Ice everywhere, a cold wasteland hostile to life from the poles to the equator. The land was frozen solid under glaciers and the oceans were frozen over. Only volcanoes towered above the shell of ice, spewing steam, smoke and lava. Life scarcely existed. But after several million years the volcanoes had hurled enough carbon dioxide into the atmosphere. The climate tipped over. Within a short time – perhaps within 1,000 to 100,000 years – the shell of ice melted away. Live awoke and developed at full speed.

Science fiction? Not at all. This hypothesis, known as the “Snowball Earth” theory (1), is a research topic that must be taken seriously, even at ETH. The theory states that between 700 and 635 million years ago the Earth might have flown through space as a snowball. Geological processes at that time removed carbon dioxide from the atmosphere on a grand scale. As a result the atmosphere became increasingly colder and glaciers began to grow. In turn the ice reflected the incoming sunlight back into space, thus cooling the atmosphere down even more. Furthermore, at that time the sun radiated up to a few percent less energy than it does today. This simplified the glaciation. At some time or other the Earth was covered over with a shell of ice.

There is a lot of evidence in favour of the snowball

“In fact there are several lines of circumstantial evidence on which the Snowball Earth theory is based,” says post-doc James L. Etienne, who is doing research at the ETH Geological Institute. Among other things, geologists have found ancient glacial debris on all the continents that still lay partially at the equator at that time. On top of these lie sharply demarcated covering layers of limestone. According to Etienne, these could have been formed only in warm or tropical oceans. This would mean that the glaciers deposited the debris and melted rapidly at some time, and the sea level rose and flooded wide areas. In the oceans the glacier debris was overlaid with sediments, the basis for the limestone. The British scientist concludes from this that “At first it was obviously very cold, then suddenly very warm.”

Furthermore, when they examined the composition of these ancient limestone covering layers, experts encountered unusual chemical compositions that are evidence against life and thus perhaps in favour of extensive glaciation of the Earth. “Although we find an abundance of multicellular organisms preserved as fossils in limestones that were formed later than 545 million years ago, there are scarcely any traces of life in older rocks,” says ETH doctoral student Ruben Rieu from the Institute of Geology. This would mean in turn that various life forms developed explosively after the glaciers had melted away. Nevertheless, Rieu and Etienne are sceptical about the hypothesis of a total ice cover. Rieu asks us to remember that “If the Earth had been entirely coated with ice, there would no longer have been any way out.” This is connected with the Albedo Effect.

Wherever sand ribs are formed, there must be waves: evidence of open water at a time when the Earth is supposed to have worn a full suit of ice armour. (Photo: J. Etienne / ETH Geolog. Institute) large

Sand is evidence of open water

The ETH earth scientists find it easier to imagine that the Earth was only nearly fully glaciated. They base their assumption on rocks from the period of time in question that show fossilised ribs of sand – an unmistakeable ind¬ication of waves and open water even during the snowball phases. In addition, based on certain rock formations, they have discovered that rivers must have existed even during the time Earth was fully covered by glaciers. “This can only mean that actually not everything was frozen." Rieu and Etienne conclude from their work that the ice ages in the early history of the Earth were probably not much different to those of the last few million years: glaciers expanded and contracted, leaving space free for rivers and open oceans.

New interest in the snowball theory

In the last few years scientists have again shown interest in the snowball hypothesis. Geologists have discovered new evidence on all the continents to support it or on the other hand to disprove it. Rieu and Etienne expect more information about the “Snowball Earth” hypothesis to emerge from the first World Congress (2) that will take place in Ascona in mid-July and which they are both involved in organising. 80 experts from several countries will present the results of their search for clues, and the intention is to merge these together to give a new synthesis on the last day. The ETH geologists hope this will yield a common position, a kind of stopover in the discussion of whether at one time the Earth orbited the sun as an entirely or just an almost completely ice-covered sphere.