Enceladus’s Fault Lines are Responsible for its Plumes

The Search for Life in our Solar System leads seekers to strange places. From our Earthbound viewpoint, an ice-covered moon orbiting a gas giant far from the Sun can seem like a strange place to search for life. But underneath all that ice sits a vast ocean. Despite the huge distance between the moon and the Sun and despite the thick ice cap, the water is warm.

Of course, we’re talking about Enceladus, and its warm, salty ocean—so similar to Earth’s in some respects—takes some of the strangeness away.

Enceladus is Saturn’s sixth-largest moon, and the Cassini spacecraft observed it during its mission to the Saturn system. Scientists discovered that plumes of water originating from Enceladus’ southern region are responsible for one of Saturn’s rings. They also discovered that the water is salty. Any place we find warm, salty water attracts our immediate attention, even when it’s covered by kilometres of ice and is 1.5 billion kilometres away from the life-giving Sun.

There’s lots of talk about a future mission to Enceladus to explore the moon and its potentially life-supporting ocean in more detail. But until then, scientists are working with their current data, and using models and simulations to understand the moon better.

Enceladus’ most defining surface features are its Tiger Stripes. They’re four parallel, linear depressions on the moon’s surface about 130 km long, 2 km wide, and 500 meters deep. They have higher temperatures than their surroundings, indicating that cryovolcanism is active. The stripes are the source of Enceladus’ plumes.

New research suggests that strike-slip faults at the moon’s prominent Tiger Stripe features allow plumes of water from Enceladus to escape into space. It’s published in Nature Geoscience and titled “Jet activity on Enceladus linked to tidally driven strike-slip motion along tiger stripes.” The lead author is Alexander Berne, a doctoral candidate in Geophysics at the California Institute of Technology.

The plumes above the Tiger Stripes aren’t stable and continuous. They wax and wane as the moon follows its 33-hour orbit around Saturn. Tidal heating keeps the moon’s water in liquid form, and according to the researchers, the same tidal forces are responsible for the intermittent plumes. Theory shows that tidal forces open and close faults at the Tiger Stripes like an elevator door, and that turns the plumes on and off.

However, those theories can’t accurately predict the timing of the plumes’ peak brightness. They also show that tidal forcing alone doesn’t provide enough energy to open and close the faults.

This research digs deeper into the question and provides an answer. The authors say that rather than acting like an elevator door, strike-slip faults at the Tiger Stripes open and close to regulate plume activity. This is similar to what happens on Earth in places like the San Andreas Fault. It’s a strike-slip fault where one side shears past the other, causing Earthquakes. The critical part of this is that strike-slip faults require less energy than the elevator opening and closing scenario.

Models are more effective as they’re fed more detailed and accurate data. Berne and his co-researchers built a numerical model that simulates the strike-skip faults on Enceladus. They included friction, compressional forces and shear forces. The numerical model showed the faults acting in concert with the changing plumes. This strongly suggests that Enceladus’ orbit and the tidal forces acting on the moon cause the strike-slip faults to open and close.

The Tiger Stripes have bent sections that pull apart under strain. Since they’re bent, an opening appears as they slide. The plumes come from these openings.

The research team’s work and previous research into the Tiger Stripes by NASA’s Jet Propulsion Laboratory both support the idea that the plumes come from these strike-slip faults.

“We now appear to have both geologic and geophysical reasons to suspect that jet activity occurs at pull-aparts along Enceladus’s tiger stripes,” said lead author Berne.

Enceladus gets most of its attention because of its potential to support life. The plumes themselves aren’t part of what life needs, but they’re a window into the moon’s potential habitability.

“For life to evolve, the conditions for habitability have to be right for a long time, not just an instant,” said study co-author Mark Simons, Professor of Geophysics at Caltech. “On Enceladus, you need a long-lived ocean. Geophysical and geological observations can provide key constraints on the dynamics of the core and the crust as well as the extent to which these processes have been active over time.”

There’s a lot more work to be done to understand Enceladus. On Earth, satellites can monitor the movement at strike-slip faults and use it to better understand Earthquakes. Once we get a spacecraft to Enceladus, it could do the same.

“Detailed measurements of motion along the tiger stripes are needed to confirm the hypotheses laid out in our work,” Berne says. “For instance, we now have the capacity to image fault slip, such as earthquakes, on Earth using radar measurements from satellites in orbit. Applying these methods at Enceladus should allow us to better understand the transport of material from the ocean to the surface, the thickness of the ice crust, and the long-term conditions which may enable life to form and evolve on Enceladus.”

When we get a spacecraft to Enceladus, it can monitor the faults and jets over multiple orbits. That will allow researchers to test their predictions.

“These observations could provide key constraints on the mechanical nature of the crust, tidal controls on jet activity and the evolution of the south polar terrain,” the authors conclude.