Hank: Our understanding of how the Earth formed has an ocean-sized gap in it. Water is virtually everywhere on this planet today. It's stored in massive, icy glaciers; it flows through mighty rivers; it laps at the coastlines as oceans cover more than 70% of Earth's surface; it even falls from the sky.

But, believe it or not, we don't completely know where it all came from. Scientists don't all agree on the same origin story. So, while they do the academic equivalent of squirting each other with water guns, let us dive into the mystery of how the planet got so wet and how, at one point, it may have been entirely covered in water.

The classic story of Earth's formation ends not with a pretty blue marble hanging out in the blackness of space; instead, it ends with a pretty dry planet. It goes something like this: After our Sun formed, a bunch of material was left orbiting it. We're talking tiny bits of stuff, like gas and dust and different kinds of ice grains.

But soon, gravity pulled all that stuff into a protoplanetary disk, and little particles started sticking together to form bigger globs. And this happened over and over until we wound up with all of the planets and moons and leftover bits that we know and love, like one of my favorite asteroids - 10731, Dolly Parton.

Of course, if you're a glob of stuff trying to turn into a planet, your distance from the Sun is gonna affect what you're actually made of, because the closer you are to the Sun, the hotter it is. Most scientists think the Earth formed in a region of the protoplanetary disk that was too hot for ice, so any water would have been vapor. But while Earth was forming, it was too small for its gravity to hold an atmosphere, so any water vapor would have been lost to space.

In other words, when the Earth was done cooking, it was effectively bone dry. And yet, now, there's water. There's water everywhere!

So, the classic solution to this mystery is that Earth's water hitched a ride on rocks that had formed out in the colder outer regions of the solar system, and then they slammed into our planet after it had mostly formed. And it turns out that there is a particular type of asteroid called carbonaceous chondrites that can have lots of water locked up within their minerals. When they hit the young Earth, the heat from that collision would release the water into our ancient atmosphere.

And we do know that they have hit the Earth, because we have the meteorites to prove it. But to test the hypothesis that they are the source of Earth's water, scientists needed to compare the types of water from both the Earth and some of these meteorites, because you might not think it, but not all water is the same.

It's just H2O, isn't it? But the H's and O's can be different! Elements come in different forms, called isotopes, that have different numbers of neutrons. If you got fewer neutrons, you are lighter; if you got more, you're heavier.

And each water molecule has three atoms, each of which can come in more than one isotope. For example, a typical hydrogen atom doesn't have any neutrons; it's just one lone proton in the nucleus. But so-called "heavy hydrogen" has one proton and one neutron. Meanwhile, oxygen almost always has eight neutrons, but other stable isotopes can have nine or ten.

So, scientists can measure what the ratio of these different isotopes is across different sources of water and see if anything matches. Because, if there is a match, it could suggest a shared origin.

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Hank: To trace the origin of Earth's water, researchers just couldn't take a glass and scoop a bit out of the nearest lake and compare to that water in ancient meteorites. They needed an equally ancient source. Luckily, geologists have unearthed some minerals that seem to have preserved the isotope signature of ancient ocean water.

Long story short, the two ratios are close, but not a perfect match. Compared to Earth's ancient water, chondrite meteorites have too much heavy hydrogen. So while some of our water almost certainly came from these kinds of space rocks, we need a source of lighter hydrogen water, too.

Over the past couple of decades, there have been a lot of ideas about what this source could be. It could have been some of the protons that our Sun is spitting out all of the time, which hit mineral grains in space dust and caused a reaction that created lighter hydrogen water. Or it could have been that, before the carbonaceous chondrites arrived, the Earth formed from another type of asteroid called enstatite chondrites.

These not only have more water than we previously thought, they also have the right isotope signature. Maybe, as Earth was first forming, it trapped a bunch of hydrogen from the greater nebula that our whole solar system was born out of. Ultimately, researchers are still debating which combination of these or other sources gave us our water.

But you might have noticed that, with all of them, Earth's water began trapped in the structure of minerals. Even today, the majority of the water on Earth is not in the oceans. There is another 18-oceans' worth of water locked away inside minerals deep within the planet. I know that's not what they taught you in earth science class, but, like, it's not important to us. We can't get it.

And during the early, hot days of the planet, before we had oceans, some of these minerals melted and released a massive amount of water vapor in the atmosphere. As the Earth slowly cooled, this condensed to form the liquid water on the surface. In fact, some scientists think that there was so much liquid water, the Earth was once a water world with little to no land at all.

A 2020 study measured the oxygen isotopes in a sample of rock that sat on the seafloor 3.2 billion years ago, and it turns out the ocean back then had a ton of heavy oxygen water. Now, when continents are weathered and eroded, chemical reactions will pull lots of water molecules with heavy oxygen out of the water cycle, so the researchers think there was very little erosion of continents at that time. Ergo, the early Earth could have been a water world, but if that's true, where did the rest of that water go?

Remember how there's way more water stored within the rocks inside our planet than what sits on the surface? Well, a 2021 study created a model of how our planet's capacity to store water has changed over time. It found that, as the Earth cools over billions of years, it can store more and more water, so some of the stuff that used to be on the surface is now locked up inside minerals deep beneath our feet.

But even if the early ocean wasn't the setting of a Kevin Costner movie, it was big, and it was also salty. Chloride ions came from volcanoes, and the water quickly dissolved sodium from the ancient sea floor.

So, in order to get its fresh water, the Earth needed a way to remove the salty stuff. One way to do that is, of course, evaporation. The salt stays behind in the ocean, and you get giant floating bags of water, called clouds, that can deposit that fresh water in places that are not a salty ocean.

And when did the first fresh water fall on Earth? Again, isotopes give us some clues. A 2024 study looked at oxygen isotopes in four-billion-year-old crystals from Western Australia, and they found very little heavy oxygen in them. When water evaporates, it tends to leave behind more of the molecules with heavy oxygen isotopes, so a lack of heavy oxygen in that crystal suggests it came in contact with fresh water, meaning our planet had a full-blown water cycle as early as four billion years ago.

So, while we are still not sure how the Earth got its water in the first place, we have a shockingly good idea of how we ended up with all that fresh stuff, and I will drink to that!