Science Questions

Is the Earth losing water into space?

Sun, 14th Jun 2009

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Nigel, London asked:

Iíve 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?



We put this question to Luca Montabone, Atmospheric, Oceanic and Planetary Physics dept., Oxford University:

Astronaut Bruce McCandless II, mission specialist, participates in a extra-vehicular activity (EVA), a few meters away from the cabin of the shuttle Challenger.  With an umbrella.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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Water molecules are too heavy to be able to just float off out of the atmosphere, but I know that hydrogen can. But even if we did lose some small amounts of water somehow I reckon on the whole we would be gaining water from icy comets. Madidus_Scientia, Tue, 9th Jun 2009

I know that the Earth's magnetic field helps to prevent dessication of the planet by deflecting the solar wind. Mars lost its magnetic field when it was about 500 million years old and dried out as a consequence.

But whether we are steadily losing water, even at a low rate, I don't know... chris, Wed, 10th Jun 2009

Do astronauts bring their water back? If so then Im stumped. davidmn, Fri, 12th Jun 2009

What if through global warming, temperatures at the highest atmosphere, where water escapes, are getting slightly higher and are letting more water out into space? This would not be good! :) Jeff, Sat, 18th Jan 2014

is all the water on the earth the same water that was here millions of years ago? eddie3x, Wed, 12th Feb 2014

Considering that we do have quite a lot of material, which should include water, crashing down to earth, do we in total actually gain water? Samuel, Tue, 13th May 2014

They have found sea plankton on the outside surface of the International Space Station. The implication is that it somehow gets into space hitching on water droplets. Ernest, Sat, 23rd Aug 2014

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