On April 3, planetary scientists announced the discovery of a subsurface ocean (at least as big as Lake Superior) sitting on top of Enceladus' rocky core at a depth of about 31 miles. The discovery was made by NASA's Cassini spacecraft that buzzed the 300 mile-wide Saturnian moon on three occasions between 2010 and 2012. This is the first strong evidence of the existence of a sub-surface liquid body of water, but ever since Cassini observed the active moon's water ice plumes, scientists have speculated about their source.
A "snowball" moon: Enceladus is bathed in light in this view from Cassini -- its icy surface exhibits huge cracks, a sure sign of an internal heat source that drives the little moon's famous geysers.
Enceladus' famous plumes are known to be composed of salty water vapor laced with organic compounds, an indication that a sub-surface liquid water reservoir is being heated and blasted through cracks in the moon's icy crust.
A mosaic of the global map of Enceladus, created using images taken during Cassini spacecraft flybys.
In 2012, Cassini captured this view of Enceladus in front of Saturn's rings and bigger moon sibling Titan in the background.
The nightside of Enceladus and backlit plumes of water ice.
A close-up of Enceladus' cracked icy terrain as observed by Cassini. The relative sparsity of impact craters is an indicator of how 'young' the moon's crust is -- ice is continuously cycled with new layers being replaced by tectonic activity.
This 2010 observation by Cassini shows the new and old surface of Enceladus -- a newly created terrain in the upper right meets older, cratered terrain in the lower left.
Small water ice particles fly from fissures in the south polar region of Saturn's moon Enceladus in this image taken during the Aug. 13, 2010, flyby of the moon by NASA's Cassini spacecraft.
This sweeping mosaic of Saturn's moon Enceladus provides broad regional context for the ultra-sharp, close-up views NASA's Cassini spacecraft acquired minutes earlier, during its flyby on Aug. 11, 2008.
A heat map of Enceladus' famous "tiger stripes" -- long fissures etched into the moon's south pole where subsurface water is released as water vapor that quickly freezes to create plumes of water ice particles.
A high-resolution view of Enceladus' vast "tiger stripes" etched into the moon's south pole as observed by the Cassini mission.