Years ago I read a science article about an idea of gathering methane gas from the atmosphere of Jupiter, and sending a stream of containers of liquid methane back to Earth orbit, theoretically making settlement of Mars and/or the asteroid belt (for metal ore) profitable.
Asimov wrote an anti-McCarthy allegory (there was a political movement to prevent the waste of water to make spaceflight possible) which had the human colony on Mars go mining the rings of Saturn for water.