Some of the moons of gas giant planets have a unique property: They are worlds that have a source of heat other than the Sun.  The huge tidal forces imparted by their host planet and fellow moons cause their crust to grow and shrink, creating huge friction and heating the rock.  It means that even if these moons are far from the warmth of the Sun, they can still host liquid water.  There are two moons in our own solar system that are especially enticing: Jupiter’s second moon Europa, and the icy moon of Saturn known as Enceladus.  Each host...