How Did Water Come to Earth?

Water is so vital to our survival, but strangely enough, we don’t know the first thing about it—literally the first. Where does water, a giver and taker of life on planet Earth, come from? When I was in junior high school, my science teacher taught us about the water cycle—evaporation from oceans and lakes, condensation forming clouds , rain refilling oceans and lakes—and it all made sense. Except for one thing: None of the details explained where the water came from to begin with. I asked, but my teacher looked as if I’d sought the sound of one hand clapping.

To be fair, the origin of our planet’s water is an intricate story stretching back some 13.8 billion years to the Big Bang. And a key part of the story, centering on two particular solar system denizens, has been hotly debated for decades.

Here’s the part we think we understand well: Just shy of a trillionth of a trillionth of a second after the Big Bang, the energy that sparked the outward swelling of space transmuted into a hot, uniform bath of particles. During the next three minutes, these primordial constituents bumped and jostled, combined and recombined, yielding the first atomic nuclei. One of the great triumphs of modern cosmology is its mathematical description of these processes, which gives accurate predictions for the cosmic abundances of the simplest nuclei—a lot of hydrogen, less helium and trace amounts of lithium. Producing copious hydrogen is a propitious start en route to water, but what about the other essential ingredient, oxygen?

That’s where stars, already plentiful about a billion years after the Big Bang, enter the picture. Deep within their blisteringly hot interiors, stars are nuclear furnaces that fuse the Big Bang’s simple nuclei into more complex elements, including carbon, nitrogen and, yes, oxygen. Later in their lives, when stars go super­nova, the explosions spew these elements into space. Oxygen and hydrogen commingle to make H2O.

So are we done? Not quite. In fact, this is where things get a little murky. Water molecules were surely part of the dusty swirl that coalesced into the Sun and its planets beginning about nine billion years after the Big Bang. But Earth’s early history, including epochs with high ambient temperatures and no enveloping atmosphere, implies that surface water would have evaporated and drifted back into space. The water we encounter today, it seems, must have been delivered long after Earth formed.

Faced with this conundrum, astronomers realized that there are two ready-made sources: comets and asteroids, the solar system’s gravel strewn among planetary boulders. The primary difference between the two is that comets typically have a greater concentration of ingredients that vaporize when heated, accounting for their iconic gaseous tails. Both comets and asteroids can contain ice. And if, by colliding with Earth, they added the amount of material some scientists suspect, such bodies could easily have delivered oceans’ worth of water. Accordingly, each has been fingered as a suspect in the mystery.

Adjudicating between the two is a challenge, and over the years scientific judgment has swung from one to the other. Nevertheless, recent observations of their chemical makeups are tipping the scale toward asteroids. Researchers reported last year, for example, that the ratios of different forms of hydrogen in asteroids appear to better match what we find here on Earth. But the analyses are based on limited samples, meaning there’s a good chance we’ve not yet heard the final word.

Even so, the next time you turn on the tap, think of the flowing water’s long and wonderful journey. It certainly makes a bottle of Fiji seem a little less exotic.