“Ultimately when we go to Europa, we’ll want to be able to tell if there’s any biological activity at all,” astrobiologist Damhnait Gleeson of NASA’s Jet Propulsion Laboratory told Wired.com here at the American Geophysical Union meeting Dec. 13.

Europa’s thick water ice crust may hide a dark liquid ocean, making the moon a favorite candidate for finding life beyond Earth in the solar system. The icy surface is crisscrossed with sulfur-rich, rust-colored lines, which could be cracks in the ice where liquid from the ocean below pushed its way to the surface.

“These geologic features on Europa indicate a plumbing system, so material gets from the bottom to the top of the ice layer,” Gleeson said.

If anything lives in the frigid subsurface brine, it could take in life-sustaining energy from chemicals brought down through the cracks, astronomers suspect. But to find these hardy bugs, astronomers need to know how to recognize them.

To get an idea of how sulfur-loving Europans might show signs of their presence, Gleeson and colleagues went to the most similar place on Earth: the Borup Fjord Pass in the Canadian arctic.

Pilots flying over the pass in the late 1990s noticed the ice was stained yellow with a pure form of elemental sulfur.

“It’s so unusual to find ice and sulfur together on Earth,” Gleeson said. “It’s really not what you’d expect to see.” On Earth, most pure sulfur reacts with oxygen to form the soft mineral gypsum. Finding so much elemental sulfur in Borup Fjord Pass suggested to Gleeson and colleagues that the supply of sulfur was being replenished, possibly by microbes. A trip to the site in 2006 confirmed their suspicions: The sulfur was built by microorganisms which derived energy from ripping electrons off another form of sulfur, sulfide.

Gleeson and colleagues brought the microbes back to the lab and let them grow on a diet of sulfide, then analyzed the elemental sulfur they produced. The researchers found that the microbe-built sulfur showed complicated sheets and filaments, structures that did not show up in microbe-free control samples.

“We can look at sulfur minerals that are formed by microorganisms, and we can tell they were formed by microorganisms,” Gleeson said. The results may help design instruments for future missions to Europa, like the proposed Europa Jupiter System Mission.

For now, though, the best observations of Europa come from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003. Gleeson and colleagues combined their fieldwork with observations of Borup Fjord from space, to help provide “ground truth” for observations of Europa from afar. But for Gleeson, these comparisons just highlight the need to actually go to Europa and start digging.

“What we can see from orbit is such a simple picture compared to the surface,” Gleeson said. “From orbit it’s just ice and sulfur. We really have to go deeper to understand the system.”

Astrobiologist Kevin Hand of NASA’s Jet Propulsion Lab, who also spoke at the AGU meeting, agreed. “Observations from orbit can provide compelling evidence for life, but not convincing evidence,” he said. “To cross that threshold from compelling to convincing requires in-situ elements.”

Images: 1) The Borup Fiord Pass at Ellesmere Island, Canada, where the snow is stained yellow by sulfur. 2) Sulfur-rich lines crisscross the surface of Jupiter’s moon Europa. 3) Gypsum deposits mark a potential sulfur spring at Borup Fjord Pass. Gleeson et al. 2010/NASA

See Also:

• A Theoretical Mission to Jupiter’s Icy Moon
• Jupiter Moon’s Ocean Could Be Rich in Oxygen
• Top 5 Bets for Extraterrestrial Life in the Solar System
• Underwater Robot Test is Practice for Jovian Moon
• Deep-Sea Vent Discovery Sets Hydrothermal Life’s New Depth Record

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Authors: Lisa Grossman

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