Stardate 12543.56. Federation ship USS Enterprise. Captain’s Log: “We are in orbit around an icy planet. The expedition I sent has informed us that it is a world completely covered by a layer of ice that is a kilometer thick. The average temperature is 40 degrees below zero. According to our geologist, the only continent that exists is slowly breaking apart. On what our sensors say is dry land, since there is no way to visually determine if we are on a continent or an ocean, vast glaciers crackle as they slide slowly down the slopes.
The ocean is also completely frozen: “We are surrounded by a flat expanse of brilliant white that shines with the rays of the sun from a cloudless sky,” says ‘Bones’, the medical officer. The truth is that you can see some, very occasionally. They are like elongated braids made of tiny ice crystals of carbon dioxide. There seems to be no life on the surface, or very little. There is free oxygen in the atmosphere, which indicates that at least there must be photosynthetic living beings. Our sensors indicate that life is found at the bottom of the sea: they are nothing more than algae and bacteria living in warm places, near submarine volcanoes and in certain places at the bottom of the oceans, where the lava from the interior escaped through a crack in the thin oceanic crust. Everything indicates that for millions of years nothing has changed.”
The previous description could very well belong to one of the chapters of the legendary Star Trek series, when the crew of the Enterprise arrive at one of those places where no one has ever been before. However, this portrait of an alien planet should not seem so strange to us because this is what our planet was like 750 million years ago, when the entire planet froze: it is the phenomenon of Snowball Earth , Snowball Earth .
It all started in the 1960s , when geologists began to discover the first evidence of tremendous glaciation in the past . As the discoveries increased, the surprise was becoming greater and greater. Slowly accumulating data indicated that near the end of the so-called Neoproterozoic era (covering from 1 billion to 543 million years ago) glaciers extended into the tropics.
How could there be ice under the equatorial sun? It was inconceivable and incomparably greater than the peak moment of what was the last and greatest glaciation that occurred on Earth , which occurred only 21,000 years ago, when most of North America and Europe were covered by glaciers 2 kilometers thick, causing the level of the sea fell around 120 meters. The cold was global; Frozen land and sea covered 30% of the Earth’s surface, more than at any other time in the last 500 million years.
In 1964, Cambridge University geologist Brian Harland postulated that our planet had undergone a great ice age in the Neoproterozoic. In fact, he had found glacial deposits from that time on virtually every continent. Interestingly, around the same time atmospheric physicists were developing mathematical models of Earth’s climate. Mikhail I. Budyko of the Leningrad Geophysical Observatory found , working with a simple climate model, that it was not that difficult for the entire planet to freeze . His calculations showed that by the time the ice reached latitudes 30 degrees north and south, the complete freezing of the planet would be inevitable.
However, few were convinced and this idea slept the sleep of the just until in 1986 Caltech paleomagnetism expert Joe Kirschvink began to put the pieces in place after investigating Neoproterozoic glacial deposits in Adelaide, Australia. Kirschvink showed that there had been ice very close to the equator 700 million years ago. Then Kirschvink noticed another detail. Many Neoproterozoic glacial deposits are associated with large accumulations of iron , known as banded iron formations, and no one knew why. It is well known that these formations appeared en masse 2.7 billion years ago (in fact, all the iron that we have used throughout history comes from them). It was due to bacterial photosynthesis , which by throwing oxygen into an atmosphere without it, reacted with iron dissolved in the water and settled on the bottom of the oceans. But 750 million years ago there was already oxygen in the atmosphere. What happened? Kirschvink’s explanation was simple. The ice cap that covered Earth’s oceans isolated iron from atmospheric oxygen. Without anyone to prevent it, it increased its concentration in the seawater until, when the ice melted… Boom! reacted with oxygen and settled on the seabed.
In 1992 he wrote an article where he exposed all his ideas and coined the term Snowball Earth . He convinced very few. Among those who took him seriously was Harvard geologist Paul Hoffman , who was digging in Namibia, right in the Neoproterozoic glacial deposits. What he found in the rocks formed when the Snowball phenomenon occurred was that photosynthesis had disappeared : the cold had caused the photosynthetic beings to enter a kind of lethargy.
Hoffman recounted his findings to a Harvard colleague, geochemist Dan Schrag , who recalled a calculation just made by Jim Kasting and Ken Caldeira of Pennsylvania State University: the amount of carbon dioxide in the atmosphere needed to thawing of the planet began was of the order of 350 times the amount that we have today. Schrag posited that as the oceans melted, the water began to evaporate . Water vapor is an excellent greenhouse gas which, together with the carbon dioxide present in the atmosphere, shot up the global temperature of the Earth to 40 and 50º C. In just a few hundred years, our planet went from the coldest climate ever known to warmer: “It was the mother of all thaws,” Schrag said.
References
Kirschvink, J.L., 1978, The Precambrian-Cambrian boundary problem: Paleomagnetic directions from the Amadeus Basin, Central Australia, Earth and Planetary Science Letters 40, 91-100.Kirschvink, J. L., 1992, Late Proterozoic low-latitude global glaciation: The Snowball Earth, en J.W. Schopf and C. Klein, eds., The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, p.51-52.
