Four decades of space exploration have taught us a great deal about the nine planets and dozens of moons that comprise our solar system. Yet Earth remains the only world on which we know for certain that life exists.

Scientists and science fiction writers alike have long speculated on that Mars might also harbor life. But images of Jupiter’s moon Europa sent back by NASA’s Galileo spacecraft have led planetary geologists and biologists alike to seriously consider adding that ice-covered world to the list of possible habitats for life.

Chris Chyba, who holds the Carl Sagan Chair for the Study of Life in the Universe at the SETI Institute in Mountain View, CA, believes that “Europa, along with Mars, is one of the two most likely places for life elsewhere in the Solar System.”

Galileo’s images reveal an enigmatic world unlike any previously known. Curious patterns of cracks and ridges can be seen in the ice, including areas where it appears that slush or liquid water may have broken through the surface. And data from the magnetometer aboard Galileo strongly suggest that a liquid ocean exists beneath the frozen surface.

Ron Greeley, of Arizona State University, agrees that life is a strong possibility on Europa. “If we think about the basic ingredients for life, we need liquid water, we need an energy source, and we need the right organic compounds. Europa seems to be a place where those three ingredients can be found.”

In an effort to bring together scientists from a variety of scientific disciplines to examine the question of life on Europa, Greeley recently convened the first meeting of the NASA Astrobiology Institute’s Europa Focus Group. Held at NASA’s Ames Research Center in Mountain View, CA, the meeting was attended by over 50 participants.

“We’re entering the synthesis stage now,” Greeley explained, “where we’re bringing in the different data sets from the [Galileo] mission to try to understand what Europa is like now and what it was like in the past. But we also want to bring into the studies people such as terrestrial sea-ice experts and biologists who are interested in life forms that exist in ice, and get a dialog going among these different disciplines.”

An ice-covered ocean may not seem an ideal place for life; human beings would certainly find it inhospitable. But microbes are far less discriminating. And two very plausible scenarios have been put forward for how microbial life might survive on Europa.

“One possibility,” said Chyba, “is at the base of the ocean, at hydrothermal vent systems.” On Earth, multitudes of organisms thrive around deep-sea hydrothermal vents. These seafloor volcanoes spew superheated water and life-sustaining chemicals into the frigid deep ocean. Indeed, hydrothermal vents are considered likely candidate sites for the place where life on Earth got its start.

Europa’s neighboring moon Io, which orbits more closely to Jupiter than Europa, is the most volcanically active world in the solar system. This volcanism is driven by tidal activity within Io. Some scientists speculate that Europa might also have volcanic activity, hidden below the surface of the ice, which could provide a source of energy and nutrients to underwater life forms.

But life in the europan deep would be difficult to detect from the surface, particularly if Europa’s ocean had a thick ice cover. “It’s not clear,” Chyba pointed out, “that even if there is an ecosystem down there that it will manifest itself in some observable way at the surface.”

The second scenario proposed for life on Europa, Chyba explained, “is that life will take advantage of nutrients created at the surface through radiation” interacting with surface chemicals to produce oxidizing and organic compounds. These nutrients then “would be introduced to the ocean at the Conamara Chaos or other regions like that, or perhaps at active cracks.” Some scientists believe that the so-called “chaos” regions of Europa Chyba referred to are areas where liquid water or slush has broken through cracks in the ice, allowing surface chemicals to mix with the ocean below.

The possibility of detection might then be greater, said Chyba. “You might hope that in the overturn of the ice whatever organisms might exist in the ocean would be detectable at the surface.”

Although some find it difficult to see how life could get started under Europa’s current frigid conditions, Kevin Zahnle of NASA’s Ames Research Center pointed out that early in its history Europa might have been much warmer, with a liquid ocean surface. The young Jupiter was a much stronger source of warming infrared radiation than it is today. If Europa formed early in Jupiter’s history, Zahnle said, “there is an overwhelming possibility that there could have been photosynthesis, for a very brief period, at the surface of a liquid ocean.” Speculation, to be sure, but intriguing speculation.

To gain a better understanding of how life might survive on Europa (if, indeed, it exists there), scientists turn to environments on Earth that most closely resemble the frozen jovian moon: the polar regions of the Arctic and Antarctica.

Chris McKay, a researcher at NASA Ames and an NAI member, has made several trips to Antarctica to study how life hangs on under the bitter cold conditions that prevail there. “I’ve always been a big proponent of studying life on Earth as a way of developing models of life beyond. Realizing, of course, that maybe life is different. But what else can we do but study life on Earth as an example. And for Europa, the best examples are really life under ice.”

McKay described “two systems in Antarctica that are particularly relevant. One is the dry-valley lakes, where we find life growing underneath ice that’s 20 feet [6 meters] thick. And the other is in Lake Vostok, which we haven’t seen yet, but we know that underneath 4 kilometers [2.5 miles] of ice, there’s a lake. And if there’s life in that lake, that might be another example of the strategy that life might use on Europa.”

At the opposite end of the Earth, Jody Deming, Professor of Biological Oceanography at the University of Washington, Seattle, and her colleagues have set up a cold-room laboratory in which they can study living organisms that inhabit Arctic sea ice. They have learned that as sea ice freezes tiny pockets and channels of liquid brine remain, even at temperatures well below the freezing point.

Deming and her colleagues have detected organisms alive and respiring at temperatures as low as minus 20 C (minus 4 F) and believes that some cold-adapted microbes may be able to survive even lower temperatures. She is interested in understanding better how these life forms remain viable under such frigid conditions. If organisms are alive on Europa, she suggests, they may well use similar survival strategies.
What Next?

NASA’s next mission to Europa, the Europa Orbiter, is currently on hold. If and when it does arrive at Jupiter’s icy moon, one of its primary objectives will be to determine definitively whether or not there is a water ocean there, and if so, how thick the ice layer is that covers it.

To answer the question of whether or not life exists on Europa, however, will most likely require landing on the surface. “If we can get a sample of the surface ice,” said Greeley, “particularly along the cracks, we might be able to find molecules that tell us — if there is life — that there is life in the ocean. And maybe even tell us how that life compares to us.” As Chris Chyba concluded, “We’re only gonna know by going there and looking.”