Chris: In that sort of time frame I reckon the answer is roughly the same. It does increase by a small amount. I think the stated geological figure is about 1 inch every 20,000 years or so. That's extrapolated over the lifetime of the Earth. Most of the water we have comes in the form of comets hitting the Earth. When the Earth first formed, of course, there was a disc of stellar debris which was basically the material left over from when the sun formed. Out of that we condensed planets as planetesimals. They slowly aggregated more material and formed bigger planets. The material left over was comets and other asteroids and other bodies which were out there in orbit. Comets are viewed as what are called, "dirty ice balls," basically lots of water with some other stuff chucked into them. Occasionally one of them's going to cross the orbit of another planet, get drawn in by gravity and crash land. Most of the water on Earth, we think, comes from comets, originally. Given that they're not actually that common these days but over the millions of years time scale I'd say the amount of water on Earth hasn't changed a huge amount. I would add that it probably is increasing very, very slightly. What do you think, Dave?

Dave: There is also the mechanism whereby the Earth loses water. What happens is, in the upper atmosphere you've got a little bit of water vapour high up in the atmosphere and it gets hit by ultraviolet light from the sun. That can split apart into hydrogen and oxygen. This light hydrogen will tend to float up really high and then get blown away by the solar wind. It's a very, very slow process but we are losing hydrogen from the water all the time. The oxygen will stay on the Earth because it is much heavier.

Chris: The same thing happened to Mars, didn't it? About 4 billion years ago, when Mars was about 400 million years old, it lost its magnetic field because the planet got too cold to have a liquid iron core. Because that's how the planet generates its magnetic field, it couldn't therefore have a magnetic field. That meant it was vulnerable to the solar wind which was just plucking all of the gas, the atmosphere and the water from the planet and it dried out.

Dave: And also to Venus. Venus is very similar to the Earth but has a much weaker magnetic field which tends to protect it from the solar wind. We think that Venus at the moment is about 500 degrees centigrade on the surface and the difference between Venus and Earth is that the Earth has a magnetic field which stops it losing water. Venus has lost all its water.