Enceladus is one of the stranger bodies in our solar system. It's a frozen, barren satellite, orbiting Saturn, with not much going on about it -- or so it would seem at a first glance.
Like Europa, Enceladus is also believed to host an ocean of subsurface water, with hydrothermal vents sending massive jets of water into space. Given that its chemistry is rich in salts, silicates, and iron, it could very well host the ingredients necessary for life to emerge. The Cassini mission also found traces of simple and complex organic molecules (such as benzene) around Enceladus.
With this alone, Enceladus has become one of the most interesting spots in our solar system. But that's not its only trick.
Slashed across its south pole are four straight, parallel fissures where water erupts. These "tiger stripes" are unlike anything we've seen in the solar system. We don't know how they were formed and why they exist in the first place. To make matters even more mysterious, they only lie in the southern hemisphere and are spaced at even distances.
"We want to know why the eruptions are located at the south pole as opposed to some other place on Enceladus, how these eruptions can be sustained over long periods of time and finally why these eruptions are emanating from regularly spaced cracks," said Max Rudolph, assistant professor of earth and planetary sciences at the University of California, Davis.
Now, researchers at UC Davis, the Carnegie Institution and UC Berkeley believe they have an explanation. Using numerical modeling, they explain that tidal heating is responsible for the fissures and their unusual distribution.
"First seen by the Cassini mission to Saturn, these stripes are like nothing else known in our Solar System," lead author Hemingway explained. "They are parallel and evenly spaced, about 130 kilometers long and 35 kilometers apart. What makes them especially interesting is that they are continually erupting with water ice, even as we speak. No other icy planets or moons have anything quite like them."
They found that there's no particular reason why the stripes emerged in the southern hemisphere -- they could have emerged in any one half, it was a coin toss -- and it just happened to be the southern one.
As it orbits Saturn, Enceladus' internal structure is tidally drawn to the planet. Just like the Earth's seas experience low tide and high tide due to gravitational attraction by the moon, Enceladus' ice, water, and rocky core are tidally drawn to Saturn. This force creates a lot of friction, which in turn produces heat -- this is the reason why Enceladus isn't completely frozen and has liquid water beneath its surface, despite being so far away from the sun.
But due to its eccentric orbit, Enceladus is sometimes closer to Saturn, and other times a little farther away -- which causes the moon to be slightly deformed. This means that in some regions, more water warms and becomes liquid, while in other parts, the opposite process takes place. As liquid water solidifies under the shell, it expands in volume, putting pressure on the surface ice; when this happens and enough force is exerted, the ice simply cracks.
The tiger stripes are essentially the Enceladus version of stretch marks.
The researchers modeled this phenomenon and found that it also explains why the stripes are spread evenly.
"That caused the ice sheet to flex just enough to set off a parallel crack about 35 kilometers away," Rudolph added.
But it gets even more interesting. The surface of Enceladus is approximately -200 degrees Celsius cold -- so it's cold enough to freeze back in place quite quickly. The continuous tidal forces would constantly generate cracks, but it would be a dynamic stretching-healing process, which does not seem to be the case. Instead, the stability of the cracks indicates that they stretch all the way down to the liquid ocean below.
In addition, if the moon was a bit larger, its own gravity would be strong enough to prevent the fractures from opening all the way down -- so these stripes could have only formed on Enceladus, researchers conclude.
"Since it is thanks to these fissures that we have been able to sample and study Enceladus' subsurface ocean, which is beloved by astrobiologists, we thought it was important to understand the forces that formed and sustained them," Hemingway said. "Our modeling of the physical effects experienced by the moon's icy shell points to a potentially unique sequence of events and processes that could allow for these distinctive stripes to exist."
