Cassini might hold secrets to life on Saturn moon
- 18:17 03 November 2008
- NewScientist.com news service
- Rachel Courtland
The Cassini probe may have already collected data that could reveal the presence of life on Saturn's moon Enceladus, a new study argues. But mission scientists say teasing out the subtle signature of life may prove difficult.
Researchers have been fascinated with Enceladus since July 2005, when Cassini revealed a dramatic plume of ice particles and water vapour shooting out from the moon's south pole.
The plume's origin is still being debated, but some models suggest the moon holds an ocean of liquid water beneath its surface. This ocean could be a potential habitat for extraterrestrial life.
Now, a team led by Christopher McKay of NASA's Ames Research Center in Moffett Field, California, says Cassini could offer up the first evidence that life exists or once existed on the 500 km-wide moon.
Though the probe was never designed to look for life, it could do so by studying organic chemicals such as methane in the plume,the team says.
"If you think about what you need for life, you need water, energy, organic material, and you need nitrogen, and they're all coming out of the plume," McKay told New Scientist. "Here is a little world that seems to have it all."
More methane
Life could take the form of methane-producing microbes, or methanogens, similar to those that have been seen buried under kilometres of ice in Greenland.
Cassini could potentially find evidence for such life by studying the relative abundances of methane and heavier organic chemicals, such as propane and acetylene.
Organic, carbon-containing molecules, including methane, are produced in various ways.
Abiological processes include the breakup of large, complex molecules called tholins, and the chemical buildup of carbon monoxide and hydrogen into organic molecules of varying weights.
None of these abiological methods should strongly favour the formation of methane over that of heavier organic molecules, McKay's team argues. Biological processes, in contrast, should produce much higher amounts of methane than heavier organic compounds, they say.
Hidden signal
Researchers have taken advantage of this disparity to trace the source of organic compounds on Earth. Earlier this year, it was used to rule out a biological origin for oils and gases released from the "Lost City" hydrothermal vents at the bottom of the Atlantic Ocean.
McKay and colleagues believe a similar study could be carried out on Enceladus using data from Cassini's Ion and Neutral Mass Spectrometer (INMS), which can measure the concentration of different molecules in the plume.
So far, flybys of the plume suggest its chemical makeup is quite comet-like.
That hints that the moon's methane was created early on, perhaps in clouds of gas that predate the solar system.
"That doesn't mean there's not a biological signal hidden under the other stuff, but we don't have any evidence to suggest that is the case," says INMS lead scientist Hunter Waite of the Southwest Research Institute in San Antonio, Texas.
Follow-up mission
"It's not a clear-cut, hands-down winner for biology," McKay acknowledges.
To better understand what a biological signal on Enceladus might look like, McKay has reconfigured a chamber previously used to simulate conditions on Saturn's moon Titan to simulate non-biological ways of making methane and other organic molecules. The signatures could help researchers interpret Cassini's results, McKay says.
Waite says the best way to search for evidence of life may be to return to Enceladus with more sensitive instruments. In 2009, NASA will choose between two competing ideas for the next mission to the outer planets.
One mission would send two orbiters to Jupiter and its moon Europa. The other would send an orbiter to Saturn and a probe that could descend to the surface of Saturn's moon Titan. The Titan-Saturn mission would also include a number of flybys of Enceladus, Waite told New Scientist.
Journal reference: Astrobiology (vol 8, p 909)
View a slideshow of Cassini's best images, narrated by imaging team leader Carolyn Porco.
Cassini: Mission to Saturn – Learn more in our continually updated special report.
Astrobiology – Learn more in our out-of-this-world special report.
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