Over the last ten years, we on Cassini have built an edifice of knowledge of Saturn's active moon, Enceladus, that has set planetary exploration abuzz.
With growing degrees of confidence, we have found, in one discovery after another, that this small world contains a liquid water environment, deep beneath the ice capping its southern hemisphere, that is laced with organic compounds, comparable in salinity to the Earth's oceans, and of all things under the Sun, venting to space in a spectacular and expansive array of 101 geysers reaching thousands of miles into the space. In all, these findings point to the solar system's most accessible extraterrestrial watery environment -- a habitat -- within Enceladus where, perhaps, a second genesis has taken hold. It is a possibility that can bewitch the mind and strike awe and exaltation in the most stolid of souls.
One unanswered question all this time has been: Just how extensive is the water layer within Enceladus? Evidence has been gathering since Cassini's first visits to this moon for a lens, or sea, of water, as wide as the South Polar Terrain ... that unique province at the south pole that is ringed by mountainous folds and ridges and slashed by 4 major fractures from which the geysers erupt. Then in 2013/2014, Cassini gravity measurements indicated much stronger evidence for such a south polar sea, about 35 kilometers below the surface and about 10 kilometers thick, but perhaps connected to a thinner global ocean. It was unclear.
Today, the members of my imaging science team, using our high resolution images of Enceladus' surface taken over the last 7 years, have confirmed that Enceladus' water layer is indeed global. How did they do it? By looking for a libration ... a small, cyclical, back-and-forth deviation from uniform rotation ... and finding that it is present and much too large to be a libration of the entire body. The conclusion: It is a libration in the thin, outer ice shell only, indicating that ice shell and rocky core are decoupled and separated by a liquid layer.
It has been a hard problem to solve, requiring persistence, painstaking analysis, an understanding of orbital and rotational dynamics, and bringing to bear the full and tedious brunt of statistical analysis. But it has yielded gold.
So here's raising a glass to our kind. We have done a remarkable thing ... to set our craft on a long-distance mission in search of lovely blue oceans like those of Earth, and have it answer us with such gratifying certitude.
Carolyn Porco Cassini Imaging Team Leader Director, CICLOPS Boulder, CO