Not far beneath the icy surface of Jupiter’s moon Europa may lurk large lakes. A study published online November 16th in Nature reveals that strange circular areas on the moon called chaos terrains come from the breakup and refreezing of ice overlying extensive pockets of water — in one case, with a volume equal to that of North America’s Great Lakes — no more than a couple miles down.
Water and ice are nothing new on Europa. Astronomers have long suspected that a liquid water ocean hides beneath the moon’s cracked, tortured ice. But so far no one’s come up with a good explanation for all the features of chaos terrains. These regions have domes or depressions on the ice and can span hundreds of kilometers. Astronomers first took a close look at them in the late 1990s with the Galileo spacecraft. The images “made us step back and wonder what’s going on,” says Geoffrey Collins (Wheaton College), a planetary scientist and Europa researcher who was not involved with the new study.
Particularly prickly is the question of how to create such tall domes, like those seen in the region called Conamara Chaos: on average the icy pimples are some 700 feet (200 m) tall, far higher than expected if they resulted from the freezing of surface water or plumes of warm ice welling up from below to fracture the surface (the two main competing theories).
The new model, set forth by Britney Schmidt (University of Texas at Austin) and her colleagues, creates a hybrid of these two ideas. The scientists took archived Galileo data and studied it with an eye toward what researchers have learned from watching terrestrial ice in action, especially subglacial volcanos and collapsing ice shelves.
In Schmidt’s scheme, deep regions of subsurface ice become buoyant compared to their surroundings, by warming from repeated tidal squeezing as Europa orbits Jupiter. The warm-ice plumes rise from the ocean-ice boundary through the thick shell. As a plume nears the surface, the pressure it puts on overlying brittle ice causes this ice to melt. The meltwater becomes what on Earth is called a perched lake, a lens-shaped body of water trapped inside the ice shell by hydraulic forces. The trapped water then creates new cracks and widens old ones in the overlying ice, which is a couple of kilometers thick. Eventually the ice breaks up into big icebergs surrounded by crushed-up ice.
“You never have liquid water on the surface,” Schmidt said in a press conference. “As those big strong icebergs move around, they can break up all the weak ice in between, which can fill with water — still not be liquid, but fill with water,” which makes the rubble-like, lumpy material salt-rich. Over the course of hundreds of thousands to millions of years, the liquid lens and icy rubble refreeze.
Here’s where the strength of this new scenario comes into play: because water takes up less space when liquid, the formation of the lens lake by melting causes the surface above to sink, like in the Thera Macula region, whose rough-and-tumble surface dips about 2,600 feet (800 m) below the surrounding terrain. But when the rubble and the lake refreeze, the forming ice “gives you the extra boost needed to pop that surface back up into a dome,” Schmidt explained, just like the domes in somewhat older Conamara Chaos.
“It is a very convincing model,” concludes Robert Pappalardo (NASA Jet Propulsion Laboratory), who is a study scientist for the Europa mission concept. “It seems to explain a wide variety of observations.”
Collins agrees. One of the many observations Schmidt’s model vindicates, he notes, is the presence of craters on top of chaos features. If the terrain was short-lived (as had been suspected), the craters shouldn’t exist. “It was sort of disturbing,” he says, “because it implied that chaos formation may last for quite a while, and the [lumpy rubble] continuously evolves over a long period of time.” A million years in which to feed the subsurface pockets with water and slush is a good thing, because it allows more time for cratering. “That to me is really interesting because it gets to one of the mysteries that I always had trouble with.”
Another observation the model explains is that ice in chaos regions is contaminated with compounds that seemingly come from below. But circulation between Europa’s various layers is important not just for what comes up, but what goes down, because what goes down has big implications for whether life could exist on Europa.
“Anybody on the street could give you a pretty good account of what it takes to stay alive: water, food, shelter, air to breathe,” Tori Hoehler (NASA Ames Research Center) said at the press briefing. There’s clearly plenty of water, and ice provides shelter. “But the big and open question has been one of energy. And when we talk about food to eat and air to breathe, [that’s] what we’re really talking about.” The cycling of ice in the model could provide a means to bring oxidants and other surface material down into the icy interior, from whence it could sneak down into the ocean and serve as fuel for organisms. “I think this really impacts the way we consider habitability on Europa.”
To watch a video animation of how lakes form inside Europa's ice shell, go to the University of Texas at Austin's press release.