biolo stro gy A & f O o u l Banik, Astrobiol Outreach 2016 4:2 t a r e n a r c u h DOI: 10.4172/2332-2519.1000153 o J Astrobiology & Outreach ISSN: 2332-2519 ReviewResearch Article Article OpenOpen Access Access Snowball Earth Indranil Banik* Scottish Universities Physics Alliance, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK Keywords: Palaeolithic; Glaciation; Neoproterozoic; Chain reaction; see later, however, small gaps in the ice cover sufficient to prevent a Snowball Earth total extinction of surface life on Earth would likely have been left for a variety of reasons. Historical Development Although a global glaciation might have been disastrous for many Our ability to unlock the secrets of the ancient earth and living organisms then present, the extreme stress put upon them may understand events that occurred long before humans had evolved is a have accelerated the pace of evolution considerably, leading ultimately to remarkable and unique accomplishment of modern science. On Earth, the Cambrian explosion. The unusual chemistry and surface processes this is achieved mainly by looking at the record of these times that is (or lack of them) that occurred during these extreme glaciations may preserved in ancient rocks. The events recorded within them would also have helped the development of complex multi-cellular life-forms be almost incomprehensible without an understanding of the laws of and of land-dwelling animals that needed far more oxygen than would physics that led to their occurrence. normally have been present in the atmosphere, having only just evolved 250 years ago, the theory that the ice sheets, now confined to mostly primitive lungs. Some key chemical reactions could have required as within the Arctic and Antarctic Circles, had in the northern hemisphere much as 40% oxygen in the atmosphere. extended very much further south and stayed there every summer Now recognised as one of the most important sites containing for thousands of years; had yet to be accepted by the vast majority evidence for a Snowball Earth event during the aptly named Cryogenian of scientists and certainly of the general public. Today, this theory of Period of the Neoproterozoic eon, the Flinders Ranges in Australia recurring Palaeolithic ice ages in the past 2.5 million years is widely were first explored in great depth by Douglas Mawson, an Australian accepted [1]. Its role in driving human evolution is also appreciated, the Geologist born in England. He identified glacial deposits, which was events having led to droughts in Africa with the resulting advantage to only really possible as he had spent a lot of time in Antarctica! Nonetheless, hunting rather than gathering (which ultimately led to a much larger before the development of the theory of plate tectonics, it was puzzling and brain size). Also important was the opening up of land bridges between suggested the possibility of a Snowball Earth event (Figure 1). continents due to a drop in sea level, allowing humans to reach them and thereby spread across the planet more easily. However, these discoveries occurred only a few years before Alfred Wegner proposed in 1912 the theory of continental drift, arguably one Although this theory of Palaeolithic glaciations is now generally of the greatest advances ever made in geology. This immediately led accepted, these events seem almost benign when compared to what to the much more plausible speculation that Australia had been much seems to have happened when our earliest ancestors, single-celled life closer to the poles at the time the evidence of glaciation was deposited. This forms; inhabited the primordial oceans in the pre-Cambrian age of seemed reasonable as the glaciations were nearly a billion years ago. earth. For it now appears that, repeatedly in the Earth’s long history, and The modern theory of a snowball Earth was first proposed in 1964 for prolonged periods of geologic duration; the entire planet may have by Walter Brian Harland, a geologist at the University of Cambridge been covered in ice. This new theory, going by the name of ‘Snowball [2]. He found evidence of glacial tillites in Svalbard (Norway) and in Earth’, has gained increasing support from mounting evidence in the last Greenland, and evidence that they were near the Equator at the time the few decades. It may have also played a key role in shaping the evolution glacial deposits were laid down. In particular, he and D.E.T. Bidgood of primitive proto-cellular life into complex multi-cellular life forms, analysed the Moelv tillite in the Sparagmite District of central Norway something that may be the key barrier to intelligent life developing on a in 1959. The deposits in Greenland were analysed in 1961 by the same planet. It took billions of years for this step to occur, whereas simple life two people. emerged relatively rapidly after the formation of the Earth. Strong evidence for seafloor spreading and continental drift came These glaciation events were so extreme that our entire planet soon afterwards. Much effort was therefore placed upon locating would have been nearly as cold as the present Mars, and certainly colder where the deposits in Greenland and Norway had actually been at the than the Moon. In fact, it would have been cold enough to sustain the time of interest. The conclusion reached by Harland as a result of his coldness, which is one key reason why it lasted so long. The mechanism palaeomagnetic studies was that the glacial deposits had been laid down responsible for this is called the ice-albedo feedback effect. Ice forms at low enough temperatures, and it reflects enough sunlight to reduce the temperature even further. Considering that almost every material is less *Corresponding author: Indranil Banik, Scottish Universities Physics Alliance, reflective than ice (usually by a huge margin), the effect is considerable. University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK, Tel: 00447806375268; E-mail: [email protected] Under certain conditions, the ice will advance further and further Received July 02, 2016; Accepted August 17, 2016; Published August 25, 2016 until it reaches within 30° of the Equator. At this point, if a small perturbation to the climate system increased the amount of ice cover Citation: Banik I (2016) Snowball Earth. Astrobiol Outreach 4: 153. doi: 10.4172/2332- by one arbitrary ‘unit’, the temperature would decrease sufficiently 2519.1000153 to allow the formation of one more unit of ice (due to the reflection Copyright: © 2016 Banik I. This is an open-access article distributed under the of more sunlight). This would be a self-sustaining geological version terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and of a ‘chain reaction’. There could then be no stopping the ice from source are credited. covering the entire planet, including the tropical oceans! As we will Astrobiol Outreach, an open access journal ISSN: 2332-2519 Volume 4 • Issue 2 • 1000153 Citation: Banik I (2016) Snowball Earth. Astrobiol Outreach 4: 153. doi: 10.4172/2332-2519.1000153 Page 2 of 18 How did it happen? The greenhouse effect Although ice ages that extend the ice cover in the northern hemisphere to the middle of Europe are known to have occurred (by geologic standards) very recently, ice ages that went much further south and covered the entire planet are incredibly rare. No evidence has been found for such events in the last 600 million years. Conditions prior to this time must have been different in such a way as to allow a global glaciation to occur. To understand this, it is necessary to understand why the Earth is not always frozen and then to understand why these Figure 1: The Flinders Ranges in Australia. warming mechanisms should have failed. It is well known that without an atmosphere, the temperature of the near the Equator, within about 10° [3]. He was forced to conclude that Earth, as well as varying enormously, would be on average –18°C. The the temperature must have been low enough to allow such ice sheets to Earth is about 35°C warmer because of the greenhouse effect. Gases like form, even this close to the Equator; and that therefore the entire Earth carbon dioxide and methane can absorb and re-radiate heat emitted by was covered in ice at the time. He also suggested a possible link to the Earth. The absorption and subsequent re-radiation of infrared radiation Cambrian explosion, just after this glaciation. within a tiny fraction of a second may not seem like much, but it has In more modern times, more evidence of this nature has been found profound consequences. This is because, in equilibrium, the Earth must elsewhere, including in the Flinders Ranges in Australia, which were lose as much energy to space as it receives from the Sun. also very close to the Equator. The pattern is not coincidental, for the This occurs at a certain temperature, and according to Planck’s Law occurrence of almost all of the Earth’s land masses near to the Equator of Radiation, a higher temperature leads to a greater amount of energy was critical to the occurrence of a Snowball Earth. The uniqueness of loss (and vice versa). In fact, part of the reradiated energy is absorbed the situation is thought to explain why it has not occurred since then. by greenhouse gases like carbon dioxide and half of this (roughly) comes back down to Earth instead of being lost to space. This means Climate models developed by Mikhail Budyko of the Main the Earth finds it harder to radiate energy away, so it needs to warm Geophysical Observatory in Leningrad in 1969 showed the sensitivity up even more to radiate as much power as it receives.