Water on earth is found in all three forms. But how did water get on earth? Where did the water on earth come from? The earth has an estimated 325 million trillion gallons of it. But earth began as a barren, lifeless, waterless ball of fire in its early history. There could not have been much water at the start. So what was the source of water?

A single molecule of water is made up of 3 atoms, 2 atoms of hydrogen, and one atom of Oxygen, much rarer than hydrogen but still abundant. The journey of the first molecule of water starts almost 13.8 billion years ago, at the time of the big bang. There were no atoms formed yet, only protons and neutrons. It took about 400,000 years for the universe to cool down enough for electrons to be captured in orbits around protons. And this is what formed the first atom, simply one electron one proton. This was hydrogen.

Oxygen is a much heavier and more complicated atom than hydrogen. It has 8 electrons orbiting a nucleus consisting of 8 protons and 8 neutrons. It took about another billion years or so before the first oxygen atoms were formed because it required nuclear fusion. This can only occur in nature, in the heart or center of stars.

The earliest stars were massive. But this also made them short lived. And about 600 million years after the big bang is when some of the first stars began to go supernova, but just before these massive supernova explosions, in their core, lighter atoms fused to form heavier atoms like oxygen, carbon, and others. When these stars blew up in huge supernova explosions, the oxygen and other heavier elements began to disperse into space and spread out. Hydrogen and Oxygen with the help of some energy, fused together to form Water.

After formation of this water, it turned to ice in the coldness of space and mixed with other dust particles that were simply floating in space for eons of time. And when enough dust from multiple supernova explosions collected over time in one spot, the gravitational pull of all this matter began to coalesce and collapse into another generation of stars. The leftover dust from the star formation coalesced into planets, planetesimals, asteroids and comets.

So you would think that since water coalesced into planets, our planet would have had all the water it has today from its early beginning. But it turns out that the most recent evidence shows that this was probably not the case, because earth had huge cosmic impacts.

One of these was a colossal impact with another early planet the size of Mars called Theia. Theia collided with earth and essentially turned it into one giant piece of molten rock. The heat of this collision would have evaporated most of the water on earth. While some of it probably remained due to gravity, a lot of the water would have spewed back out into space. So most scientists believe that the massive amount water we see on earth today must have come from another source, long after earth was formed.

Up until recently, most scientists thought that the most likely water delivery candidates were comets. But in 2012, researchers found that the chemical make up of the water on Comets doesn’t fit with the chemical make up of the water on earth. They measured the ratio of hydrogen isotopes deuterium and protium. The levels of these isotopes on comets is twice what we find on earth.

The source of earth’s water is most likely asteroids, like the one that killed the dinosaurs. Asteroids had huge amounts of water too, since they had not been exposed to the heat of the sun for as long as they have now. The isotope ratios of near earth asteroids, matches almost perfectly with the isotope ratios in water that we find on earth. So the dinosaur-killing asteroid was not the only one that hit the earth. Many such asteroids, probably hundreds of thousands of them hit the earth early in its history.

And these asteroids were formed from the same dust, ice and other substances as the early earth – which came from multiple supernova explosions over billions of years since the beginning of the universe.

So the next time you drink a glass of water, think about the journey that every single molecule of that water has taken, from the center of stars to floating among dust clouds for billions of years in space, to the formation of asteroids, to its collision on earth, to the formation oceans, to evaporation into clouds, to rain into rivers, to your bottle and into your body.

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