There are two reasons why scientists think there is water in Enceladus: heat and geysers.
Despite its distance from the Sun and its high albedo (Enceladus reflects almost all of the sunlight it receives), the southern polar region of this moon is -- relatively speaking -- unusually warm. Recent observations by the Cassini spacecraft measured the surface of Enceladus' south pole to be minus 116 degrees Celsius (minus 177 degrees Fahrenheit), considerably warmer than the minus 205 degrees Celsius (minus 337 degrees Fahrenheit) typical of celestial bodies that far from the Sun.
Scientists believe that the heat causing the thermal anomaly at the south pole is coming from deep within the moon's interior. Combine this heat with the geysers observed by Cassini erupting from Enceladus' southern regions, and the evidence for liquid water increases.
Images from the spacecraft show geysers throwing off huge jets of vapor, revealed by Cassini's Ion and Natural Mass Spectrometer (INMS) to be composed mostly of small ice particles and water vapor. The great power of Enceladus' geysers makes the presence of liquid water near the surface very likely.
Cassini scientists theorize that heat from the moon's core raises the temperature enough to melt some of the ice underneath Enceladus' south pole and create underground reservoirs of liquid water, which in turn feed the geysers. It's still not clear how big these reservoirs of water are.
