Enceladus, the icy moon that circles Saturn and shoots out jets of water, emits a much larger amount of water at the farthest point in its orbit, according to a new study published in the journal Nature. The discovery backs up a years-old theory and provides the researchers with fresh insight into this geophysically intriguing body.
Enceladus, named after one of the giant children of mother Earth in Greek mythology, has long intrigued planetary scientists (as well as astrobiologists wondering if primitive life could exist in extreme environments).
The water-rich moon is "one of the few places beyond Earth where we can watch geology happen in real time, giving us a primer for understanding other, less active, icy worlds," John Spencer, a scientist at the Southwest Research Institute in Boulder, Colo., who was not involved in the study, wrote in a commentary.
Any two bodies held in each other's gravitational thrall are subject to tidal forces. That's how the moon causes Earth's oceans to rise and fall. Enceladus also gets kneaded as it travels around Saturn, but a larger moon named Dione throws a wrench into the works, making its orbit slightly eccentric.
"The daily variations in the tidal stresses from Saturn due to that eccentricity distort Enceladus and dump gigawatts of frictional heat into its interior," Spencer wrote.
All that distortion causes Enceladus to heat up, just as rubbing your hands together will quickly warm them. That heat causes water vapor and ice to squirt out of its south end, from four parallel 'tiger stripe' fractures around its southern pole. But it didn't seem to happen consistently -- scientists saw hints of variation in the plume that the fractures created.
To figure out exactly when and why Enceladus would start to spit, a team of researchers led by Matthew Hedman of Cornell University studied 252 images taken by NASA’s Cassini spacecraft between 2005 and 2012. With so many images, they were able to figure out when extra activity from the tiger stripes linked to Enceladus’ orbit.
They found that when Enceladus was at the farthest point in its orbit from Saturn, the plume was about four times brighter than when the moon was at its closest approach. This dramatic difference is probably caused by tidal forces. When it’s closest, the moon is under compression -- being squeezed. But at its farthest point, it’s under tension -- being stretched. Under tension, the tiger stripe cracks open wider, letting more water vapor and ice bits come out.
Understanding how these jets works may help researchers understand more about what’s happening beneath the icy moon’s surface -- whether it has an underground ocean, for example.
"Geology, dealing as it does with the complex behaviors and long memories of materials in the solid state, tends to be a messy business," Spencer wrote. "So it is always startling and instructive when simple patterns like this emerge."