Plants growing in rain © Getty Images

This is why water is essential for life on Earth… and perhaps the rest of the Universe

When we look for extra-terrestrial life, we look for planets with liquid water. But are we overlooking places where life could thrive?

Water, and specifically liquid water, is deemed so important to the creation and sustenance of life that few scientists entertain the possibility of life existing on worlds without it. The search for extra-terrestrial life by organisations like NASA often boils down to one simple strategy: “follow the water”.

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So why is water so important? Well, there are several reasons, but they all hinge on water’s unique chemical properties. The chemical known as H20 is a simple molecule composed of two small positively charged hydrogen atoms and one large, negatively charged oxygen atom. This gives each molecule, and the substance itself, what is called ‘polarity’.

The opposing charges mean the different bits of nearby water molecules stick to each other, but it also means that water interacts with the charged elements of other molecules, often helping to break them apart and dissolve them.

For the thousands of chemical reactions going on in our cells to happen quickly and efficiently, the molecules involved need to be able to mix freely – they need to be dissolved in something. Water is so good at dissolving substances that it is known as the ‘universal solvent’. While other substances have similar dissolving power to water, they do not have its chemical stability and its ability to neutralise strong acids and bases.

The polarity of water also helps the formation of the delicate membranes that encapsulate all living cells. In water, special fats called lipids line up with their water-loving ends facing outwards and water-hating ends facing inwards, forming a continuous but flexible two-layer film, like the outside of a soap bubble. These lipid membranes play a fundamental role in keeping the complexity of life concentrated in one place, creating individual entities separate from each other and their environment.

The polarity of water also helps water molecules to stick to each other, giving water another useful property called cohesion. This means water will get drawn through very thin tubes, even against the pull of gravity, enabling water to flow hundreds of feet from the ground to the tops of tall trees. Handily, H20 also helps transport nutrients, clean away waste, and provides pressure for structural support.

Anything else? Ah yes: photosynthesis, the process in plants that creates sugars from sunlight, and which creates the food that feeds the planet’s entire food chain, requires – you guessed it – water.

In fact, there are so many reasons why water is crucial to life that entire books have been dedicated to it.

Could there be strange forms of life out there that don’t require water? Of course – it is possible that there is life so different to ours on Earth that we can’t even imagine how it works. But it seems highly likely that if there is life out there, it will need a solvent to lubricate the chemical processes that create energy, movement and replication. And we know of no other substance that does that half as well as water.

Water’s importance as a supportive base substance led the Nobel Prize-winning biologist Albert von Szent-Györgyi to describe it as life’s “matrix, mother and medium”, while the science writer and physicist Philip Ball wrote this of biology: “You could be forgiven for concluding that the subject is all about proteins and genes, embodied in DNA. But this is only a form of shorthand; for biology is really all about the interactions of such molecules in and with water.”

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