How Enceladus got its stripes

By Nick Carne

Saturn’s icy moon Enceladus has a unique appearance, and now US researchers think they know where the “tiger stripes” come from.

Writing in the journal Nature Astronomy, a team led by Doug Hemingway from the Carnegie Institution for Science says the answer lies in the physics governing the fissures through which ocean water erupts from the moon’s icy surface.

First seen by the Cassini mission to Saturn, the stripes are parallel and evenly spaced, about 130 kilometres long and 35 kilometres apart. They are named after places referred to in the collection of stories we know as the Arabian Nights.

“What makes them especially interesting is that they are continually erupting with water ice, even as we speak,” says Hemingway. “No other icy planets or moons have anything quite like them.”

With Max Rudolph and Michael Manga from the University of California, Hemingway developed models to investigate the physical forces that allow the fissures to form and remain in place. 

Enceladus experiences internal heating due to the eccentricity of its orbit. Its distance from Saturn varies, which causes it to be slightly deformed as it responds to the planet’s gravity. This process keeps it from freezing completely.

Key to the formation of the fissures, the researchers say, is the fact that the moon’s poles experience the greatest effects of this gravitationally induced deformation, so the ice sheet is thinnest there. {%recommended 9368%}

During periods of gradual cooling on Enceladus, some of its 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. However, it didn’t just freeze up again. It stayed open, allowing ocean water to spew from its crevasse, which in turn caused three more parallel cracks to form.

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 35 kilometres away,” Rudolph says.

That the fissures stay open and erupting is also due to the tidal effects of Saturn’s gravity, the researchers say. 

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, the researchers say, these stripes could only have formed on Enceladus.

And why did they only form on the south pole? Just chance, they suggest. The fissures could have formed at either pole, but the south split open first.