THE idea of a snowball Earth – a period when the planet was completely enveloped in thick sheets of ice – has received a knock. Evidence of prehistoric photosynthetic algae indicates that tropical oceans did not freeze over completely at the time.
Geologists accept that Earth was in a deep freeze twice during a period from about 750 to 580 million years ago. But they disagree over whether it was a snowball or a “slushball” with areas of thin ice or open ocean.
Now, Alison Olcott of the University of Southern California in Los Angeles and her team have found evidence for…
