How did Saturn’s moon get its tiger stripes? A ‘just so’ space story

Saturn’s moon Enceladus is an intriguing oddity. The tiny moon, only about 300 miles across, contains a rocky core and global subsurface ocean beneath an icy shell. Scientists believe the icy water world could be another potential spot for life in our solar system.

And along the south pole, four “tiger stripes” can be seen.

The stripes are large cracks that allow jets of water to shoot up through the icy shell, creating plumes that have intrigued scientists since the Cassini mission flew through them in 2015.

A new study analyzing the tiger stripes published Monday in the journal Nature Astronomy. The cracks are named for cities referenced in the folk tale “One Thousand and One Nights.”

“First seen by the Cassini mission to Saturn, these stripes are like nothing else known in our solar system,” said Doug Hemingway, study author at the Carnegie Institution for Science. “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.”

Researchers wanted to understand the activity they were observing and used numerical modeling in an attempt explain the stripes and plume activity.

“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, study author and assistant professor of earth and planetary sciences at the University of California, Davis.

Enceladus is Saturn’s sixth largest moon and its tides are at the mercy of the gas giant’s gravity. Those tides cause heating and cooling. Enceladus feels those forces especially at its poles, stretching the icy shell thin. The tiny moon has an eccentric orbit that brings it closer than usual to Saturn at times but also pushes it out further from Saturn than the other moons as well. Saturn has 82 moons.

Cooling can cause the liquid water beneath the moon’s ice shell to freeze and expand, causing the ice shell to crack.

Enceladus is already a cold place in our solar system with an average surface temperature of negative 328 degrees Fahrenheit. This implies that any cracks in the ice would immediately freeze shut — but the cracks at the south pole creating the tiger stripes remain open and reach the subsurface ocean.

The researchers used their model to determine that Saturn’s tidal forces cause the internal ocean to remain incredibly active, which generates heat through energy and won’t allow the cracks to seal.

After the first crack formed and didn’t freeze, the plumes rising up through it allowed new, parallel cracks to form.

“Our model explains the regular spacing of the cracks,” Rudolph said. “That caused the ice sheet to flex just enough to set off a parallel crack about 35 kilometers away.”

The researchers also determined that the tiger stripes are unique to Enceladus because they only could have formed there. A larger moon would have stronger gravity to prevent cracks from forming.

“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.”