Mysterious depressions and domes on Jupiter's ice-encrusted moon Europa may signal the presence of subsurface lakes, at least one of which holds enough water to fill the American Great Lakes.
The finding, if confirmed, could be a boost for those who speculate that simple forms of life could exist on the moon.
For several decades, scientists have known that an ocean of liquid water exists beneath Europa's crust of ice. But some research suggests that Europa's ice shell could be miles thick. The subsurface lakes described by NASA scientists Wednesday, however, appear to exist within that icy mantle, meaning they would be at a far shallower depth than the underlying ocean.
Moreover, the lakes could be part of a planetary dumb-waiter system that transfers potential nutrients from the surface to the subsurface ocean, the team suggests. That transfer of nutrients could be crucial to any potential life on Europa.
With liquid water, Europa already has one necessary ingredient for life. That has made it "a compelling object for study for decades," says Tori Hoehler, an astrobiologist at NASA's Ames Research Center in Moffett Field, Calif., who was not part of the research team.
But another requirement for life, he adds, is a source of energy, a.k.a. food. The moon's upper crust is rich in compounds that could be a food source, but the potential nutrients would need a system to deliver them to the ocean deep below.
The mechanism that forms the subsurface lakes – involving rising plumes of heat – could provide that service, the team suggests.
If the team is right, "you've moved from a system that checks [off] one of the requirements for life to a system that checks two requirements for life," Dr. Hoehler says.
The results, set for publication in Thursday's issue of the journal Nature, grew out of an attempt to explain two different areas within Europa's "chaos terrain" – a landscape of jumbled surface features visible in images from the Voyager and Galileo spacecraft.
The two areas that have puzzled scientists cover significant large patches of the moon's surface and host icebergs with ground-up ices filling the channels in between – much as jumbled ice chunks fill gaps between icebergs as they calve from ice shelves in polar regions on Earth.
One of the two areas appeared as a depression in the surface. The other appeared as a dome.
"Why are these two features so similar but so different?" says Britney Schmidt, a planetary scientist at the University of Texas at Austin who led the research team.
In trying to solve the puzzle, the team was struck by parallels on Earth. Similar features are apparent in ice that forms above under-ice volcanoes, as well as at ice shelves where warm sea water is thought to be thinning the ice from underneath.
The team suggests that plumes of warmer water, heated by Europa's core, migrate up through the thick icy crust until they reach a "sweet spot" where a combination of pressure, temperature, and the composition of the ices allows a subsurface lake to form. Since liquid water takes up less volume than water ice, the icy lid on top of the lake begins to crack and collapse in a kind of frigid sink hole. The surface ice breaks into chunks in the process. As it does, some of it overturns, mixing the potential nutrients into the water below.
That explains the depressions. But what about the domes?
Over time, the lake refreezes and expands. This pushes remaining large ice blocks upward. The brine surrounding the blocks also refreezes, expanding as it does, adding to the dome.
One of the features, Thera Macula, is a depression, suggesting that a Great Lakes-scale body of water some two miles below currently is sculpting the surface, the team says. A dome feature, known as Conamara Chaos, represents the end of another lake's thaw-freeze process.
Over time, researchers have debated whether the icy shell covering Europa is thin or thick – thick roughly defined as more than six miles top to bottom. But neither camp can adequately explain how the chaos terrain forms, the team posits.
The team's new explanation "is a really interesting half-way point that is much more realistic," says Geoffrey Collins, a planetary scientist at Wheaton College in Norton, Mass. "It's not just 'only liquid down here and only ice up there.' There are perched lakes or slushy areas in the ice shell that may be having a huge effect on the surface geology," he says.
The thin-thick argument is of interest to astrobiologists, says Dr. Collins. If the crust is thick – say, 18 miles or so – that would leave little hope of ever being able to sample the ocean and check for signs of life. If it's thin, hope would spring eternal for sending a robotic mission to study what's beneath.
The new explanation leaves open the possibility for exploring potentially habitable environments even if the icy shell turns out to be a thick one.
Collins, however, is less convinced that the process can bring nutrients in the upper crust down to any hidden ocean. In the scenario the team lays out, the heat plumes that form the lakes – and the lakes themselves – don't come into contact with an ocean at any depth below the surface.
But Curt Niebur, NASA's program scientist for the outer planets, explains that it's possible the overturning ice can transfer any potential nutrients to the water or slush below. That leaves the enriched material free to migrate back down through cracks in the ice.
These ideas require missions to Europa to test them, researchers acknowledge.
Even so, "I'm really excited" about the team's proposed solution to the chaos terrain conundrum, Collins says. Thick shell or thin, if the presence of the lakes can be verified, they could be "interesting, accessible bodies of liquid water."