NASA | JPL-Caltech | Cassini | 2017 Feb 16
How a puzzling sensor reading transformed NASA's Cassini Saturn mission and created a new target in the search for habitable worlds beyond Earth.On Feb. 17, 2005, NASA's Cassini spacecraft was making the first-ever close pass over Saturn's moon Enceladus as it worked through its detailed survey of the planet's icy satellites. Exciting, to be sure, just for the thrill of exploration. But then Cassini's magnetometer instrument noticed something odd.
Since NASA's two Voyager spacecraft made their distant flybys of Enceladus about 20 years prior, scientists had anticipated the little moon would be an interesting place to visit with Cassini. Enceladus is bright white -- the most reflective object in the solar system, in fact -- and it orbits in the middle of a faint ring of dust-sized ice particles known as Saturn's E ring. Scientists speculated ice dust was being kicked off its surface somehow. But they presumed it would be, essentially, a dead, airless ball of ice.
What Cassini saw didn't look like a frozen, airless body. Instead, it looked something like a comet that was actively emitting gas. The magnetometer detected that Saturn's magnetic field, which envelops Enceladus, was perturbed above the moon's south pole in a way that didn't make sense for an inactive world. Could it be that the moon was actively replenishing gases it was breathing into space?
Thus began a hunt for clues that has turned out to be Cassini's most riveting detective story. ...