Saturn’s 300-mile- (500 km) diameter moon Enceladus is thought to have an outer ice shell covering a liquid water ocean. Slashed across the moon’s south pole are four straight, parallel fissures or ‘tiger stripes’ from which water erupts. Using numerical modeling, a team of planetary researchers from the Carnegie Institution for Science and the Universities of California Davis and Berkeley now explains how tidal heating causes Enceladus’ erupting stripes and their spacing.
Enceladus’ tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s south pole and creating Saturn’s mysterious E-ring. Evidence for this has come from NASA’s Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. Tiger stripes are visible in false-color blue. Image credit: NASA / ESA / JPL / SSI / Cassini Imaging Team.
“First seen by the Cassini mission to Saturn, these stripes are like nothing else known in our Solar System. They are parallel and evenly spaced, about 81 miles (130 km) long and 22 miles (35 km) apart,” said Dr. Douglas Hemingway, a researcher at the Carnegie Institution for Science.
“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.”
“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,” added Dr. Max Rudolph, from the University of California, Davis.
The scientists used models to investigate the physical forces acting on Enceladus that allow the tiger stripe fissures to form and remain in place.
They were particularly interested in understanding why the stripes are present only on the moon’s south pole, but were also keen to figure out why the cracks are so evenly spaced.
They revealed that the fissures that make up the stripes could have formed on either pole, the south just happened to split open first.
“Enceladus experiences internal heating due to the eccentricity of its orbit. It is sometimes a little closer to Saturn and sometimes a littler farther, which causes the moon to be slightly deformed as it responds to the giant planet’s gravity,” the study authors explained.
“It is this process that keeps the moon from freezing completely solid.”
“Key to the formation of the fissures is the fact that the moon’s poles experience the greatest effects of this gravitationally induced deformation, so the ice sheet is thinnest over them.”
“During periods of gradual cooling on Enceladus, some of the moon’s subsurface ocean will freeze. Because water expands as it freezes, as the icy crust thickens from below, the pressure in the underlying ocean increases until the ice shell eventually splits open, creating a fissure. Because of their comparatively thin ice, the poles are the most susceptible to cracks.”
The researchers believe the fissure named after the city of Baghdad was the first to form. (The stripes are named after places referred to in the stories of One Thousand and One Nights, which are also called Arabian Nights.)
However, it didn’t just freeze back up again. It stayed open, allowing ocean water to spew from its crevasse that, in turn, caused three more parallel cracks to form.
“Our model explains the regular spacing of the cracks,” Dr. Rudolph noted.
The additional splits formed from the weight of ice and snow building up along the edges of the Baghdad fissure as jets of water from the subsurface ocean froze and fell back down. This weight added a new form of pressure on the ice sheet.
“That caused the ice sheet to flex just enough to set off a parallel crack about 22 miles away,” Dr. Rudolph said.
That the fissures stay open and erupting is also due to the tidal effects of Saturn’s gravity. The moon’s deformation acts to keep the wound from healing — repeatedly widening and narrowing the cracks and flushing water in and out of them — preventing the ice from closing up again.
For a larger moon, its own gravity would be stronger and prevent the additional fractures from opening all the way. So, these stripes could only have formed on Enceladus.
“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,” Dr. 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.”
The team’s findings were published in the journal Nature Astronomy.
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D.J. Hemingway et al. Cascading parallel fractures on Enceladus. Nat Astron, published online December 9, 2019; doi: 10.1038/s41550-019-0958-x
