Researchers say there is reason to believe that large lakes lurk just under the surface of the large icy moon of Jupiter, making the possibility of life that much stronger.
For explorers searching for life beyond Earth, the siren song of Europa, Jupiter's icy moon, trills sweetly.
"Europa has the best chance of having life there today," said Britney Schmidt, who studies the moon at the University of Texas, Austin.
Astrobiologists think so because NASA's Galileo probe found strong evidence for the existence of a deep, briny ocean covering the entire moon deep under the icy surface.
A theory published by Schmidt and colleagues Wednesday in the journal Nature is sure to raise the volume of the siren call. The scientists suspect liquid lakes lurk just under the moon's cracked and mottled surface.
The lakes themselves could provide a second habitat for life, said Don Blankenship, a geophysicist and Europa specialist also at the University of Texas.
More intriguingly, such lakes would also provide channels for organic compounds on Europa's surface to be drawn into the deep ocean. Without that material from the surface, the ocean would not contain the elements needed for life.
"If Europa is habitable, we need to get material from the surface down into the deep interior, down into the ocean," Schmidt said.
Subsurface lakes would also explain Europa's "chaos terrains." These huge expanses of jumbled-up icebergs cover half of Europa's surface. But they have puzzled scientists since 1995, when Galileo began beaming pictures of Europa back to Earth.
"If you didn't know you were looking at Europa, you would think you were looking at icebergs calving off Greenland or the Antarctic ice sheets," said Kevin Hand, who studies Europa at NASA's Jet Propulsion Laboratory and was not involved in the work. "These iceberg-riddled features have presented quite a conundrum."
Schmidt and Blankenship say they've solved the mystery. To do so, they point to glacier-covered volcanoes in Iceland.
Deep warmth from these volcanoes, especially one known as Grimsvotn, melt the glaciers from below. A lens of water forms under the icy surface, eating away at the ice and moving upwards. Eventually, the warm water breaks through, and a flood gushes.
The surface of Europa is too cold for any liquid water. But as warm water seeps up, heated by Europa's hot core, it would break the surface ice into a jumble of miles-long icebergs. The icebergs flip, float and freeze in place.
"It's a vigorous process," Schmidt said. "Material is getting thrown around. Icebergs are flipping over. Brines are going up and coming back down."
Blankenship used a kitchen analogy. "It's like a Cuisinart operating at the surface," he said.
"It's very simple physics that just wasn't considered before," Schmidt said. "It's really just the melt action."
Schmidt arrived at the theory after she traveled to Antarctica, which hosts a hundred or more subsurface lakes. One, Lake Vostok, holds intrigue as a possible home for exotic life on Earth. A Russian team has repeatedly tried to drill through 3 miles of ice to reach the lake for study.
If such subsurface lakes exist on Europa, they would hold more water than all five Great Lakes, Schmidt said.
"I think they present a compelling story," said Hand of the research group. "Unfortunately, we have very limited data from Europa."
Schmidt acknowledges the idea is still a theory.
But there is a way to test it: Fly another probe to Europa and peer beneath its surface with powerful radar.
Blankenship has spent more than a decade helping NASA plan such a mission. So far, funding has not arrived.
"I'm 57 now," he said. "I'd love to see the subsurface of Europa before I die."