- You can't see the ice line, but you can see its effects.

- When small bodies that are primarily composed of ice cross inside of the ice line, the ice within them begins to vaporize.

- [Narrator] Sometimes icy rocks are ejected from the frigid outer regions of the solar system and pulled towards the sun.

These are known as comets.

- Comets are basically dirty snowballs, mostly water, ice, with a little bit of dust.

And as they come into the inner solar system, they get heated up by the sun.

- [Narrator] As comets cross this invisible ice line, their ice starts turning into gas.

- And that's what forms these long tails that we can see when you look at a comet, and those tails stream away from the sun as the comet moves in towards it.

- [Narrator] Inside the ice line, ice is rare, but there are places where it can hang on.

(mysterious music)