If we back to this one, this is Mars,
it starts out colder and you add water vapor and it goes up.
And maybe it hits the phase diagram, phase boundary of ice plus vapor.
So on Mars it would snow.
But again, the water vapor feedback would be limited by this phase boundary.
Now, Venus is a different story.
Venus originally had water.
It doesn't anymore, because Venus suffered a runaway greenhouse effect.
If you start from a condition that's warmer, and
you raised the water vapor concentration and raised the temperature,
it can curve over and miss the phase boundary for liquid or
solid entirely, and this is called a runaway greenhouse effect.
So all of the water on the planet would go into the atmosphere.
It would all boil.
[SOUND] And this is a one-way street.
If this ever happens on a planet, it never can go back,
because if you get too much water vapor up in the upper atmosphere where
it's exposed to the ultraviolet light from the Sun,
this ultraviolet light can split the water molecule into hydrogens and oxygens.
And these hydrogens are so small, they travel quickly with their allotted kinetic
energy just put into that small mass they go very fast, and so
they can be lost to space.
So a runaway greenhouse effect is the end of a biosphere for a planet.
So looking at a picture of Earth from space is always astonishing to me,
because the atmosphere is so thin.
And the ocean is just right there,
spread out through the whole surface of the planet.
And what keeps this from happening on Earth is sort of a cold trap.
The fact that it gets so
much colder in the upper atmosphere, the water vapor can't get through it.
It all rains or snows out.
So it's like this very, very thin layer of cold air that has protected and
guarded all of Earths of water for these billions of years, it looks so
fragile to me looking at a picture of Earth from space.
[MUSIC]