Introduction
Europa, one of the four large satellites of Jupiter, is nearly the size of Earth's Moon. Tidal flexing driven by Jupiter's gravity and sustained by an orbital resonance with two other jovian satellites, Io and Ganymede, results in significant heat dissipation within Europa. Calculations indicate that this tidal heating is sufficient to maintain liquid water beneath Europa's ice crust. Moreover, observational evidence suggests that it is indeed probable, but not yet completely certain, that Europa harbors a subsurface ocean of liquid water whose volume is about twice that of Earth's oceans. The likely presence of abundant liquid water places Europa among the highest priority targets for astrobiology.
To support life, Europa would also require an inventory of biogenic elements and a source of sufficient free energy. The ability to support life does not of course guarantee that the origin of life took place on Europa, or that life is present today; answering these questions will require further exploration. This chapter considers Europa in an astrobiological context, distinguishing among what is known, what is supported by evidence but still uncertain, and what remains more speculative. We conclude with a discussion of future missions that will be needed to address current geological and astrobiological questions.
Jupiter and its satellites
The planet Jupiter, orbiting the Sun at 5.2 AU, is more massive than the other planets in the Solar System combined; it is 3.3 times more massive than Saturn and 318 times more than the Earth.