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Why do scientists think Enceladus has a subsurface ocean?

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Saturn Saturn and its spectacular rings, in a natural-color composite of 126 images taken by the Cassini spacecraft on October 6, 2004. The view is directed toward Saturn's southern hemisphere, which is tipped toward the Sun. Shadows cast by the rings are visible against the bluish northern hemisphere, while the planet's shadow is projected on the rings to the left.

Scientists have concluded that Enceladus likely harbors a global subsurface ocean based on several key lines of evidence gathered by the Cassini spacecraft:

Cryovolcanic Activity: The moon exhibits high heat flow and erupts plumes of water vapor and ice from tectonic fractures known as "tiger stripes." The high speed of particles within these jets suggests they originate from a liquid source.1
Chemical Composition: The presence of sodium and other minerals in the ejected ice particles indicates that the water must have been in contact with a rocky ocean floor to dissolve these materials.1
Hydrothermal Vents: Analysis of silicate dust grains spewed from the plumes points to the existence of hydrothermal vents, where water is heated by contact with hot rocky material at the bottom of the ocean.1
Rotational Measurements: Observations of the moon’s rotation provide evidence that an ocean exists beneath the icy crust and covers the entire globe.1

This subsurface environment is considered a potential habitat for life.2