Two new studies hint at a richer picture of what’s happening on Saturn’s extraordinary icy moon Enceladus.
At about 500 kilometers in diameter, Enceladus is a diminutive natural satellite. But when it comes to scientific discovery it punches way, way above its size.
Since 2005, when NASA’s Cassini mission took a close look at this ice-covered moon, we’ve known that it spews interior material out into space – helping form Saturn’s vast E-ring. This active cryovolcanism seems to point to a subsurface ‘pocket ocean’ (less kindly we might call it a lake) towards the southern polar regions of Enceladus. As time goes by, more and more evidence is accumulating to suggest that this internal body of water has been – and perhaps still is – in direct contact with a rocky, mineral-rich, core.
Cassini observations indicate the presence of more than just water in Enceladus’s geyser-like emissions. There appear to be sodium salts in E-ring material and hints of simple carbon compounds from flyby measurements of the geysers themselves.
Now a new study by Glein, Baross, and Waite in the journal Geochimica et Cosmochimica Acta applies a detailed chemical model to the constraints of the Cassini data to ask what the internal body of water has to be like. The results suggest that it has to contain plenty of good old sodium chloride (table salt) and a big helping of sodium carbonate – akin to some of Earth’s chemically extreme environments like Mono Lake in California.
This ‘soda ocean’ should be strongly alkali too – with a pH of 11 to 12. How does it get to be this way? If terrestrial oceans are a good proxy then these conditions suggest a process called serpentinization, where raw mantle material rich in iron and magnesium reacts with water. The scientists also point out a highly desirable side-effect of serpentinization – the production of molecular hydrogen which could serve as a potent energy source for analogs to Earth’s microbial fauna living in abyssal realms.
The catch is that we don’t yet know if such chemical activity is ongoing or whether the soda ocean that may exist in Enceladus is simply a remnant of times past.
But up at the surface there is still plenty of action. A new study published in Nature by Spitale et al. takes a clever and contrarian look at the famous geysers and argues that instead of ‘jets’ of material, in many cases these are much more likely to be sheets or curtains erupting along the great tiger-stripe cracks at the moon’s southern end.
In a nutshell, curtain-like eruptions can play optical tricks on the observer – in this case the Cassini images. Viewing angles and the wiggly geometry of the surface cracks can conspire to enhance the brightness of the material along vertical directions, presenting an illusion of bright jets – the way a wavy curtain can appear to have vertical stripes.
If correct, this means that we can get a better handle on the physical conditions that create these outbursts. The nature of the sheets or curtains relates to how this watery vapor is being primed beneath the surface, whether in smaller or larger cave-like cavities above the interior ocean for example.
Both of these investigations help advance our understanding of little Enceladus, and further bolster its status as one of the most interesting and important targets for solar system exploration and the search for life elsewhere.