Radiolysis from jupiter's magnetosphere produces hydrogen peroxide and oxygen from the surface ice (this was detected spectroscopically) and the theory is that since the ice sheet seems to undergo something like tectonic behavior, when a plate subducts to the ocean and melts the oxygen will be released and the hydrogen peroxide will decompose the problem though is that nobody knows the thickness and dynamics of the ice sheet, and also i think its unknown how the surface contaminants and environment etc would influence the ice structure's ability to retain the gas and also how much makes it to the ocean so you have estimates from the oxygen level of earth's oxygen minimum zones to estimates exceeding even the oxygenation of earth's oceans. If it’s water-ice-water, there’s nothing. if it’s water-ice-rock, then something is possible, but maybe just battery acid. but europa is so 2010 and at least enceladus has geysers spraying its ocean out into space where we could reasonably manage to sample it. now all they just need to figure out is how to LAUNCH A FUCKING MICROSCOPE INTO SPACE FOR THE FIRST TIME EVER!! >we've been broadcasting radio signals over hundreds of light years for the last two hundred years. Radar is brighter so if someone out there is looking at the right time they'd see it much more clearly. But biosignatures should be visible to anyone within 50 light-years. We're close to being able to detect the presence of life on earthlike planets with JWST. Nearby aliens with something a little better would be able to tell that there's life on earth. Discovering evidence of life on another planet or moon in the solar system would mean life in another star system is far more likely than not. There's about a few thousand star systems close enough to see our biosignature so they'd know something is here. I believe it is likely someone may have already tried to contact us.