I’ve always taken the world’s oceans for granted and for always ‘being there.’ But how did they come into existence?
Schenectady. The same guys that make all the ideas.
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They came from water. Earth has quite a lot of it. I don’t know if you’re going to get a more coherent answer than that unless you can narrow down exactly what you’re asking.
We’re not completely certain.
There are a number of competing theories, the two main ones being A) The water was already here, in the form of ice, when the Earth coalesced from the nebula that eventually formed the entire solar system, or B) The water was added later by collisions with ice-rich asteroids.
You can find more information about these two theories and the supporting evidence in this article:
It’s not exhaustive but it’s a place to start.
Icy comets. Billions and billions of them from the early days of the solar system. Where they came from is a different mystery.
It’s still a matter of some debate among scientists.
If you look at the outer planets, they all have a lot of water. The inner planets don’t. This makes some folks think that most of our water came from the formation of the planets, and Earth just managed to hang onto its water, where water on the other inner planets evaporated away into space. Mars has ancient stream beds and other geological formations that indicate that it once had a lot more water than it does now. Venus doesn’t have much water now, but many scientists think that it did have a fair amount of water in its past. These days, Venus’s thick atmosphere is mostly carbon dioxide, and the whole thing is a mad greenhouse gone wild type scenario.
Another theory is that the Earth’s water came from space, in the form of comets and asteroids. This theory is otherwise similar to the above, with Mars and Venus losing the water that originally came from space, and Earth managed to retain its water.
There is evidence to support both theories. How much water came from which source though is still a matter of debate.
ETA: Dang ninjas.
Okay, then where did all the water come from? It’s essentially the same question.
Thank you, I will take a look. 
Okay, many thanks. 
Fascinating.
What is the most common element in the universe?
Hydrogen.
What’s second most?
Helium. OK, forget helium because it’s chemically inert.
What’s next? Carbon, Oxygen, and Nitrogen.
Combine those with hydrogen, and you get the most common compounds in the universe: methane, water, and ammonia.
When we look at the outer solar system, most of the bodies out there contain mostly ices of various sorts. These compounds were formed early in the formation of the solar system out of the primordial nebula, and the nebula had the CHON because that’s what the universe is mostly made of.
Most of the nebula ended up either in the Sun, or blown out of the solar system by the energy of the newly formed Sun. Some bits of it remained though. And those bits formed the various planets, moons, comets, and dwarf planets.
In the inner solar system there’s a lot of rocky stuff. Probably because it’s hot there, and most of the volatiles boiled off. In the outer system there’s a lot more. The gas giants are mostly hydrogen. Their moons are various mixtures of rock and ices. And when you get out to the far reaches you find that comets and are mostly balls of ice.
So Earth either retained more of the water from the very early history of the solar system when the Earth was formed, or it retained more of the water from the early history of the solar system from various cometary collisions.
The point is that water is actually very common in our solar system. It’s just that most of the time that water is frozen.
I find the “always there” and “came from comets/etc.” distinction somewhat artificial.
Stuff just got clumped together. Tiny bits of ice and dust early. Bigger chunks of those later. Where do you draw the line?
One thing that creates such a distinction somewhat is the Theia Hypothesis. If that happened, then a lot of water might have gone away and then replaced over time. Then the question of where that water came from arises. But that’s a good chain of what-if’s to discuss.
As Lemur866 correctly points out, water is relatively common in our solar system … but not all of it exists in it’s basic form H[sub]2[/sub]O, some of it is weakly bound to minerals to form mineral hydrates and incorporated into the crystal structures of these minerals … for example Cobalt(II) chloride hexahydrate CoCl[sub]2[/sub]•6H[sub]2[/sub]O, six molecules of water weakly bound to each molecule of CoCl[sub]2[/sub] …
Enter Theia and the shock wave she produces and much of this water of hydration is broken free and this water floats up to the top of what is largely a molten sphere of rapidly differentiating materials …
The bigger question is why this water is still here after 4.7 billion years whereas it is all but gone on Mercury, Venus, the Moon and Mars … here I believe the leading theory is Earth has a rather intense magnetic field compared to these other inner solar system bodies and this deflects the nasty cosmic rays away …
Exploding stars. Same place that everything else comes from.
I have a feeling next you’re going to ask where the stars came, and then where did the hydrogen come from and on and on until you get to what you want to hear, and satisfy the whole point of your question.
On a clear night, go to somewhere well away from artificial light, lie on your back, look up and wonder.
“Watson, you fool! Someone has stolen our tent!”
“I wonder if I should go to the clinic and see if this rattlesnake bite needs to be treated”
One way or another, wouldn’t *condensation *be involved? Not the best answer, but I’m not seeing that word in this thread.
“Condensation” only describes a change in state of water … from a gas to a liquid … we’re not really asking why water is liquid on Earth, but how the water got here in the first place … be it as vapor, liquid or solid …