Europa's plumes may not originate from subsurface ocean
Jupiter’s moon Europa is a fascinating world, with a global subsurface ocean encased in an icy shell and plumes of water vapor that vent into space. A new study has simulated how these plumes might originate, revealing a surprisingly dynamic ice ball.
With an abundance of water and energy sources, Europa has long been considered one of the most promising places in the solar system to search for extraterrestrial life. The problem is, any signs of life would most likely be hiding in that subsurface ocean, buried beneath some 10 to 15 miles (16 to 24 km) of solid ice.
But in recent years a shortcut seemed to present itself. In 2012 Hubble spotted what looked like plumes of water vapor erupting from the south pole, and further evidence for them was gathered in follow-up studies in 2016 and 2018. The belief was that these plumes were gushing up from the ocean below, potentially carrying vital minerals and other clues to life and scattering them into space and across Europa’s icy surface, where we could study them more easily.
But do these plumes really originate in the ocean? That was the question tackled in the new study, by researchers at NASA and the Universities of Arizona, Texas and Stanford. And the answer might, unfortunately, be “no” – some of the plumes could be coming from much shallower lakes.
Starting with images from the Galileo probe, which observed Europa in the late 1990s and early 2000s, the team developed a model that could explain what may be causing the plumes. They focused on an 18-mile-wide (29-km) crater called Manannán, which has a spider-shaped feature that could be a smoking gun for a past plume.
According to the model, the impact that created the crater would have melted much of the ice in its path, and the center of Manannán would have been relatively warm for a while, before cooling back down.
Briny water would have persisted under the surface in scattered pockets, and the team showed that these little lakes could move sideways through the ice from colder to warmer areas. As a result, eventually all the water would have gathered at the center of the crater.
“We developed a way that a water pocket can move laterally – and that’s very important,” says Gregor Steinbrügge, lead author of the study. “It can move along thermal gradients, from cold to warm, and not only in the down direction as pulled by gravity.”
Over time, that central lake would begin to freeze too, pressurizing the remaining water until it burst forth into a plume more than a mile high. That model has a few implications for Europa as a whole.
“Even though plumes generated by brine pocket migration would not provide direct insight into Europa’s ocean, our findings suggest that Europa’s ice shell itself is very dynamic,” says Joana Voigt, co-lead author of the study.
That said, the team acknowledges that this mechanism can’t explain all Europan plumes, so we might still have some hope of getting a handy glimpse into the inner workings of this water world.
The researchers were also able to estimate how salty the ocean and the ice were. According to their calculations, Europa’s ocean may only have about a fifth of the salt content as Earthly oceans.
The study presents a bit of a mixed bag for future exploration of this icy moon, including NASA’s proposed Europa Clipper mission currently penciled in for 2025. On the one hand, the plumes may not necessarily be scattering minerals and life signs within reach. But on the other, the lower-than-expected saltiness could make it easier for spacecraft radar to penetrate the ice.
The research was published in the journal Geophysical Research Letters.