Saturn’s Enceladus is one of the most attractive moons due to its unique appearance. And now new research reveals the origin of its “tiger stripes.”

Since 2005, astronomers have struggled to understand Enceladus’ fissures, known as tiger stripes, through which ocean water erupts from the moon’s icy surface. But new research explains how these so-called tiger stripes formed.

These stripes, about 35 kilometers (21 miles) apart, lie only at the moon’s south pole. They’re unlike any features found on other icy moons.

“First seen by the Cassini mission to Saturn, these stripes are like nothing else known in our Solar System,” lead author Douglas 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.”

Now, Douglas and his colleagues developed models to investigate the physical forces acting on Enceladus. These forces allow the tiger stripe fissures to form and remain in place.

The eccentric orbit of Enceladus causes the moon to experience internal heating. Because its distance from Saturn varies, the planet’s gravity slightly deforms the moon. This process creates heat and keeps it from freezing completely. And that’s why Enceladus is able to maintain liquid water underneath its icy crust.

This gravitationally induced deformation is felt most strongly at the poles, where the ice is thinnest. During periods of gradual cooling on Enceladus, some of the moon’s subsurface ocean froze beneath the caps. And because water expands when it freezes, as the icy crust thickens from below, the pressure in the underlying ocean increases until the ice shell eventually splits open, causing fissures to appear at the south pole.

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“Our model explains the regular spacing of the cracks,” Rudolph said. He further explained that the weight of the icy material falling back to the edges of the first crack “caused the ice sheet to flex just enough to set off a parallel crack about 35 kilometers (22 miles) away.”

The researchers also found that the cracks stay open and continue to erupt in part because of the tidal effects of Saturn’s gravity which changes with the moon’s strange orbit. The fissures continue to widen and narrow, bringing water through them. And this prevents them from closing up again.

According to the models, the tiger stripes could only happen on a moon with the mass of Enceladus, which measures roughly 500 kilometers (310 miles) in diameter.

“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.”

The researchers published their findings today (Dec. 9) in Nature Astronomy.

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