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If there is O2 trapped as bubble inclusions in the Europan ice crust,
it would be extremely strong evidence for photosynthetic life
occuring either in, or just below, its crust.
Couldn't photolysis alone do that? There's no or almost no atmosphere,
so hard UV and harder can directly reach the ice, and hydrogen would
escape more quickly than oxygen. That's also why there are traces of
oxygen in the atmosphere of Venus and of ozone around Ganymede.
Photosynthesis below the crust is very hard to imagine, because the
crust is simply too thick to be reasonably transparent. But absence of
photosynthesis wouldn't mean anything. Even on Earth, photosynthesis is
just one of a very large variety of ways to make ATP and pump protons.
That would leave the uppermost parts of the crust, insanely cold and
probably not well protected from cosmic radiation.