When Earth is seen from the depths of space it appears as a blue dot. This is because just over 70 per cent of its surface is covered with water. Water is present on many of the planets too, and several moons of Jupiter and Saturn are thought to have significant water resources. But Earth is, without doubt, the one body in our Solar System where water has a defining presence. It’s thanks to water and its physical peculiarities that life has flourished in the first place. Water is so familiar to us that we often forget just what a remarkable substance it is.
What exactly is water?
Water is H2O, a simple molecule consisting of two atoms of hydrogen and one of oxygen. It is the only substance that exists as solid, liquid and gas in the temperature ranges found naturally on Earth. Water is transparent, but isn’t entirely colourless. Just as the sky is blue because molecules in the atmosphere scatter blue light more than other colours, so large quantities of water have a similar blue tinge, whether it’s the ocean or the dramatic blues of glacier ice (oceans and lakes also reflect a blue sky, making them appear even bluer). We are extremely lucky to have so much water on Earth because it has remarkable properties.
What’s so special about it?
Water is an impressive solvent, which means it is extremely good at dissolving things. This is partly why it’s so valuable for living organisms, acting as a transport fluid for a whole host of chemicals in living cells. What makes water such a good solvent is its ability to stick onto and separate the atoms of a substance, which is thanks to unusually strong hydrogen bonding. This is the effect that makes water so special: an electrical attraction between hydrogen atoms and other atoms such as nitrogen, oxygen, and fluorine. Hydrogen bonding between water molecules also makes them hard to separate, pushing up the boiling point. Without this effect, water would boil at around -70°C. That would mean no liquid water on Earth – and no life.
Another essential side effect of hydrogen bonding is that when water freezes, the hydrogen bonds between the molecules pull the crystals into a particular shape. This is why snowflakes form with six points, and it means that water crystals have more space in them than they otherwise would. They form tetrahedrons – shapes with four triangular sides. As a result, solid water, or ice, is less dense than the liquid form, which is why it’s not recommended to put a glass bottle of water in the freezer (the water will expand and can shatter the bottle), and why ice floats on a pond.
It’s often said that this property of water is unique. This is not quite true, as acetic acid and silicon, for example, are both less dense as a solid than as a liquid. But it is unusual, and it’s important. If ice were denser than water, lakes would freeze from the bottom, not the top, making it far less likely that aquatic life could survive cold winters.
Where in the Universe have we found water?
The chemical elements making up water (hydrogen and oxygen) are plentiful in the Universe. In fact, they’re the first and third most common by mass. Therefore, it’s not surprising that water shows up in many places. Every planet in the Solar System has at least some water, though the furnace-like Venus only has tiny amounts of vapour in its atmosphere.
Similarly, some moons are well-provided. Our own Moon appears to have ice deposits, while a number of the moons of Jupiter and Saturn, such as Europa, Ganymede, Callisto and Enceladus, are thought to have salty liquid water under surface ice. Comets, which plunge towards the Sun from the outer Solar System, usually contain large amounts of water ice. Further out, we find water in vast clouds of material between the stars, in the atmosphere of planets in distant solar systems and in the rotating discs of matter where new stars are forming. Water is indeed common, though rarely as dominant as it is on Earth.
