The Dirty Snowballs

If the water rode in on ancient ice, the ice had a vehicle: comets. Not the tidy painted streaks of children's books — mountains of frozen dust and water, some miles across, older than the ground beneath your feet, drifting in from the cold edge of everything.

Out past the last planet, a vast swarm of them hangs in the dark, billions upon billions, left over from the building of the solar system and never warmed enough to change. Now and then one falls inward, toward the Sun, and as it nears the heat it begins to boil — streaming a tail of gas and dust millions of miles long, which is the only part we ever see. The comet is the small dirty heart; the glory is just its breath, escaping.

And here is the part that touches your cup: the early Earth was born far too hot to keep any water at all. So the leading answer to where the oceans came from is that they did not rise from the rock — they fell, on comets and their wetter cousins, the water-rich asteroids.

You may be drinking a comet.

And if the oceans truly fell out of the sky — when, and how hard?