SPACE dust found deep below central Africa has boosted the controversial idea that Earth was once completely sheathed in ice.
The 鈥淪nowball Earth鈥� theory claims that our planet was entirely frozen over about 630 million years ago. Most geoscientists agree that the Earth cooled significantly at that time, but some favour a 鈥淪lushball Earth鈥� scenario in which equatorial oceans did not freeze.
Bernd Bodiselitsch of the University of Vienna in Austria and his colleagues evaluated the two theories by measuring iridium levels in three cores drilled from central Africa. Iridium is a component of the space dust that continually falls on Earth. They predicted that if the Earth was completely frozen the iridium would have accumulated on top of the ice and precipitated in one go when the ice melted. Otherwise, it would have settled gradually on the equatorial sea floor.
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The researchers found a sharp spike in iridium levels in the cores that date to the end of the cooling period, favouring the snowball theory. The size of the spike suggests a global glaciation lasting between 3 and 12 million years (Science, vol 308, p 239).
