18 December 2009
As a small, icy moon of Saturn, Enceladus has been getting its close-up from the Cassini mission. In 2005, spectacular geysers were observed erupting from the surface, fueling speculation that a liquid ocean—and maybe even life—lay just beneath the ice. Unfortunately, geologist Susan Kieffer of the University of Illinois splashed a dose of cold water on those fanciful ideas at the P41C Shoemaker Lecture, Enceladus: Oasis or Ice Ball?
The controversy stems from the fact that here on Earth, geysers such as Old Faithful result from water boiling beneath the crust exploding out at tremendous speed, as seen in the bright bursts from the surface in the image from Cassini above. Some scientists proposed a similar mechanism for the geysers on Enceladus, a model dubbed “Cold Faithful.” In contrast, Kieffer and her colleagues mathematically simulated a competing idea; the “Frigid Faithful” model, in which water vapor and gases trapped in icy cracks forcibly erupt due to contact with the vacuum of space.
It works like this: tidal forces stretch and squeeze the little moon and create a tectonic “hot spot” at the south pole. The pressure creates long cracks known as tiger stripes (seen as blue bands in the picture on the right), which expose underlying layers to space. A reservoir of gas and shredded ice travels to the surface via long vents and explodes outward with great force. The smoking gun in this model is the gases that have been detected with the water vapor. Kieffer said that there is no way to dissolve them in water at the temperatures that exist on Enceladus.
Keiffer believes that her models offer a much more plausible scenario, more in keeping with observations from Cassini. Her calculations indicate enough that “Frigid Faithful” would have energy to vaporize water in accordance with the detected geysers.
While this news might disappoint extraterrestrial life enthusiasts, it shows how interesting and varied a chilly environment can be. As Kieffer explained, you cannot equate a cold environment with a lack of energetic phenomenon.
-Adam Mann,UC Santa Cruz Science Communication Graduate Student