Life and water are inextricably linked. Life as we know it needs a solvent in order to exist – a liquid carrier that can dissolve biological molecules, allowing them to come into contact and therefore enabling the chemistry of life to occur. Water is the only molecule known to be able to perform this function.
Water is quite unlike any other solvent in the known universe. Its chemical structure means that one side of each molecule is positively charged and the other is negatively charged. This makes water molecules sticky, attracting anything else with a positive or negative charge, including other water molecules and charged particles, like salts, for example.
There are other liquids that have been suggested as possible biological solvents, including methane, but none are quite like water. Because water is polar and its molecules are sticky, it can hold together as liquid at temperatures that would turn other similarly sized molecules to gas. Water remains liquid over a large range of temperatures, a property that has been vital for life on Earth.
In order for methane to be found in its liquid form, the temperature needs to be lower than -161 degrees Celsius. Liquid methane seas do exist on Saturn’s moon Titan, but molecules at this temperature move around so little that biological reactions would have to occur in extreme slow motion.
The arrival of water on Earth was the catalyst that enabled life to evolve. Early Earth was hot and inhospitable, but around 400-600 million years after its formation, in the period known as the Late Heavy Bombardment, Earth was pummelled by rock and ice flung from the far reaches of the Solar System by the immense gravitational interactions of Jupiter, Saturn, Neptune and Uranus. The rocks brought ice with them, which melted to form liquid water.
All life discovered on Earth so far depends on this water for survival, so in the search for life elsewhere in the Solar System, scientists are focusing on finding water. If the conditions are right, liquid water on other planets or moons could have supported extraterrestrial life in the past, or possibly to this day.
