Ice geysers erupt on Enceladus, the bright and shiny inner moon of Saturn. Shown in this false-color image, a backlit view of the moon’s southern limb, the icy plumes were first discovered by instruments on the Cassini Spacecraft during close encounters with Enceladus in 2005. Eight source locations for these geysers have now been identified along substantial surface fractures in the moon’s south polar region. Researchers suspect the geysers arise from near-surface pockets of liquid water with temperatures near 273 kelvins (0 degrees C). (NASA/ESA/ SSI/JPL/Cassini Imagining Team)


As if the prospect of billions of potentially habitable exoplanets wasn’t enough to get people excited, what about all those watery exo-moons too?

The question arises as the Cassini mission makes its final pass near the now famous geysers at the south pole of the moon Enceladus.  The plumes are currently in darkness and so it’s a perfect time to tease out a particularly compelling aspect of the Enceladus story:  how hot is the inside of the mini-moon.  Earlier measurements of the water ice spray took place when the sun was on that southern pole, so this will be the first time Cassini can measure precisely how much of the already detected heat comes from the moon’s interior.

The expectation is that much of the heat does indeed come from inside, warmed substantially by tidal forces and perhaps hydrothermal vents that together serve to keep liquid a subsurface ocean all around the moon.  As a result, the evolving scientific view is that tiny Enceladus, one of 63 moons of Saturn, just may have the ingredients and characteristics that put it into an improbable habitable zone.

“Step by step, we’re learning about an environment that seemed impossible not long ago,” said Cassini Mission Scientist Linda Spilker.  “We know that Enceladus has some rocky core, and that it touches the liquid water.  We also know that some of the compounds identified in the geysers can only be formed when rock is in contact with hot water, and that must be happening at the bottom of the moon’s ocean.  All the pieces are coming together to tell us that the moon has an ocean that might be able to support life.”


NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015. The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.

The Cassini spacecraft, sponsored by NASA, the European Space Agency and the Italian space Agency,  captured this view on Oct. 28 as it neared Enceladus. The wavy boundary of the moon’s active south polar region — Cassini’s destination for this flyby — is visible at bottom.

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