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BETWEEN FIRE AND ICE

ONE OF THE best things about Finnmarksvidda was the sky in winter. There were neither high mountains nor tall houses, so the stars and northern lights played freely across an endless 360-degree horizon. Best of all, there was no electric light to dull the light show in the heavens. The weather also tended to be clear on winter evenings.

Nowadays, the northern lights have become a tourist magnet, and people flock here from all over the world because you can enjoy them without freezing in northern Norway, thanks to the Gulf Stream. But for me, the clear starry sky was an equally unique show. And walking there, eyes turned upward, it was easy to wonder: Are there any other souls out there? Anyone who—right now—is asking whether there’s another planet like ours, with living creatures who go about wondering the same thing? Perhaps because the starry sky was so clear, thoughts like this occupied my mind so much in those days that I decided to study astronomy and physics when I grew up.

With the passage of time, though, my fascination ebbed away. Astronomy felt a bit otherworldly and I found other interests. When I returned to science, not as a scientist but as a communicator, what most preoccupied me were the mysteries of life. And not least the ultimate mystery: how life came about and how living organisms assumed ever more complex forms until, eventually, creatures emerged that were capable of pondering their own existence. Darwin became more important to me than Einstein, and the evolution of the human brain became more exciting to me than black holes. This preference also applied to my job as a science journalist, because the brain was still a newly discovered, unexplored continent.

So when NASA and other organizations began to report the discovery of Earth-like planets where there might be life, I was skeptical. Of course, the thought could stir your imagination: What if there really was somebody out there for us to talk to? But my reading about the development of life told me that we are the result of a series of almost impossible, or at any rate improbable, events. Life did not simply arise of its own accord, especially not complex life. This was something the evolutionary biologists John Maynard Smith and Eörs Szathmáry wrote about in The Origins of Life.⁴ They described eight transitions or revolutions life had to undergo before creatures like us could come about, living beings it was possible to communicate with. And to get all the way to this point, it was necessary to undergo all the transitions: there were no shortcuts.

The first transition was the emergence of self-replicating molecules, which created copies of themselves. Even this is still a mystery to biochemists, but the assumption is that RNA (the slightly less complex relative of DNA) may have been the first stage. We do not know if this was how it happened, and self-replication requires a combination of two mechanisms: not just a method for the actual replication (copying), but also a means of acquiring the energy needed to carry it out. Life must therefore have emerged in the vicinity of an energy source. And remember, this was long before life’s usual means of capturing energy, photosynthesis (which converts solar energy to biological energy), was “invented.” Some scientists, like biochemist Nick Lane, have therefore argued that the first living organisms must have arisen in or close to submarine hot springs or volcanoes.⁵

I won’t go through all eight stages proposed by Maynard Smith and Szathmáry, or Lane’s version of the development of life. Suffice it to say that it is theoretically possible to provide an explanation of how life on Earth evolved from simple, single-celled organisms, somewhere between 3.5 and 4 billion years ago, to more complex beings. That is not to say there is perfect clarity about all of the steps.

The story I will try to tell here—in a very short, simplified version—is how this development and life’s different revolutions have been intertwined with the history of the cryosphere.

The connection appears to have been there from the outset. It all started several billion years ago with a chunk of ice that came sailing through space and collided into a blazing hot Earth. This chunk of ice was a comet, and it was followed by a whole swarm of other comets and various celestial objects during the highly unstable early phase of our solar system’s history. These celestial objects brought many things with them—of which more later—but one crucial contribution was the substance that actually forms the cryosphere: water.

Because water is what it’s all about. Water in its many frozen forms: transparent ice, clear as glass, on the lakes; slop and slush on the roads; fern frost on frozen winter windowpanes; snowflakes drifting slowly through the air; compressed crystals beneath thousands of feet of glacier ice; black ice that suddenly springs up on the road sending cars into ditches; rime on withered straw in October; old spring snow that makes it impossible for animals and humans to get about; icebergs that strike ships in the night, sending hundreds of passengers out into the waves. And snow and glaciers that store water through the spring and melt in time to allow thirsty humans and beasts to drink. This is what makes the Earth unique: that we, here, can find water in all these strange frozen variants.

Water is an unusual substance, and all the more remarkable when it freezes. It isn’t a question of magic but of water’s physical properties, which result from the water molecule’s distinctive form. This form creates especially strong bonds between water molecules, giving the substance unique properties, especially in frozen form. Water molecules are formed of hydrogen and oxygen atoms, which are bonded in such a way that the two hydrogen atoms attach to one side of the oxygen atom. This makes the water molecule “lopsided,” giving it strong polarity, with a positive charge on the side where the hydrogen atoms are and a negative charge on the oxygen atom’s side. This polarity creates powerful bonds between the water molecules, binding them together tightly in a “bent” form in a liquid or gaseous state, and as symmetrical, hexagonal crystal structures in a solid state.

These crystals, which can vary dramatically in shape but are mostly hexagonal under normal conditions, are bonded in a way that gives water several remarkable properties—among others, that of being lighter in solid than in liquid form, which is why ice floats on top of water. This property is shared by only a few other substances, including diamonds, which are actually a form of carbon. Under the right temperature conditions—on another planet or moon—we might see “icebergs” of diamonds looming up from a sea of liquid diamond.

However, we will never see this on Earth. Where we live, water is the only substance that can occur in all three states, solid, liquid, and gas, under conditions we can live in. Indeed, the three states can actually occur at the same temperature—32 degrees Fahrenheit, or 0 degrees Celsius (ice and snow can evaporate directly, without taking the “detour” via liquid water). This is because the strong bonds between the water molecules make it difficult to separate them, which gives water unusual boiling and freezing points. In thermodynamic terms, water is described as being extremely resistant to phase change. It takes a great deal of energy to melt ice into water, and also to make water evaporate. Water’s special structure in frozen form, especially when it occurs as snow, gives it other unusual properties: it becomes white and light when it freezes, and snow is one of the substances that best retains heat. This is why you can sleep in a snow cave without freezing to death.

But in the earliest days of Earth’s history, there wasn’t much snow or ice to be seen. After Earth came into existence, during the turbulent beginnings of our solar system some 4.5 billion years ago, our planet was a ball of fire with a temperature of over 14,000 degrees Fahrenheit—hotter than the surface of the sun is today. Bombarded by a constant rain of comets, meteors, and other celestial objects, it was truly hell on Earth. Gradually things calmed down. After half a billion years, the gravitational fields of the sun or the planets had drawn in the solar system’s stragglers, which had either landed or settled into a stable orbit, like the asteroids we might come across between Mars and Jupiter. Earth had begun to cool and now at last it could enjoy a gift brought here by all this bombardment. As I’ve noted, the comets and rocks had brought with them water, that singular substance with its unique properties on which we are so reliant. And not just water: scientists have now discovered that comets may have brought with them everything that is needed for life to emerge, perhaps even life itself—all bundled up in a packaging of ice.

What does it take to create life? First of all, there must be complex organic molecules, such as amino acids (the building blocks for proteins), nucleobases (the building blocks for genetic material), and carbohydrates. One of the prerequisites for the emergence of life is the presence of such molecules. But scientists do not believe these complex molecules existed on Earth at the time when living organisms are supposed to have appeared here. So how can life have emerged? One possible explanation, recently backed up by observations and experiments, is that these types of molecules actually came tumbling down from the heavens, from outer space. And they were apparently brought here by large chunks of ice, comets. If this is true, all of us have our origins in ice.

The idea that life came from outer space is not new in itself; indeed, it is so widespread that it has a name: panspermia. Renowned scientists such as Francis Crick and Enrico Fermi have written about this, and it is a familiar theme in books and films.⁶ Panspermia comes in different versions. One is that someone intentionally sent these “seeds” to Earth. Another is that living organisms survived their journey through space and landed here by chance. It has, in fact, been proved that certain tiny animals called tardigrades or water bears can survive such conditions. Scientists have tested this theory by sending them into space, where they go into a kind of hibernation but can be woken up again afterward.⁷

A more sober version is the one supported by recent discoveries: that what came to Earth with the comets was not living organisms but the building blocks of life. And precisely these types of building blocks have now been found on a comet, 67P/Churyumov–Gerasimenko, which has been extensively studied using instruments on the Rosetta space probe. The substances found to date are the amino acid glycine and the mineral phosphorus, which is also a necessary ingredient in living organisms. In addition, comets and other celestial objects must have brought water—also absolutely essential for life—to Earth, in the form of ice.⁸

According to Kathrin Altwegg of the University of Bern, a lead scientist on the Rosetta project, this shows that comets may contain everything that is needed to create life apart from energy (it is too cold on a comet). It is hardly likely that the glycine originated on the comet itself; it probably came from dust clouds that existed before the solar system was formed. The dust particles were, in fact, a good place for organic molecules to be formed, as demonstrated in laboratories. At that time, however, Earth was too hot for such fragile amino acids to be able to come into being here. What Earth could contribute, though, and what the comets lacked, was the energy needed for life to emerge from these organic molecules. They needed heat to begin to react with each other. This was why the encounter between frozen organic molecules and the heat of the Earth may have been what kick-started life.

These sorts of “start-up packs,” or perhaps even frozen single-celled organisms, may have arrived on Earth early on, via comets or other celestial objects. We know Earth was heavily bombarded by such objects in its infancy, and water must also have been involved in this bombardment: we know there are still celestial objects out in space, like the asteroid/dwarf planet Ceres, which have large quantities of frozen water. Just a few collisions with such celestial objects would be enough to provide Earth with all the water we have today.

But it took a long time for these “start-up packs” to be opened. About a billion years had to pass before the conditions were ripe for life to develop here. The surface of the Earth had to cool, and the steam had to condense and fall as rain, allowing liquid water to form on the Earth’s surface, and eventually oceans. Because the ocean is where life began.

Not only did life’s building blocks come to Earth with ice, but the ice that had melted in its collision with the blazing world had to return, as the cryosphere, in order for life to begin developing here. Life and the cryosphere appear to have tracked and influenced each other through billions of years, although their dance was a very slow one in the earliest days. And it took a long time for the Kingdom of Frost to send its first snowflake down to Earth.