Home North pole ice The geyser on Enceladus, Saturn’s moon, explained

The geyser on Enceladus, Saturn’s moon, explained

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Enceladus, Saturn’s sixth moon, is about the size of the United Kingdom and covered in ice several kilometers thick. Below is a liquid ocean, which erupts from the surface through a series of fissures at the south pole, sending a constant geyser into space. This geyser contains traces of undersea vents, salt water and even methane, a sign that the pitch black sea could contain life.

The moon is unique in the solar system, says Maxwell Rudolph, a geophysicist at the University of California, Davis. A new study he led, published in Geophysical Research Letters, offers an explanation for “tiger stripes” and the geyser running through them. As Enceladus heats and cools in orbit, its icy crust warps under pressure, allowing water to spill out to the surface.

[Related: Saturn has a slushy core and rings that wiggle]

The heating and cooling cycles are driven by the movement of the moon itself. Every hundred million years or so, the shape of Enceladus’ orbit around Saturn changes, going from more circular to more oval and vice versa. When the orbit is more oval, the moon is squeezed tighter by Saturn’s gravity and then released. “This causes the whole ice pack to stretch, almost as if it were kneaded,” Rudolph explains. The slight lifting of this ice is enough to heat the whole moon and slightly melt the shell.

Then, when the moon’s orbit becomes circular, its crust cools again. The pack ice extends down into the hidden ocean. What happens next should be “obvious to anyone who’s ever put a bottle of soda in the freezer,” Rudolph says. “The change in volume when water freezes into ice causes a buildup of pressure. [On Enceladus]it’s happening globally.

When the pressure becomes too intense, the outer ice cracks, much like the crust of a bread splits when the soft interior expands. The buildup takes millions of years, but the crack appears in seconds, starting at the surface and descending about nine miles to the subterranean ocean.

This illustration is a depiction of the interior of Saturn’s moon Enceladus with a global liquid water ocean between its rocky core and icy crust. NASA/JPL-Caltech

The cracks can only reach the ocean at the poles, where the ice is thinnest. “It’s a bit of a matter of luck if the first crack forms at the [Enceladus’s] north pole or the south pole,” says Rudolph. “But once it’s done, it sets up a series of events that lead to the cracks in the tiger stripe.” It is possible that the process repeated itself with each cold cycle.

But pressure alone doesn’t quite explain the geyser. According to models run by Rudolph and his team, not even all of the newly formed ice is enough to bring water to the surface. And photos of Enceladus show no telltale traces of liquid water pouring onto the smooth exterior of the moon.

Here, a process described by Miki Nakajima, a University of Rochester astronomer who was not involved in the new study, could come into play. In 2016, Nakajima demonstrated that if liquid water pushes part of the way up a crack in the ice, it could begin to boil spontaneously in the vacuum of space. “We know how thick the cracks are on the surface, but we don’t really know what’s going on inside,” Nakajima explains. “It could be a straight line; it could be weird. Either way, the steam could escape because boiling can occur even in the tightest cracks.

Any fissure leading to the ocean would not produce much water. At the bottom of the cracks the water is probably simmering and a person standing on the surface would be surrounded by a fine mist, not a jet. It would take many crevices and tubes to produce the geyser that humans have photographed from space. Nakajima says that if the explanation is correct, there are many cracks smaller than the tiger stripes on Enceladus that astronomers haven’t yet gotten close enough to see.

Saturn's moon Enceladus backlit by the sun with jets coming from the southern end in a space probe image
Saturn’s moon Enceladus drifts past the rings and the lesser moon Pandora in this view captured by NASA’s Cassini spacecraft November 1, 2009. The entire scene is backlit by the sun, providing striking illumination for the particles ice that make up both the rings and the jets emanating from the south pole of Enceladus. NASA/JPL-Caltech/Institute of Space Sciences

Other forces could also be at play. Dissolved carbon dioxide or other gases could rise to the surface like the carbonation of soda, throwing water to the surface. Unpublished work presented at the American Geophysical Union meeting last year also suggests that the water could be released by melting ice in the crevasse, rather than by the buried ocean.

“I think everything is on the table,” Nakajima says.

Rudolph’s study also looked at Europa, one of Jupiter’s most famous moons, which has both an ice crust and a liquid ocean beneath the surface. But Europa is about five times the radius of Enceladus, and the hot-cold cycles of gravity just aren’t strong enough to break through its thicker crust.

“I’m really intrigued that the model can explain Enceladus, but not Europa,” says Nakajima.

[Related: NASA’s next Jupiter mission will hunt for life’s ingredients under Europa’s frozen shell]

Elodie Lesage, Europa expert at NASA’s Jet Propulsion Laboratory, writes in an email to popular science that there is no direct evidence of cryovolcanoes on Jupiter’s moon, although some plains on its surface appear to be the result of liquid water But based on the new findings, she notes, any volcano from Potential ice on Europa is probably forming for completely different reasons than on Enceladus. “It seems unlikely that liquid water could rise from the ocean of Europa to its surface,” she explains. Instead, pockets of water could become embedded in the crust and explode when they begin to freeze, again “like a can of soda someone forgot in a freezer.”

Enceladus was last studied by the Cassini space probe in 2015 – it flew 30 miles above the moon’s surface – while data on ice eruptions from Europa come mainly from images of distant telescopes. The launch of the next mission to Europa is scheduled for 2024. “Whatever happens [there] is less understood,” says Rudolph.

This difference means that any hunt for extraterrestrial life would take place on the two moons separately. If there are organisms living deep under the ice, the Enceladus geyser seems to give us direct access to their world. Anything inside Europa might be a lot harder to figure out.