The discovery of volcanic eruptions on Saturn’s moon Enceladus tantalised astronomers with questions of how these eruptions occur, and whether they are a semi-permanent phenomenon or something we were lucky to see. New research offers a theory, and indicates that the moon’s distinctive “tiger stripes” can sustain eruptions over geological timespans.
We know that Enceladus has been erupting continuously since at least 2005, when the Cassini spacecraft made its first close approach. Moreover, ice grains thrown into space from the small moon are the source of Saturn’s E-ring, and the ring’s stability since its 1966 discovery demonstrates that volcanic activity has been going on far longer.
The only apparent source of heat is the tidal flexing caused by the non-circularity of Enceladus’ orbit and the influence of nearby larger moons. This has created a liquid ocean beneath Enceladus’ southern hemisphere, covered by an icy shell kilometers thick. It is thought that boiling water rises through fracture lines to become geysers that are dotted along the tiger stripes.
The tiger stripes are long thin lines near Enceladus' south pole dotted with active geysers. NASA/JPL-Caltech/Space Science Institute
Puzzlingly, however, this does not seem sufficient to supply enough energy to keep Enceladus so active. Exposed to the icy cold of space so far from the Sun, the cracks should freeze over quickly in the absence of a powerful energy source to maintain them. This has inspired the theory that we are seeing a temporary outbreak of volcanism, expending stores of energy built up in previous eras.
However, modeling published in the Proceedings of the National Academy of Sciences shows that if the long thin cracks sit atop slots that penetrate Enceladus’ ice shell down to the ocean, the same tidal flexing that keeps that ocean liquid will hold them open.
Successive bouts of flushing and filling with ocean water, driven by Saturn's tidal influence would dissipate enough heat to keep the slots open, argue the authors Dr Edwin Kite of the University of Chicago and Professor Allan Rubin of Princeton.
Enceladus’ eruptions cycle with its 33-hour orbit around Saturn, but lag peak tidal extension, something which previously puzzled astronomers. However, Kite and Rubin show that if the eruptions are caused by turbulent water flow this lag is exactly what we would expect. This turbulent flow generates heat when encountering the narrow slots, preventing them from freezing over.
When water near the moon's surface partially evaporates it cools and becomes more saline, sinking to the ocean depths like Antarctic bottom water to be replaced by warmer liquids. The authors conclude the system is sustainable for periods of more than a million years.
The material thrown up from the sunless sea is salty water, with ammonia, silica and simple organic molecules mixed in. Combining liquid water and organic molecules, Enceladus has the basic materials for life, and some astrobiologists consider it our best chance for finding extraterrestrial organisms.
Any lifeforms would be microscopic, but might be vented into space, rather than trapped under ice or rock, as could be the case on Europa or Mars. NASA is considering a mission to Enceladus for this reason.