Life extracts energy from its environment in order to carry out biological processes like maintaining cellular structures, growing and reproducing. Most living things on Earth's surface depend (directly or indirectly) on energy supplied by the sun, but there are many organisms that extract their energy from chemical sources like those produced by hydrothermal activity.
Europa's constant tidal flexing provides heat energy to drive chemical reactions in the rocky interior, recycling the elements and making them available for potential use by living things. If Europa's seafloor has volcanoes (as its sibling moon Io does) or hydrothermal vents, they may drive the chemistry of the ocean and play an important role in cycling nutrient-rich water between the ocean and the rocky interior. Tidal flexing of the ice shell could create slightly warmer pockets of ice that rise slowly upward to the surface, carrying material from the ocean below. Jupiter's intense radiation also provides a source of energy by ripping apart chemicals on the surface, where they can recombine to form new compounds.
The greatest uncertainty about energy as it relates to Europa's habitability is in how material cycles between the ice, the ocean and the rocky mantle on the ocean bottom. There are, potentially, sources of chemical energy for life being created on the surface and in the rocky interior, but their availability for use by living organisms depends on how well Europa's different layers are able to exchange material. In essence, the more energetic Europa is, the more energy would be available for life. Determining the balance of all these forces - Europa's energy balance - is a major hurdle toward understanding the icy moon's habitability.
What is the evidence for an ocean within Europa? >