Callisto is the outermost of the Galilean moons of Jupiter.  Recent discoveries by the Galileo probe suggest that a saltwater sea may exist beneath the rock and ice surface.  This raises some interesting questions about the possibilities of life on Callisto.  Where there is liquid water, life can exist.  After Europa, Callisto may be the best candidate for extraterrestrial life in the Solar System.

Evidence for there being such an ocean comes from fluctuations in the moon's magnetic field which match the rotation of Jupiter.    The most likely explanation for this is that Jupiter's magnetic field, which rotates with the planet, induces electrical currents within Callisto which in turn creates its own magnetic field.

Callisto displays signs of ancient and sustained bombardment with space debris - asteroids, meteorites and comets - and is in fact the most heavily-cratered object in the entire Solar system.  It displays no sign of hot or cold volcanism, as do its sister Galilean moons - Ganymede, Europa and Io - and it is something of a mystery as to the source of the heating that maintains the suspected sub-surface ocean in a liquid state.  However residual heat from radioactive decay in Callisto's rocks, or less likely heating from the limited tidal actions associated with Jupiter and the other Galilean moons, may be sufficient to maintain a reasonable core temperature.  In essence Callisto may work the same way as Europa is thought to, but the energy input will be very much lower.  The variations in the tidal pull that Callisto undergoes as it orbits Jupiter are tiny compared with the forces that the inner moons - Io and Europa - experience.

A liquid saltwater ocean is one of the main prerequisites for life, and planetary scientists are now eager to explore Callisto at close quarters to investigate the possibilities.

See Callisto's salt water© All images NASA


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