I just finished the "Muse Online Writers' Conference," including a session about keeping the science part of science fiction reasonably accurate. By coincidence, a friend passed me a link to this story at physorg.com: Jupiter's Moon Europa Has Enough Oxygen For Life. According to the article, scientists have estimated that the frozen surface of Europa turns over and remakes itself rapidly enough that oxygen, produced by cosmic radiation striking the surface, would make it down through the ice to the 100-kilometer-deep ocean believed to exist between the surface and the rocky core. Given abundant oxygen, or so the thinking goes, Europa should be able to support complex and interesting life, things equivalent to the fish found in our oceans.
I'm not so sure. Europa apparently has a steady supply of oxygen, but does it also have a mechanism for recycling the oxides (primarily carbon dioxide) produced by living creatures? On earth, we don't have simply water, oxygen and carbon; we've got a system of unstable equilibrium in which life constantly cycles the components around: CO2 plus water plus sunlight plus plants creates carbohydrates (like sugar) and free oxygen; carbohydrates plus oxygen plus animals (and plants at night) returns us to CO2, water, and waste heat. So while energy passes through the system, downgrading from light to heat in accordance with the second law of thermodynamics, mass cycles endlessly around within the system. Oxygen, water and carbon dioxide levels remain approximately the same, in a wonderful unstable equilibrium.
But what of Europa? Here, if the scientists are right, we have a constant influx of oxygen, courtesy of cosmic rays interacting with something on the surface. The article doesn't say what, but I assume it's water (since Europa's surface is ice). The oxygen dissolves in the moon's abundant water, while the hydrogen most likely leaks off into space. So, over time, the concentration of oxygen in Europa's ocean increases--unless that oxygen reacts with something.
That something would be carbon and other reactive elements being released from Europa's core, which is supposedly made of rock, much like Earth. Okay... so, with or without life (oxygen doesn't need life in order to react with carbon; it's just doin' what comes naturally), as carbon-containing materials are brought up from Europa's core (the core is heated by tidal forces as Europa orbits Jupiter), they react with oxygen to form CO2, which goes... where?
Well, CO2 dissolves in water, forming a weak acid solution. As far as I can tell, there's no mechanism described for turning that CO2 back into carbon and oxygen, so the dissolved CO2 will just pile up, rendering Europa's ocean more and more acidic, and consuming carbon from the planet's core until either the carbon's all gone (at which point any life starves) or the water's too acidic to support life (in which case any life expires). Either way, the situation doesn't look good for Arthur Clarke's Europan creatures.
Of course, how quickly this happens and how far it goes depends on a number of factors. How much water is in Europa's ocean? How quickly do the oxygen (introduced at the surface) and carbon compounds (introduced by hydrothermal events at the sea floor) mix--is it slow diffusion in a largely stagnant ocean, vigorous circulation due to a warm core, some form of stratification, or what? Bodies of water on earth display all of these behaviors--which one characterizes Europa? And just how much carbon dioxide can be dissolved in an ocean of that size? Is there enough carbon in Europa's rocks to reach this limit? How acidic will the water eventually get? I suppose there is a window--which, if life is lucky, is several billion years in length--between the start of these processes and their eventual end in carbon exhaustion or acidification.
And one more thing to think about: after enough CO2 builds up, Europa would have an ocean of carbonated water, making it the solar system's Coke bottle...