This illustration depicts a plume of water vapor that could potentially be emitted from the icy surface of Jupiter’s moon Europa. New research sheds light on what plumes, if they do exist, could reveal about lakes that may be inside the moon’s crust. (NASA/ESA/K. Retherford/SWRI)
It’s been some years since Europa scientists agreed that the Jovian moon has a large global ocean beneath miles of ice. More recently, scientists have identified what they view as pockets of water surrounded by ice but much nearer the surface than the ocean below. And there has been research as well into what may be salty, slushy pocket of water further down in the ice covering.
With NASA’s mission to Europa scheduled to launch in about two years, modeling of these all potential collections of liquid water has picked up to prepare for the Europa Clipper arrival to come.
The latest research into what the subsurface lakes on Europa may look like and how they may behave comes in a recently published paper in Planetary Science Journal.
A key finding supports the idea that water could potentially erupt above the surface of Europa either as plumes of vapor or as cryovolcanic activity — flowing, slushy ice rather than molten lava.
Computer modeling in the paper goes further, showing that if there are eruptions on Europa, they likely come from shallow, wide lakes embedded in the ice and not from the global ocean far below.
“We demonstrated that plumes or cryolava flows could mean there are shallow liquid reservoirs below, which Europa Clipper would be able to detect,” said Elodie Lesage, Europa scientist at NASA’s Jet Propulsion Laboratory and lead author of the research.
“Our results give new insights into how deep the water might be that’s driving surface activity, including plumes. And the water should be shallow enough that it can be detected by multiple Europa Clipper instruments.”
A minimally processed version of this image was captured by JunoCam, the public engagement camera aboard NASA’s Juno spacecraft. It was taken during the mission’s close flyby earlier this fall, almost 950 miles above the moon’s surface. The raw image was processed by “citizen scientist” Navaneeth Krishnan to add enhanced color contrast that allow larger surface features to stand out more.
The question of whether or not Europa has plumes is not settled. While the plumes coming from Saturn’s moon Enceladus have been well studied and even had a spacecraft fly through one, Europa has only some fuzzy Hubble Space Telescope, Galileo mission and ground-based telescope images that suggest a plume.
“We’re still in the space where there’s really intriguing evidence but none of it is a slam dunk,” said Matthew McKay Hedman, a member of Europa Clipper’s Mapping Imaging Spectrometer for Europa (MISE) science team and associate professor in the Department of Physics at the University of Idaho.
Like Enceladus, Europa is geologically dynamic, meaning both moons generate heat inside as their solid layers stretch and flex from the gravitational pulls and pushes coming from their host planets and neighboring moons.
This, instead of heat from the Sun, keeps subsurface water from freezing on these ice-covered moons. The heat may also help produce or circulate life’s chemical building blocks at the seafloors, including carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.
But that’s where the similarities end.
“A lot of people think Europa is going to be Enceladus 2.0, with plumes constantly spraying from the surface,” said Lynnae Quick, a member of the science team behind Clipper’s Europa Imaging System (EIS) cameras. “But we can’t look at it that way; Europa is a totally different beast,” said Quick, who’s based at NASA’s Goddard Space Flight Center.
These composite photos, which combine images by NASA’s Hubble Space Telescope and Galileo spacecraft, show a suspected plume of material erupting two years apart from the same location on Europa. The images added to the view that the plumes are a real phenomenon, flaring up intermittently in the same region on the moon, but the issue remains open and debated. (Science Credit: NASA, ESA, and W. Sparks (STScI). Illustration Credit: NASA, ESA, W. Sparks (STScI), and the USGS Astrogeology Science Center)
But if plumes do come off Europa, scientists want to know whatever they can about where they may come from and how they can be identified and studied — in other words, what the Europa Clipper team should be looking for.
Lesage’s computer modeling lays out a blueprint for what scientists might find inside the ice if they were to observe eruptions at the surface. According to her models, they likely would detect reservoirs relatively close to the surface, in the upper 2.5 to 5 miles of the crust, where the ice is coldest and most brittle.
As described in a NASA release, the reason for potential eruptions in this region is that subsurface ice there doesn’t allow for expansion. As the pockets of water freeze and expand, they could break the surrounding ice and trigger eruptions, like a can of soda in a freezer explodes. And pockets of water that do burst through would likely be wide and flat.
Reservoirs deeper in the ice layer – with floors more than 5 miles below the crust – would push against warmer ice surrounding them as they expand. That ice is soft enough to act as a cushion, absorbing the pressure rather than bursting. Rather than acting like a can of soda, these pockets of water would behave more like a liquid-filled balloon, where the balloon simply stretches as the liquid within it freezes and expands.
What potentially is at stake if plumes can be found and studied on Europa is surely significant. This is what the authors of the recent Planetary Science Journal article wrote in their paper:
“These reservoirs, if they exist, could represent the most accessible liquid water bodies in the outer solar system and are key for the exploration of ocean worlds and the search for life beyond Earth.”
