"I think Europa is the best place to return back with another spacecraft to learn about extra-terrestrial life in the solar system as life is based on liquid water solvent, a suite of "biogenic element" and a source of a free energy, which seems to present on Europa. The assumptions and some observations suggest that Europa may have been these life supporting factors.
Europa's putative subsurface the ocean suggest that the traditional view of planetary is strengthened by elucidation of the terrestrial life exists or could exist independently of surface photosynthesis, then the possibilities for extraterrestrial biosphere greatly expand. Processes at hydrothermal vents may have been important in Earth's origin of life but it remains unclear whether the entire origin of life could have been independent of sunlight driven surface conditions and photochemistry. Galileo spacecraft gravity measurements indicates that Europa has a combined ice and liquid water shell ≈80-170 km thick overlying metallic and rocky core and mantle. Models indicate sufficient geothermal and tidal heating to maintain much of the ice shell in liquid form.
It is common to assume Europa's composition to be that of a carbonaceous chondrite meteorite, in which case biogenic elements would be abundant .Little is known observationally. Spectral evidence reveal's certain organic functional groups (C—H,C≡H) on Jupiter's moons Ganymede and Calisto and hints at their presence on Europa. I think internal heat and has a thick upper atmosphere that traps most of the electromagnetic radiation would have sustain life by converting heat energy into some useful chemical form of energy . Methanogenesis at hydrothermal vents at the bottom of Europa's ≈100km deep ocean could supply similar amounts of energy to that which supports ecosystem at terrestrial vents although the potential annual biomass production would be≈10^8-10^9 times below terrestrial primary production based on photosynthesis. It is also possible that niches might exist within Europa's ice shell where transient near surface liquid water environments could permit photosynthesis or other metabolic processes.
Charged particles accelerate in the Jovian magnetosphere should simultaneously produce oxidants and simple organics at Europa's surface. A scientist suggested that these molecules ,if delivered to the liquid water layer could provide a surface of free energy sufficient to sustain a Europan ecosystem. The charged particles interactions with water ice should produce molecular oxygen, hydrogen peroxide and other oxidants. So, I think Europa sustains life."