Extraterrestrial liquid water
The Gliese 581 planetary system contains multiple planets that may be candidates for surface water, including Gliese 581c, Gliese 581d, might be warm enough for oceans if a greenhouse effect was operating, and Gliese 581e.
Hydrogen economy
Molecular hydrogen is not available on Earth in convenient natural reservoirs. Most hydrogen in the lithosphere is bonded to oxygen in water. Manufacturing elemental hydrogen does require the consumption of a hydrogen carrier such as a fossil fuel or water. The former carrier consumes the fossil resource and produces carbon dioxide, but often requires no further energy input beyond the fossil fuel. Decomposing water, the latter carrier, requires electrical or heat input, generated from some primary energy source (fossil fuel, nuclear power or a renewable energy). Hydrogen can also be produced by refining the effluent from geothermal sources in the lithosphere. Hydrogen produced by zero emission renewable energy sources such as electrolysis of water using wind power, solar power, hydro power, wave power or tidal power is referred to as green hydrogen. Hydrogen produced by non-renewable energy sources may be referred to as brown hydrogen. Hydrogen produced as a waste by-product or industrial by-product is sometimes referred to as grey hydrogen.