropped enough to freeze the carbon dioxide into dry ice, robbing us of this greenhouse escape from Snowball Earth."
Carbon dioxide concentrations became so high over millions of years that the temperature soared to 50 degrees Celsius. As the solar system aged, Earth's mood swings became less extreme.
Scientists have known for a number of years that Earth plunged into one of its periodic ice ages 2.3 billion years ago, a change reflected in glacially formed rocks in Canada. The cause, however, has been the subject of debate. Many experts have said that cyanobacteria evolved to break down water between 3.8 billion and 2.7 billion years ago, too early to cause the deep freeze.
The Caltech team believes the answer lies in the movement of glaciers at the time around the middle latitudes of the planet, which in part is borne out by magnetic readings of the Canadian rocks. The glaciers scraped iron, phosphorous and other nutrients off existing rocks. These ran into the ocean and provided food for a massive algal bloom.
"We could still go into Snowball if we goof up the environment badly enough," Kirschvink said. "We haven't had a Snowball in the past 630 million years, and because the sun is warmer now it may be harder to get into the right condition. But if it ever happens, all life on Earth would likely be destroyed. We could probably get out only by becoming a runaway greenhouse planet like Venus."
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