Enceladus’ plumbing explains ‘tiger stripe’ geysers

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Enceladus’ distinctive pattern of continuous, slightly curved and roughly parallel faults within the moon’s southern polar latitudes. Informally called ‘tiger stripes’, they mark the source of the moon’s long-lived geysers. – NASA / JPL / Space Science Institute

Saturn's icy moon Enceladus' decade-long frosty eruptions can be explained by a network of slots under the ice, according to a new study of the moon's plumbing.

A computer model by Edwin Kite from the University of Chicago and Allan Rubin from Princeton University suggest that the erupting fissures, known as "tiger stripes", sit atop slots that connect to oceans beneath the surface. These slots fill and empty as Saturn's gravitational pull drags the liquid around, and blast geysers through the surface.

American planetary scientist Carolyn Porco and head of Cassini’s imaging science team said she was "very happy" to see the explanation that "brings to the fore a process that had escaped notice".

That tides are behind Enceladus' geysers isn't a new idea. Just as the Moon exerts a tidal force on our oceans, so does Saturn on its moons.

The Cassini spacecraft has seen geysers erupting on Saturn’s sixth largest moon Enceladus for more than 10 years, but how they're sustained – and why the cracks don't clog up with ice – has remained a mystery.

Kite and Rubin were particularly interested in the moon's weird timing between maximum tides and eruptions. Some eruptions peaked five hours later than they should have, even when taking into account the 40 minutes needed for the icy particles to reach Cassini's orbiting altitude.

The continual eruption was another stumper. At various points of its orbit around Saturn, the tiger stripes should clamp shut. But they don't. Something must be keeping them open.

So the pair modelled a 500-kilometre icy moon with a subsurface ocean and fissures – a la Enceladus – but added slots beneath the fissures, which were connected to the underlying ocean.

The slots were partially filled with water, but as Saturn's gravity jerked the oceans around, they overflowed into the fissures and erupted as plumes of ice and water vapour.

Wide slots, they found, responded quickly to tidal forces to produce eruptions, whereas narrow ones weren't so fast to fill.

Tidal forces turned water motion into heat, which not only kept the cracks from freezing over, but produced eruptions that matched the five-hour lag. This, Porco says, is "the best thing in my mind about this new work".

And it's not just a short-term effect. The model, published in the Proceedings of the National Academy of Sciences, suggest that the ocean-surface connection may be sustained not for decades, but for millions of years.

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