Not long after arriving at Saturn, NASA's Cassini orbiter flew past the moon Enceladus and observed enormous geysers of ice particles spewing from its surface. But the existence of these towering fountains caused astronomers to scratch their heads — how could a frozen world like Enceladus ever be hot enough for geysers to form? According to mission scientists reporting this week from the Lunar and Planetary Science Conference in Houston, Texas, the answer lies in the moon's ancient past, and with molecular nitrogen (N2) seen in the spray coming off the surface.
Astronomers believe that all the inner moons in the Saturnian system formed together from essentially the same stuff. So when the Huygens probe descended to Titan's surface in early 2005 and found it to be nearly bereft of N2, the implication was that other Saturnian satellites shouldn't have N2 either. So Enceladus needs to be generating N2 internally to have it at all.
Models created by Julie C. Castillo-Rogez and Dennis L. Matson (NASA/Jet Propulsion Laboratory) show that free atoms of nitrogen can form by breaking down ammonia (NH3) in the presence of liquid water (H2O). The nitrogen atoms, says Matson, would combine into N2 molecules and dissolve in the water. The trick is generating the nitrogen in an enclosed, confined space so that the free atoms aren't lost to space. The region between the bottom of Enceladus's icy outer layer and the top of its rocky core would be the ideal setting.
But there's a catch: Castillo-Rogez and Matson say that the reaction needs heat, and lots of it, to work. Somehow the ammonia needs to reach around 850K (1,070°F). How does a frozen world get that hot?
The most likely heat source is the decay of radioactive isotopes in the core. However, says Castillo-Rogez, long-lived isotopes — the kind that would be decaying today — can't produce enough heat to drive the reaction. Only short-lived radioisotopes (such as aluminum-26 and iron-60) will work.
Putting the pieces together, Castillo-Rogez and Matson suggest that Enceladus must have accumulated ample ammonia and short-lived radioactive isotopes when it formed. Those isotopes disappeared quickly, within the first 7 million years or so of the moon's life. But their rapid decay yielded a powerful burst of heat that broke down the ammonia and melted the ice layer nearest the core. After that, tidal forced induced by Saturn kept the interior of Enceladus warm and slushy enough for the N2 reservoir to bubble out slowly. That's why we see molecular nitrogen today.
Not everyone agrees with the hypothesis, however. Icy-satellite expert William B. McKinnon (Washington University) is skeptical. He says ammonia breaks apart at much lower temperatures than what Castillo-Rogez and Matson require, and it is unnecessary to invoke such high heat to explain the N2. According to McKinnon, a period of extreme tidal flexing (like what Jupiter's moon Io now undergoes) might be the only trigger needed. And Enceladus once occupied an orbit that made it particularly susceptible to internal heating from tidal stress.
One last question remains: If all Saturnian moons are made of the same stuff, why aren't geysers found on Enceladus's neighbors? Certainly tidal flexing has something to do with it, but that can't be the whole story because other Saturnian moons have endured similar tidal conditions in the past. Subtle differences in overall size, core size, and composition may also play a part.