Is the Earth losing water into space?

14 June 2009

Question

I’ve always wondered, is the Earth leaking? Could water evaporate into space? Given enough time could the Earth end up like Mars: a desolate wasteland with not a drop of water to be found?

Answer

We put this question to Luca Montabone, Atmospheric, Oceanic and Planetary Physics dept., Oxford University:On the Earth, water can exist in all three forms namely as a solid, liquid, or gas. Evaporation transforms liquid water into water vapour which can then freely move in the atmosphere as a gas. Now, atmospheric molecules including water vapour molecules are in perpetual motion in all directions. Without the gravitational field of the earth, those moving away from the planet would be lost. Even with the gravitational field, in the upper thin part of the atmosphere, a molecule moving outwards has little chance of colliding with another and would therefore,be able to escape if it has sufficient speed. The average speed of the gas, for example water vapour, depends on its temperature. The conditions of temperature at the altitude from which water molecules are able to escape indicated the earth can retain water vapour over geological time scales that, is over several billion years. The retention of water vapour on our planet is also favoured by the fact that it can condense, form clouds at an altitude well below the one from which water molecules can escape and precipitate back to the ground as rain or snow. Adding to all these, we have to remember that water is also introduced in the hydrological cycle from the interior of the planet, for example, every time that a volcanic eruption occurs. So, to summarize, even if few water molecules are continuously off to space, the average level remains fairly constant over geological times which is what we want.

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