Geologists have found evidence that ice covered the whole world 716.5 million years ago.
By analysing ancient tropical rocks that are now found in northwestern Canada, the team concluded this chilly state lasted for at least five million years.
"This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a 'snowball Earth' event," says lead author Francis Macdonald of Harvard University.
According to Enriqueta Barrera, program director in NSF's Division of Earth Sciences, the Sturtian glaciation, along with the Marinoan glaciation right after it, are the greatest ice ages known to have taken place on Earth. "Ice may have covered the entire planet then," says Barrera, "turning it into a 'snowball Earth.'"
The survival of eukaryotes - life forms such as bacteria - suggests that sunlight and surface water remained available somewhere. The earliest animals arose at roughly the same time.
The implication, says Macdonald, is that the sea ice was slushy, leaving patches of open water in some places.
The rocks analyzed by the team showed glacial deposits and other signs of glaciation, such as striated clasts, ice-rafted debris, and deformation of soft sediments.
The scientists were able to determine, based on the magnetism and composition of these rocks, that 716.5 million years ago the rocks were located at sea-level in the tropics, at about 10 degrees latitude.
"Climate modeling has long predicted that if sea ice were ever to develop within 30 degrees latitude of the equator, the whole ocean would rapidly freeze over," Macdonald says. "So our result implies quite strongly that ice would have been found at all latitudes during the Sturtian glaciation."
The team doesn't know exactly what caused or ended this glaciation, but says volcanoes may have had something to do with it - there's evidence of major volcanic activity at the time.
There's a full report in Science.