[SciShow intro plays]

Reid: When you were a kid, you probably learned all about the water cycle -- evaporation, condensation, and precipitation. You know the drill. Maybe you were an especially smart kid, and you started to think: this water must be pretty old. If it just keeps getting recycled by a closed system on Earth, there’s a chance it was sipped by Alexander Hamilton and Alexander the Great. And at some point, maybe it was dinosaur pee. But how did water get here in the first place? Where did the cycle begin? Let’s just say: it might be out of this world.

First let’s start with the components of water. You know: H2O, two hydrogen atoms bonded to an oxygen atom. Hydrogen is really old -- like, almost as old as the universe. About 14 billion years ago, just a few minutes after the Big Bang, particles that formed began to bump into each other, combining to create the first atomic nuclei: mostly hydrogen, some helium, and a little bit of lithium.

Oxygen arrived on the scene about a billion years later, when stars began to form. In the dense, hot cores of stars, the older, simpler atoms -- like hydrogen and helium -- fused together to make heavier elements -- including oxygen. A couple billion years after that, these atoms were all swirling around with a bunch of dust grains, made of carbon and silicon and other things. Every once in awhile, hydrogen and oxygen atoms would both be on the grain long enough to form chemical bonds. And as time went on, the dust grains kept building up more H2O molecules as layers of ice. Soon the universe had lots of water! Icy, dusty, spread out water. But how did it get to Earth?

Scientists have a few different theories, but they all have some problems. And we still don’t really know how to account for those problems. One theory is based on the idea that as this icy dust swirled around, it would have condensed and collided into bigger rocks and meteorites and even rocky planets like Earth. So it’s possible that water ice was part of the matter that crashed together and created our planet in the first place, and what if it never left?

At first, the planet would’ve been incredibly hot, and water on the surface would have evaporated right back into space. But these scientists think that some water stuck around, or that there was water that seeped up to the surface of the planet as the Earth cooled. After around 500 million years, Earth’s atmosphere and temperature were probably stable enough that some water could condense and fall back to the surface. But that doesn’t necessarily account for all of the water, because a lot would’ve been initially lost.

Another popular theory is that water came to Earth riding on celestial delivery trucks -- otherwise known as comets and asteroids. Around 3.9 billion years ago, there was a period called the late heavy bombardment. During that time, lots of comets and asteroids carrying icy water slammed into Earth and other celestial bodies, forming craters and maybe forming our oceans.

However, recent studies analyzing the composition of water on certain comets might shake up this theory. See, there are different forms, or isotopes, of water. Hydrogen’s nucleus is usually made up of one proton. But sometimes hydrogen can have a nucleus with one proton and one neutron -- called deuterium. Water with lots of deuterium is sometimes called “heavy water.” And our oceans are made up of a mixture of this “heavy water” and the lighter isotope. These studies found that when compared to our oceans on Earth, many comets had twice as much heavy water on them.

So either the comets that hit young Earth had very different compositions from the ones we’ve studied -- meaning that they probably came from a different region of space -- or the water mostly came from asteroids. It’s also possible that both of these theories fit together somehow -- maybe Earth had some original planetary water and we got more water later from bombardment. We just don’t know exactly how that would have gone down. So, scientists are still on the hunt for the exact sources of Earth’s water. But in the meantime, you can appreciate what we do know about the wet and wild history of your drinking water. It was born out of the Big Bang, fostered in stars, and after billions of years in cold, dark space it eventually, landed in your glass.

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