The Second Snowball

M ORE than 600 million years ago, Earth went through a very long period of extreme cold. This was no ordinary ice age like the one  that ended a few thousand years ago. It involved nothing less that the freezing of the entire planet from the equator to the poles. In Snowball Earth, continents, oceans and all but the highest mountain ranges, would be under a mile or more of ice, and this frigid state of affairs lasted for some ten million years. What could have caused it, and how could it ever have ended? The Sun's brightness was six per cent less than it is today, but this is unlikely to have been the main cause, since Earth had existed for billions of years before that without suffering such a freeze-over.

Alternately, the solar system might have passed through a cloud of cosmic debris that would have blocked the Sun's radiation and induced a period of freezing, an event that is bound to happen every few hundred million years. Or there could have been a relatively sudden and mysterious shortage of carbon dioxide in the atmosphere. This would have halted the world's natural greenhouse effect by which global temperatures maintain themselves in equilibrium. But it is hard to see how this could have happened without the interference of some external cause. And a preference for simple explanations over complicated ones inclines one towards the theory of a cosmic cloud. After all, there are many such clouds in the galaxy. We can see them through our telescopes. For a long time experts refused to believe in Snowball Earth. It was not the beginning of the freeze-over that baffled them but its end. They reasoned that once Earth covered with ice it would remain in that state for ever. It would be a crisis from which there was no escape. Life would never evolve beyond single-celled organisms, and we would not today exist. Then the geologist Californian Joe Kirschvink came up with a brilliant explanation which resolved the matter. Volcanoes, protruding above the level of the ice, would have continued to pour the warming gas carbon dioxide into the atmosphere. There being no rain to wash it away, the gas would have built up slowly but relentlessly, heating the planet and eventually causing most of the ice to melt. Might there exist a twin of Snowball Earth out in the cold, dark regions beyond Pluto? We know of at least two large planetary objects out there the size of Pluto, known respectively as Sedna and Makemake, and there has been unconfirmed talk of a giant planet, larger than Jupiter, out in the distant Oort Cloud. (Such a planet, if it existed, would be 30,000 times further away than the distance between the Earth and Sun, and each of its days would in consequence last six million years!) But an Earth-sized planet beyond Pluto would be a second Snowball. It would be covered with ice because it was so far from the Sun and so cold, and it unlikely that any volcanic emissions would become warm enough to melt the ice. Finding a present-day twin of ancient Snowball Earth is thus an exciting challenge for those who explore the remotest reaches of the solar system..