Europa's surface is blasted by radiation from Jupiter. That's a bad thing for life on the surface – it couldn't survive. But the radiation may create fuel for life in an ocean below the surface. The radiation splits apart water molecules (H2O, made of oxygen and hydrogen) in Europa's extremely tenuous atmosphere.

Scientists are almost certain that hidden beneath the icy surface of Europa is a saltwater ocean with about twice as much water as Earth's global ocean. Europa may be one of the most promising places in our solar system to find present-day environments suitable for some form of life beyond Earth.

A new analysis of data from Juno shows Europa generates 1,000 tons of oxygen every 24 hours. While this is enough to allow 1 million people to breathe for 1 day, it's still less oxygen than scientists previously thought existed on the moon. Image via NASA/ JPL-Caltech/ SwRI/ MSSS/ Kevin M. Gill (CC BY 3.0).

Subsurface oceans such as those on Europa are generally expected to lack nutrients because they are so isolated; without sunlight to cook up prebiotic chemistry or to power biological photosynthesis, these buried bodies of water could be bereft of life.