The article itself make it clear that they aren't saying that; but they are saying that such life, if it exists, must be even scarcer and more limited in variety than scientists had thought.

Their reasoning is as follows: Virtually everybody is certain that the total amount of geothermal heat (from both tidal and radioactive sources) reaching each square kilometer of Europa's surface rocks must be, at absolute most, only about one-half that of Earth (and about the same as that on Mars).

This doesn't prove that Europa has no volcanic vents today, but it does mean that they must be few and far between. And it also means that Europa (like the Moon and Mars) doesn't have Earth's crustal tectonics ("continental drift") -- which is the process that keeps dragging the oxidized minerals of Earth's crust back down into its mantle.

Without this "crustal recycling", the minerals of Europa's mantle must be highly depleted in oxygen. And that means that whatever volcanic vents it does have will be belching forth only hydrogen, methane and sulfides, and not carbon dioxide, sulfur dioxide or elemental sulfur -- which are by far the most important energy sources for chemosynthetic bacteria on Earth (including those found at the "black smokers" on Earth's ocean floor).

As Gaidos et al. point out, though, the situation is by no means hopeless. It's possible that bacteria might have evolved that could pull oxygen out of the iron oxides in Europa's crustal rocks and combine it with hydrogen, methane or hydrogen sulfide to produce metabolic energy.

There are many types of Earth bacteria that combine oxygen with those chemicals for that purpose, and two groups of bacteria that can extract energy from iron minerals (including some that then react that oxygen with hydrogen).

"These organisms may metabolically resemble those that could function in a Europan biogeochemical cycle." But it's a very weak source of metabolic energy.

It should also be kept in mind (as I pointed out in my previous column here) that Europa in its earlier days was a far friendlier place. It certainly had far more volcanic activity, and its mantle had not yet belched most of the supply of oxygen in its minerals out into Europa's crust -- so, during those days, its volcanic vents certainly did emit much CO2, SO2 and sulfur which could have served as a better food supply for microbes.

Furthermore, as the environment deteriorated, it is very possible that some species of Europan bacteria evolved to adapt to their new, more paltry food supply. But, if Gaidos et al. are correct, we'll have to look very hard indeed to locate currently-existing Europan life (as opposed to the remains of past Europan microbial life frozen into the upper ice, which may be relatively common).

Bruce Moomaw is retired and lives in Cameron Park east of Sacramento. He has been an avid reader of planetary exploration research and provides occasional public talks on the solar system.

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