Since liquid water may well be essential for life beyond Earth, a critical component of the search for life beyond Earth naturally becomes: the search for liquid water beyond Earth. Evidence points to stable liquid water existing not just elsewhere in the universe, but even elsewhere in our own astronomical “backyard” – the Solar System, such as in the (likely) subsurface liquid water oceans on moons in the Solar System. Where we find it (and, ideally, examine it), we may indeed ultimately find alien life.
This leads to the ancillary queries: what is required for liquid water to be present on an astronomical body (a planet or a moon)? And also, on what astronomical bodies might these conditions be met? These issues overlap with the much broader topics of planetary and natural satellite (moon) habitability, but this article focuses only on (critical) issues relating to liquid water.
While water is fairly common in the solar system and the universe generally, it mainly exists in solid form – ice. However, liquid water requires heat and pressure within certain ranges.
Heat sources may include sunlight (particularly for planets) as well as gravitational tidal forces (particularly for moons), as well as geological or volcanic activity or irradiation.
Liquid water generally requires sufficient mass above it, essentially “pressing” down on it. The mass can be a sufficient gaseous atmosphere (such as on Earth, which allows for surface liquid water) or a solid such as ice (such as on the moon Europa, which can allow for subsurface liquid water). As such, generally, astronomical bodies with no (or insufficient) atmosphere cannot retain liquid water on their surface.
From an astrobiological perspective, surface liquid water is especially intriguing, since the interplay and chemical exchanges at the liquid water/atmosphere boundary can be very friendly to the reactions necessary for life. However, under the right conditions, subsurface liquid water may also provide for life, particularly if it is in contact with the rich chemistry that may be afforded by underwater hydrothermal vents (witness life around hydrothermal vents in the oceans of Earth), or even just rocky interior surfaces.
The first image, below, summarizes some of the basic pertinent physics behind liquid phase water, including examples relating to Earth and Europa, moon of Jupiter.
The second image, below, is a phase diagram for water, including ranges for Earth and Mars. It is notable that the thin atmosphere on (present day) Mars leads to a pressure that is very close to the limit for water to exist as a liquid, which is a key reason why liquid water exists on Mars only minimally and transiently, if at all.