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.
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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 first snow fell? It is difficult to establish a precise date. Nobody was there to witness it. In the unlikely event that it left any trace fossils, we wouldn’t know where to look for them anyway because the continents have shifted so much over the ages. But what we can say something about is when snow first lay so long on the ground that it became ice, a glacier that created identifiable striations in the bedrock. We can see these striations and we have ingenious methods for dating them. It’s all to do with the fact that the rock contains certain radioactive variants of minerals (isotopes): based on the amount of radiation they emit, we can calculate their age—take a reading of when they were formed. This is because we know the half-life of the different isotopes (how long it takes for their radiation to diminish), just as we can tell a glass of beer has been standing around for a long time when there are almost no bubbles left in it. If the minerals are magnetic, the scientists can even find out how they have moved by checking the direction of their magnetism. Earth has a magnetic field that creates patterns in certain minerals.
No snow fell in the earliest part of Earth’s history, that much is certain. As noted earlier, Earth was hotter then than the sun’s surface is today. But as the solar system calmed down, Earth had some respite from the bombardment, and the steam that had gathered in the atmosphere began to cool, falling as rain. Rain has a chilling effect, as we can tell when there’s an afternoon shower on a hot summer’s day. And gradually, Earth became cooler. Increasing amounts of water made the transition from steam to liquid form, creating pools of water, then lakes, and eventually an entire ocean. At the same time, the amount of steam in the atmosphere diminished, thereby reducing the greenhouse effect, which we now know keeps Earth warm.
A CLOSER LOOK
The Greenhouse Effect
The much-discussed greenhouse effect is so called because it is reminiscent of the conditions in a greenhouse, where the heat of the sun enters through the transparent walls of glass or plastic, but less of it gets out. This is because the radiation that is reflected has a different wavelength than the radiation that enters. The Earth’s atmosphere