The large jets of salty water found at the south pole could emerge from wet pockets inside the icy shell of Saturn’s moon itself.
Since a series of flybys by the Cassini probe in 2005 revealed the existence of geysers of water vapor at the south pole of Enceladus, researchers believe they are due to the presence of a large ocean that extends under the icy surface of that Saturn’s moon. Now, however, a series of simulations carried out by a team of researchers from Dartmouth College in Hanover and the Universities of California at Santa Cruz, Oxford, and New York suggest another possible source for the geysers: wet pockets in the icy crust of Enceladus itself. The idea was proposed during the meeting that the American Geophysical Union (AGU) held in New Orleans between December 13 and 17.
According to planetary scientist Jacob Buffo, one of the authors, “We may not yet have gotten the ‘straw’ through the ice sheet to the ocean. We may have only gotten as far as this weird pocketing.” The researcher regards the finding as a warning. In fact, the subterranean ocean of Enceladus is one of the best places to search for life in the Solar System, and designs for future missions to visit it are based on the idea that samples taken from geysers would directly test the contents of the ocean, without the need to drill or melt the ice.
But if Buffo and his colleagues are correct, the data collected by those missions would not be from the subterranean ocean but could come from these moist areas in the icy crust itself. “The simulations -explains the researcher- suggest that we could be sampling this muddy region in the middle of the ice sheets and that it could not be the same chemistry that is lower down, in the ocean”.
Ideal for looking for life
Enceladus has captured the imagination of planetary scientists ever since NASA’s Cassini spacecraft revealed spectacular plumes of water rising from its south pole in 2005. At the time, the researchers already wondered if these vapor jets could originate from Enceladus’s own icy surface, where the friction caused by earthquakes could melt the ice and let it escape as pure water vapor into space. But subsequent evidence collected by Cassini convinced most scientists that the geysers originate from fractures in the icy shell that reach into a large salty underground sea.
One of the most convincing pieces of evidence was, according to physicist Colin Meyer, co-author of the research, the fact that geysers contain salts, and early versions of the earthquake idea could not explain the presence of those salts.
But Meyer, who has studied the physics of sea ice on Earth, realized that pockets of meltwater in Enceladus’ own icy shell could concentrate salts and other compounds. He, Buffo, and their colleagues applied computer simulations developed for sea ice on Earth to icy conditions observed on Saturn’s moon. And the team found that Enceladus could easily generate pockets of this ‘frozen mush’ within its ice crust and vent the contents of that mush into space, salts and all.
Of course, Meyer clarifies, that doesn’t mean Enceladus doesn’t have an ocean. On the contrary, it is practically certain that it does. And it doesn’t mean the ocean isn’t habitable either.
Despite this, the implications of this study “are enormous”, especially for future missions to search for life on this moon of Saturn. Because if these geysers do not come from the ocean but from the ice cover itself, our ideas about the information they reveal to us about the interior of the moon will have to change radically. And that can become a big problem when planning future exploration missions.
