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How did complex life survive when the Earth turned into a giant snowball hundreds of million years ago? A bizarre region of 鈥渄irty鈥� Antarctic ice, discovered a century ago by British explorer Captain Scott鈥檚 team, might hold clues.

There is evidence that Earth experienced at least two astonishingly severe ice ages between 717 and 636 million years ago. Some researchers say conditions were so extreme that ice reached the equator 鈥� so our planet effectively became a giant snowball.

If it happened, Snowball Earth creates a puzzle. We know that complex life and perhaps even early animals appeared before the glaciations began. So how and where did these organisms survive when Earth became a frozen snowball?

at MIT, at University of Waikato in Tauranga, New Zealand and their colleagues have found a possible solution.

Dirty ice

It lies in a strange corner of Antarctica explored by Robert Falcon Scott鈥檚 Discovery Expedition between 1901 and 1904. Infamously, Scott later died in Antarctica in 1912 after being beaten to the South Pole.

Scott鈥檚 team found a region of the McMurdo Sound ice shelf that was littered with large deposits of salt and lots of marine animal remains, from sponges to shellfish. It was, according to the team鈥檚 geologist Hartley Ferrar, a 鈥渇reak of nature which is difficult to explain鈥�.

. Ice is constantly being frozen to the base of the ice shelf and evaporated from the surface, so over time each layer of ice moves upwards. Because the ice shelf directly overlies a very shallow sea, animals and seafloor mud are frozen into the base of the ice and carried upwards. They concentrate at the surface when the ice in which they were trapped evaporates, creating a thick layer of litter. Geologists call it 鈥渄irty ice鈥�.

There are also small ponds on the dirty ice, although it鈥檚 not completely clear why they stay liquid. Summons says these ponds are teeming with life, from bacteria to complex 鈥渆ukaryotic鈥� microbes. The ecosystems keep going thanks the vertical flow of material 鈥� including vital nutrients 鈥� through the ice, he says. 鈥淵ou鈥檝e got a continuous flow of phosphate and trace elements.鈥�

A refuge from the Snowball

Summons and his colleagues now argue that similar communities could have existed during Snowball Earth. 鈥淚f these places exist on Earth today, they must have been prolific on an ice-covered planet,鈥� he says. 鈥淲herever you have shallow continental shelf and sea glaciers, there must have been dirty ice.鈥�

In other words, relatively complex ecosystems might have survived the harsh conditions of Snowball Earth by living on the surface of the ice itself.

at Laval University in Qu茅bec City, Canada, and his colleagues were the first to suggest that . 鈥溾�楧irty ice鈥� is a compelling analogue for Snowball Earth ecosystems,鈥� he says.

Life might actually have thrived and evolved on Snowball Earth, if the impressive communities in modern dirty ice are any guide. 鈥淭his is not just an invisible microbial world, eking out an existence at the edge of life,鈥� says Vincent. 鈥淚t鈥檚 the polar equivalent of an Amazonian rainforest.鈥�

at Harvard University has also suggested that . 鈥淭he important thing is that such environments would have been extensive on Snowball Earth, approximately 12% of global surface area,鈥� he says.

Dirty ice also seems to be very stable. Summons says his colleague Hawes has been studying ponds on the dirty ice for years. And we know from the observations Scott鈥檚 team made in 1903 that the dirty ice has been there for at least a century.