My supposition is that within the distance of the so called frost line around a star, water in empty space sublimates, evaporates. I further suppose that this means that vaporized water is pushed to outside the frost line by solar wind or some other mechanism. (Please correct me if this is wrong)
But there are some ways for water (not necessarily liquid) to exist within the frost line. I wonder under what conditions that is possible. What the origin or history such water can have. And how common or likely it is.
I suggest some possibilities to be extended, elaborated or refuted:
- Water under enough atmospheric pressure to keep it from escaping into space.
- Water in cold shadow, as in some craters on the lunar poles.
- Water inside rock, as in the mantle of the Earth (and other planets?)
- Water ice transported by comets across the frost line.
- Water vapor hanging around in empty inner inter-planetary space, for example after a planetary collision which melts most matter into a cloud. Or maybe permanently.