Volatile Molecule with two hydrogen atoms and one oxygen atom. Required for all known forms of life. NASA currently follows the mantra "follow the water" when trying to find life on other planets. The solar system has a "frost line", where most of the objects have little water closer than it, and objects are composed mostly of frozen water beyond it. Water is thought to come from comets in objects closer in than the frost line.
Ss:Water Web Pages
Water on Worlds
Mercury/Water - Water ice is thought to be frozen in Mercury's eternal shadow craters like the Moon. Smooth areas detected by Earth-based radar are an early evidence. Messenger found that the shadow craters are sufficiently deep to trap water ice there.
Mars/Water - Life giving substance generally found today on Mars in the form of ice at its caps and as vapor in the sky. Large amounts of it could be stored underground in areas outside the polar regions, such as under the craters at Sinus Sabaeus. Salty water, which has a lower freezing temperature, could occasionally flow on the surface today. Parts of Mars' interior may be warm enough to allow liquid water to exist. It may come up in a mud volcano. Nasa's current mantra is "follow the water", which its spacecraft do to try to find evidence of past and present habitability.