1. Water exists in space in the interstellar clouds, in proto-planetary disks, in the comets and icy satellites of the solar system, and on the Earth.
2. In particular, the ortho-to-para ratio (OPR) of water in space has recently received attention. Like hydrogen, water can also be classified into ortho-H2O and para-H2O, in which the directions are antiparallel.
3. At the temperature conditions of the Earth (300 K), the OPR of H2O is 3.
4. At low temperatures below (< 50 K) the amount of para-H2O increases. It is known that the OPR of water in interstellar clouds and comets has more para-H2O (OPR = 2.5) than on Earth.
