Morning dew and roaring waterfalls inspire poets. Hurricanes and typhoons wreak havoc. Melting glaciers and rising tides pose a challenge to all of us, even in an increasingly thirsty world.

Water is critical to our survival, but we do not know the first thing about it—literally the first thing. Earth has approximately 70 per cent of its surface covered with water, and yet the origin of this water remains a puzzle.

Where does water come from, the giver and taker of life on Earth?

Various Studies and Theories about the Origin of Water on Earth

Big Bang

To be fair, the origins of our planet’s water are a complicated story that dates back 13.8 billion years to the Big Bang. Moreover, a key part of the story, centred on two specific solar system denizens, has been the subject of decades of heated debate.

Here’s the part we think we’ve figured out: The energy that sparked the outward swelling of space transmuted into a hot, uniform bath of particles just shy of a trillionth of a second after the Big Bang. These primordial constituents bumped and jostled, combined and recombined for the next three minutes, yielding the first atomic nuclei. The 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—is one of modern cosmology’s great triumphs. Producing a lot of hydrogen is a good start on the way to water, but what about the other important component, oxygen?

That is where stars, which were abundant about a billion years after the Big Bang, come into play. Stars are nuclear furnaces that fuse the Big Bang’s simple nuclei into more complex elements like carbon, nitrogen, and, yes, oxygen deep within their scorching hot interiors. When stars go supernova later in their lives, the explosions spew these elements into space. H2O is formed when oxygen and hydrogen combine.

So, are we finished? That is not the case. This is where things start to get a little murky. Water molecules were almost certainly part of the dusty swirl that coalesced into the Sun and its planets nine billion years after the Big Bang. However, surface water would have evaporated and drifted back into space during Earth’s early history, which included epochs with high ambient temperatures and no enveloping atmosphere. It appears that the water we have today was delivered long after the Earth was formed.

Comets and Asteroids

When faced with this problem, astronomers discovered two ready-made sources: comets and asteroids, which are the solar system’s gravel-strewn among planetary boulders. The main difference between the two is that comets have a higher concentration of ingredients that vaporise when heated, which accounts for their distinctive gaseous tails. Ice can be found on comets and asteroids. In addition, if they added the amount of material some scientists believe they did when colliding with Earth, such bodies could easily have delivered oceans’ worth of water. As a result, each has been named as a possible suspect in the case.

Adjudicating between the two is difficult, and scientific judgment has swung from one to the other over the years. Nonetheless, recent observations of their chemical compositions are tipping the scales in favour of asteroids. Last year, for example, researchers reported that the ratios of different forms of hydrogen in asteroids appear to be more similar to what we find on Earth. However, because the analyses are based on small samples, there is a good chance we have not yet heard the final word.

Sun’s Solar Winds

One more study suggests that the sun’s solar wind may have played an important role in delivering water to Earth, helping to solve a long-standing mystery about the origins of our planet’s seas and oceans.

Meteorite studies have revealed that they are surprisingly rich in water, implying that incoming asteroids delivered water early in our planet’s history, 4.6 billion years ago, allowing Earth to become the habitable world we see today. The composition of the water in meteorites, however, does not exactly match that of Earth’s: the extra-terrestrial version contains more deuterium, a heavier form of hydrogen, indicating that our planet must have had another source of water.

Studies Still Going On

Water is a defining feature of our planet, and it is so important in our daily lives. Understanding how and when water arrived on Earth is critical to understanding how and when life evolved here. But we have no idea how it got there or where it came from. Scientists are still trying to figure out how our planet got so wet in the first place.

Also published on Medium.