Where does water come from?

Evan Gough reports in Science alert,

We have comets and asteroids to thank for Earth's water, according to the most widely-held theory among scientists. ...As asteroids and comets struck this hot Earth, the water and the hydrogen in it were boiled off into space. As Earth cooled over time, water from comet and asteroid collisions was allowed to condense on Earth, and not be boiled off into space. The water stuck around.

The evidence for this lies in isotope ratios. The ratio of the heavy hydrogen isotope deuterium to normal hydrogen is a chemical signature.

Two bodies of water with the same ratio must have the same origin, the thinking goes. And Earth's oceans have the same ratio as water on asteroids. That's a very simplified version of the widely-held theory of how water got to Earth.

But scientists are malcontents, always trying to have a better, more thorough understanding of things. They were questioning the "water from comets" theory before this newest study came out.

Back in 2014, some scientists studied the issue by looking at meteorites of different ages. (Meteorites are just asteroids that have struck Earth.) First they looked at what are known as 'carbonaceous chondrite meteorites'.

They're the oldest ones we know of, and they formed about the same time as the Sun did. They're the primary building blocks of Earth.

Go here to read what the new model is suggesting.