Question: why is the world 70% water ?

Keith Brain answered on 23 Jun 2010:

About 70% of the surface area of the earth is covered by water, but the earth itself is much drier – plenty of magma down deep. The amount of surface area covered depends on how irregular the surface of the earth is. Imagine if the earth was completely smooth – then 100% of the earth would be covered with water (even though there was no more water physically present). The roughness of the earth is a balance between the movement of continental plates, which crash together (very very slowly) to make mountains, balanced by erosion, which tends to make everything smooth again. These things are studied in subjects like geology and geography.

So, why do we have quite a bit of water on the earths surface? Well, I suppose that hydrogen is very abundant in the universe, and whenever it finds oxygen it will tend to form water. So, water forms very easily, but on most planets it doesn’t stay on the surface in liquid form for one of several different reasons:
– if a planet’s atmospheric pressure is too low, then the water evaporates;
– if the planet is too small, then its gravitational pressure is too low to keep hold of air/atmosphere (yes, without gravity the air would diffuse off into space), including the water, is lost;
– if the planet is too hot, the water boils off into the atmosphere (where it is more easily lost);
– if too cold, the water freezes solid.

So, the conditions on earth are very special (compared to elsewhere in the solar system) and allow water to pool on the surface and hence support (and probably initiate) life.

PS: (Advanced concept; feel free to ignore): The existence of these special conditions on earth isn’t so remarkable – each planet has its own remarkable characteristics. The interesting thing about earth is that life developed on it – life capable of asking “why is the earth special?”. There is an argument called the “weak anthropic principle” which, when applied to the earth, basically means that the earth is only special because we evolved on earth to ask that question. Gets me confused, too.