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.
Important clues to the presence of an ocean within Europa:
- Observations by NASA's Galileo spacecraft confirmed that Europa's surface is sparsely cratered and therefore young. (Heavily cratered surfaces are older.)
- Models for the formation of the many linear ridges and fractures on Europa's surface suggest that the moon's icy shell is relatively thin and flexes in response to tidal forces as the moon orbits Jupiter.
- Flexing of the icy crust above an ocean could create pockets of salty impurities and partially melted areas leading to features seen in spacecraft images.
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.
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