An explanation has been offered for the geyser curtains on Enceladus discovered by the Cassini spacecraft in 2006. These geysers allow the ocean inside the small moon of Saturn to escape into space, providing an opportunity for future missions to sample the composition without having to drill inside. New evidence suggests the geysers are made possible by fissures produced during a cooling phase in Enceladus's long orbital cycles.
When astrobiologists try to imagine life on other worlds, there are few things they agree on – but there's a near-consensus that liquid water is key. Yet, since Mars dried out, there has been no liquid water found on the surface of any body in the Solar System other than Earth. However, a number of Moons (and perhaps Pluto) have confirmed or suspected internal oceans, representing the best chance for extraterrestrial life we can reach.
Europa may have better prospects of supporting life in its depths, particularly complex life, but detecting it could prove hard if it requires drilling through kilometers of ice. On the other hand, Enceladus spills its guts all over the inner-Saturnian orbit. If there is life within, traces may be shot out through those geysers where we can sample it.
This makes a new paper in Geophysical Research Letters explaining why the geyser curtains exist rather important.
The geysers emerge from long, thin “tiger-stripes” near Enceladus' south pole – but the origins of these cracks, thought to run 5-14 kilometers (3-8 miles) through the moon's icy crust, have been mysterious.
Rudolph and co-authors suggest they are the product of changes to that crust resulting from variations in Enceladus' orbit around Saturn.
Even Earth's orbit varies between being more rounded and more elongated, one of the Milankovich cycles that controlled the climate for millions of years until humans caused more rapid changes. With so many moons of Saturn – some much larger than Enceladus – exerting their gravitational pull, it is inevitable its orbit experiences more dramatic changes over periods of 100 million years or so.
These changes, Rudolph argues, produce warming and cooling cycles on Enceladus, as the squeezing force produced by Saturn strengthens and weakens. During warmer phases, the internal ocean expands, and the crust of ice thins, only to thicken again as things cool down.
During cooling periods the expanding ice puts pressure on the ocean, which in turn pushes back against the ice, cracking the shell to create fissures where the ice is thinner.
Europa experiences a similar expansion and contraction cycle, but Rudolph's modeling indicates the changes there are not large enough to produce cracks that reach even halfway to the internal ocean. If we want to sample the interior for life, we're probably going to have to drill.
Modeling done by Rudolph finds the previously favored explanation, of pressure from the ocean, might be sufficient to create the cracks, but not to push water to the surface. Instead, they endorse an existing theory on that question, that water is getting into the cracks and boils away when exposed to the vacuum of space.
The conclusions go some way towards explaining one of the major mysteries about Enceladus's geysers – why have they not caused the little world to boil away over its four billion-plus year lifespan. If the cracks only exist during certain periods of its cycle, much less material would be lost.
The work also implies we are very lucky to be visiting Saturn's moons in a period where this is happening – if our probes had arrived at a different stage, Enceladus might look like a much less exciting object.