Freaked out about the prospect of global warming? A few hundred million years ago, the planet warming by a few degrees might have been the least of your problems. As National Geographic reports, a Harvard researcher and his team have uncovered new evidence that corroborates the "snowball Earth" hypothesis. The magazine goes so far as to say the findings confirm that hypothesis, which would mean our world was once a big ball of ice.
Study leader Francis Macdonald, an Earth scientist at Harvard University, and colleagues worked with volcanic rocks in Canada that were found sandwiched between glacial deposits. Such deposits are recognizable by the presence of debris left behind by melting glaciers and sediments deformed by glacial movement.
Using extremely precise uranium-lead mass spectrometry, the researchers determined that both the volcanic rocks and glacial sediments were deposited about 716.5 million years ago—during the purported snowball-Earth period.
The team then matched their findings to previous magnetic studies that had found these rocks had formed when Canada was situated near the Equator.
As I understand it, the hypothesis says "snowball Earth" was precipitated by the combination of a regular ice age and the positions of the continents at the time. As more and more ice accumulated on the surface, more and more of the Sun's energy was reflected back into space, and eventually, the planet reached a tipping point where it became cold enough for the poles to freeze, as well. The ice only melted millions of years later, after volcanoes pumped massive amounts of carbon dioxide—hundreds of times current levels—into the atmosphere and caused a formidable greenhouse effect.
For a more long-winded explanation, a BBC Horizon special about the subject can be found on YouTube.
As Macdonald points out, primitive life could have survived global glaciation: "modern-day ice cracks off Antarctica are 'chockablock' with single-celled life-forms," he told National Geographic. Earth may not have been an immaculate ball of white ice, either. Macdonald believes "dusty messes" created by volcanic eruptions resulted in more of a "mud ball," and darker parts of the planet might have captured more of the Sun's energy and served as havens for surviving life.
Perhaps, long ago, Earth wasn't too different from Jupiter's icy moon Europa—minus the massive cracks caused by Jupiter's tidal forces.