Water, water… Everywhere?

Earth contains an awful lot of water. It’s what makes our planet unique in the solar system — copious amounts of liquid surface water. In fact 70.9% of our planet’s surface is covered in water, and all of it helps to maintain Earth’s climate. Though its ubiquitous nature on our planet makes us think of it as mundane, water is actually a very interesting chemical. With an incredibly high heat capacity (the amount of energy required to increase its temperature), a high heat of vapourisation (the energy needed to make it change state from liquid into gas) and the ability to remain liquid over a temperature range of 100°C, water is unique among small molecules.

But the funny thing is that even though it may be everywhere, for its size Earth doesn’t actually contain much water at all. Observe:


This graphic is a visualisation of how much water is on the surface of our planet. If you could hypothetically scoop all of the water off Earth and suspend it in space for comparison, all of Earth’s oceans would fill up that small blue sphere to the right of the Earth there. The interesting thing is that that small blue sphere isn’t even that small. It represents 1.332 billion km3 of water. Which is rather a lot. It’s easy to forget how big Earth is in relation to all of the things that exist on it! The vanishingly small speck to the right of that blue sphere represents the volume of all of Earth’s fresh water. Every river and lake on Earth only make up that tiny volume. In all honesty, I was surprised! That tiny speck is responsible for all land-based life as we know it.

These two small spheres — this small volume of water? It’s this that helps regulate our entire environment and stop Earth from becoming a dustball like Mars or a pressure cooker like Venus. Humble though it may be, this mighty little molecule has a profound effect.

Think about that the next time you find yourself reaching for a glass of water…

A nod goes to Life’s Little Mysteries for sharing the image!

Image Credit: David Gallo/WHOI

No other small molecule stays liquid across a range that broad, and the only substance with a higher heat capacity is ammonia.
Incidentally, it’s a common misconception that water is actually completely transparent. Thanks to a couple of high energy vibrational overtones in its spectrum, pure water is indeed very slightly pale blue. It’s too pale to notice if you fill a bathtub or a fish tank with it, but en masse it appears very blue indeed — which is the real reason why the sea is blue.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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