[thedoc (OP)]

Ghayath asked the Naked Scientists:
   Since earth's total water is finite , what would happen if we added a considerable amount of water from outer space into earth ?!  Would it jeopardize the equilibrium of our planet ?
What do you think?

[chiralSPO]

This is of course a function of how much we increased the amount of water, and how quickly it was introduced...

The actual amount of water on the earth does actually change with time. Ignoring for the moment chemical consumption and production of water, which is essentially a steady state that oscillates somewhat with time, water does actually leave our atmosphere, and is introduced from space. (I don't know the exact ammounts, but I think it is on the order of several kg per day either going or coming, worldwide)

Adding enough water to raise the oceans a few meters would be terrible for civilization, but I don't know, ultimately, how much effect it would have on the chemical and ecological equilibria on the planet. The oceans would be diluted to some extent, but I don't think it would be that significant. Species that the ocean is saturated in would probably stay saturated, there's more than enough CO₂, Mg and Ca to go around for that type of increase.

Increasing the oceans by a few km would have much, much more significant effects. Now the ocean will cover the entire surface of the planet, and will have absorbed almost all of the CO₂, NO_x etc. from the atmosphere.

After this point, all of the added water will be spherically distributed about the globe, so I don't think it would have any major tectonic effects, it would just compress the earth underneath as the mass of water increases.

We could theoretically add a few earth masses of water before much interesting would happen, but eventually the Earth would be massive enough to capture hydrogen and helium. Heck if added enough water, the Earth could turn into a star in a very explosive way (this would have to be a stellar amount of water)