Alexis Madrigal of Wired Science outlines the state of current scientific thought on the possibility of life on the Saturnian moon Enceladus: "The plumes of gas and ice shooting from the south pole of the Saturnian moon Enceladus contain sodium salts, which is the best evidence so far that the satellite harbors a liquid water ocean.
"NASA's Cassini probe
observed the salts in Saturn's outermost ring, which is believed to be composed of material ejected from Enceladus. That news, published Wednesday in Nature, is sure to excite life-hunters hoping to find extraterrestrial microbes within our solar system.
"Those salty grains provide our current best smoking (or steaming) gun pointing to present-day liquid water near the surface of Enceladus," space scientist John Spencer of the Southwest Research Institute, who was not involved with the research, wrote in an essay accompanying the findings.
"Since 2005, when Cassini spotted plumes jetting out from Enceladus, the moon has become one of the hottest topics in solar-system science. In 2008, water vapor was discovered in the plumes, and Enceladus joined Mars and Jupiter's moon Europa as the likeliest places to find liquid water — and therefore life as we know it — outside Earth. Though the planet is covered with ice and too far from the sun to derive much warmth, the gravitational field in the Saturnian system is believed to warm the moon by a frictional process called tidal heating, possibly allowing it to maintain a deep liquid water reservoir.
Betsy Mason, writing in Wired Science has recently suggested that: "The tiny Saturn moon of Enceladus may be among the best candidates for extraterrestrial life in our solar system.
"Scientists for the first time have gathered comprehensive evidence suggesting Enceladus may have all the necessary ingredients to harbor life in the ocean beneath its icy crust.
"Particles in a large plume of water vapor emanating from the surface suggest that the moon has an active ocean that circulates life-sustaining nutrients picked up from the rocky interior below.
"The plume is our smoking gun," said astrobiologist Christopher Parkinson of the University of Michigan. "It gives you a hint about what's going on inside."
"Life could arise in these conditions, or it could arrive from elsewhere in the galaxy, Parkinson said Monday at the annual meeting of the American Geophysical Union.
""If we sent a probe with the idea that there was microbial life on it that we were going to infect the place with, it would likely be a successful experiment," he said. "I'm not suggesting we do it, but it would be very cool."
"So far, space missions have found evidence in the solar system for liquid water and organic molecules on Mars, Jupiter's moon Europa and now Enceladus. Scientists hunting for life outside of the solar system are looking for planets with atmospheres with the right chemistry. But if Enceladus could host life, it shows that worlds without atmospheres could also be candidates.
"Chemical analyses of the moon's vapor plume made by the Cassini-Huygens spacecraft in a close flyby in October showed that it was mostly water, but also contained methane, carbon dioxide, carbon monoxide and silicate dust.
"Some of the vapor from the plume falls back to resurface the crust, Parkinson said. There the molecules could react with charged particles from the sun to create things like hydrogen peroxide and methanol — unappetizing to humans, but a nutritious buffet for microbes.
"The plume emanates from an area of Enceladus that has long dark lines, known as tiger stripes, that may mark where the ice is slowly circulating up to the surface. That would mean that in between the stripes, the ice would make its way down to the ocean, taking the nutrients with it.
"The circulation of the ocean could continue to bring more silicate dust from the rocky floor of the ocean up to the base of the ice where it would circulate through the tiger stripes to the surface and out into the plume, starting the process over. The whole cycle would take less than a million years — fast enough, Parkinson said, to keep a microbial colony alive.
""There could be ocean processes that could create a sustainable or complete geochemical cycle required for life," Parkinson said. "We have analogs on Earth that show this. Certainly we know from Lake Vostok in
Antarctica (that) there's water and life stays fairly dormant, but it's there."
"But going from the possibility of life on Enceladus to life actually taking hold is a leap, said planetary scientist Juergen Schmidt of
Potsdam University, who has also studied Enceladus' plume.
""We say we need water and we need this chemistry, but that could just reflect our geocentrism," he said.
""And even if we do know the right ingredients for life, Enceladus might not be the best candidate to host it.
""There are three candidates — Mars, Europa and now Enceladus, and all that is based on the presence of liquid water and organic molecules," Schmidt said. "But I don't think Enceladus is more likely than the others."
""For now, Parkinson is content with just the possibility.
""I can show it's life friendly. Is there life? No guarantee," he said. "If we put it there, it could thrive. It's possible."
"But the presence of the hypothesized subsurface ocean isn't as simple to confirm as it sounds," Alexis Madrigal explains.
"A second study in Nature Wednesday that looked at the same plumes from ground based telescopes found no evidence of sodium vapor. That rules out the possibility that the plumes are just near-surface ocean water blasted out into space, complicating our understanding of the moon's internal dynamics.
""I'm still a little skeptical," said astronomer Nick Schneider of the University of Colorado, Boulder, lead author of the second study. "There are other ways to explain the results."
"Taking both papers together, we now know that an intermediate step is necessary to explain the plume, if liquid water is indeed present under the tiny planet's icy glaze. Scientists had hoped that water from a deep ocean was simply making its way up through cracks in the ice towards the surface, where it was erupted into space.
""That scenario is out according to both of our results," Schneider said.
"Explaining both results requires some sort of distillation process that would give you pure vapor and then some salt pieces that are carried along during eruptions.
""Our picture of its subsurface must now be expanded to include the possibility of misty ice caverns floored with pools and channels of salty water," Spencer wrote. "What else may lurk in those salty pools, if they exist, remains to be seen."
"There are other explanations of Enceladus' behavior. For example, Susan Kieffer of the University of Illinois said the simplest explanation of the planet's internal structure doesn't require water at all. The presence of clathrates, ice-like lattice structures that can trap gases, could just as easily explain what we see on the moon.
""We proposed that the crust of Enceladus was composed of two layers: One, a surface layer of ice with carbon dioxide, and two, starting at no more than 3 kilometers, a mixture of icy clathrates that overlaid the core," Kieffer said. "Gas is released from the clathrates by the earthquakes associated with the tectonic activity at the south pole. In that way, we were able to account quantitiatvely for the gases observed in the plume. "
"Support among scientists for the ocean hypothesis has gathered a lot of steam from the range of Cassini observations.
"Fortunately, we might be able to settle the disputes with further observations. Cassini will be flying by the moon at least four more times by mid-2010, and could make as many as 12 fly-bys before 2015, if NASA extends its mission.
""Unfortunately, NASA has no current plans to send a probe crashing through the ice anytime soon. Enceladus lost out in the most recent round of mission planning to the Jovian moon, Europa, to be NASA's marquee outer planet mission."
