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The basic ingredients for life as we know it:
Water is essential to life, serving as a perfect liquid medium for dissolving nutrients for ingestion or wastes for excretion, and for transporting chemicals living things can use. Several lines of evidence strongly suggest that the planet-sized moon contains an ocean of liquid water many tens of miles deep. If it does exist, the ocean lies beneath an ice shell that is at least a few miles thick, and perhaps tens of miles thick. At the ocean bottom lies a rocky seafloor in direct contact with the water, possibly supplying chemical nutrients into the ocean by hydrothermal activity.
There are several important clues to the presence of an ocean within Europa.
Favorable environments for the chemistry of life (or even life itself, in microbial form) could exist in areas within Europa's ice shell that contain salty fluids or around possible hydrothermal systems driven by tidal heating. An ocean rich with chemistry conducive to life could be maintained by a cycle that moves water through the moon's ice shell, ocean and rocky interior.
Keep reading to learn more about the next essential ingredient for life: Chemistry.
Studying Europa's chemistry -- on the surface and within the suspected ocean -- is important for understanding its habitability because living things extract energy from their environments via chemical reactions. Interactions between materials from Europa's surface and those in an ocean environment beneath the ice could produce elements essential for life such as carbon, hydrogen, nitrogen, oxygen, phosphorous and sulfur.
Europa's surface is mostly water ice (H2O), but the surface is bombarded by intense radiation from Jupiter, which can alter the chemistry of the ice. Through this process, the hydrogen and oxygen from water ice can combine with other materials on the surface to create a host of molecules like free oxygen (O2), hydrogen peroxide (H2O2), carbon dioxide (CO2) and sulfur dioxide (SO2).
If these compounds are finding their way into an ocean as part of an ongoing cycle, they could be used to power the reactions living things depend upon. Meanwhile, cycling of ocean water through minerals in the seafloor could replenish the water with other chemicals that are crucial for life.
The last essential ingredient for life is: Energy.
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.