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.
— 3 —
THE FIRST SNOW
WHEN DID THE first snow fall? No, not the kind that falls one day in November only to melt as swiftly as it came. I mean the very first snow here on Earth. The first snowflake to come drifting down upon an unprepared Earth, which had no idea this singular phenomenon would recur each winter. This first flake would be joined by many others, so many in the end that some remained on the ground throughout the summer, marking the start of the cryosphere, the frozen part of Earth.
Snowfall is light and quick to vanish, so how can we say when the
