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John Hill asks two questions. (1) Is there as much water on Earth today as when it formed? Formation took a longish time and we don’t know.

But look at Mars, which is now nearly dry. Rounded rock grains, sedimentary layers, and river-eroded canyons all show that there was once abundant water on the surface.

Where has it gone? Water vapour at the top of the atmosphere ‘evaporates’ into space, and on Mars its water has left that way. The same must be happening slowly on Earth. (2) Why is the sea salt but rain is fresh? The rain falls and flows over and through rock and soil, during which it dissolves tiny amounts of ‘salts’ including sodium chloride and carbonates of calcium and magnesium. T

his solution reaches the seas. From them water evaporates into the atmosphere, condenses as clouds, and rain falls, completing the cycle. The water evaporates but the salts do not: they gradually accumulate, so over billions of years the seas become saltier.

Of course the processes are more complicated than this simple account. For example, why is sodium chloride the main salt in seawater? What became of the calcium carbonate?

RS Clymo, Robertsbridge.