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NASA's Cassini spacecraft has discovered the best evidence yet for a large-scale saltwater reservoir beneath the icy crust of Saturn's moon Enceladus.
The data came from the spacecraft's direct analysis of salt-rich ice grains close to the jets ejected from the moon.Data from Cassini's cosmic dust analyzer show the grains expelled from fissures, known as tiger stripes, are relatively small and predominantly low in salt far away from the moon. But closer to the moon's surface, Cassini found that relatively large grains rich with sodium and potassium dominate the plumes. The salt-rich particles have an "ocean-like" composition and indicate that most, if not all, of the expelled ice and water vapor comes from the evaporation of liquid salt water. The findings appear in the journal Nature.
Enceladus does not orbit in a perfectly circular motion, as our Moon does(*); instead its orbit is slightly elliptical – so the gravitational force on the surface changes throughout its orbit, constantly reshaping it. Hurford's theory suggests that when the moon's orbit is furthest from Saturn, tensile stress forces the fractures to open, releasing liquid water from beneath. As the moon's orbit moves closer to Saturn, compressive forces cause the fractures to close.
Rifts would open periodically, based on the position of the moon in its orbit, so "when a rift opens, it may not open all at once but instead open incrementally, like opening a zipper," said Hurford.
In an accompanying commentary, also published in Nature, Andrew Dombard, of Johns Hopkins University, Maryland, said that while the studies provide new directions for understanding Enceladus, we don't yet have all the answers as to why the moon behaves this way.
"Mimas, another icy satellite of Saturn with a closer orbit and a larger eccentricity, has followed a benign evolution," said Dombard. "Why Enceladus has ended up in such an active state whereas Mimas has not, are questions that will exercise planetary scientists for some time to come."
This is currently undecided. We are waiting until we see what targets may turn out to be environmentally sensitive, and then make a decision regarding the final disposition of the spacecraft. Going into Saturn's atmosphere as Galileo did at Jupiter may be difficult to accomplish because of the need to fly through the rings for an orbit or two and yet maintain a functioning spacecraft capable of going the rest of the way down to the atmosphere. The orbit of the spacecraft won't degrade by itself. We would have to actively control it to a Saturn impact if that is the way the mission is finally ended.