Saturn’s icy moon Enceladus is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it. “It has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source,” says Chris McKay, an astrobiologist at NASA’s Ames Research Center in Moffett Field, California. Besides Earth, he says, “there is no other environment in the Solar System where we can make all those claims.”
In addition, geyser-like jets spew ice crystals and gases into space, allowing a spacecraft to sample the subsurface by flying overhead. The current Cassini mission has done that several times already, but it’s only equipped to find the building blocks of life, not more complex molecules. “We want biomarkers,” says Larry Esposito, a planetary scientist at the University of Colorado, such as Earth-like organisms that might survive in Enceladus’s underground lakes, ponds or oceans, where there is no sunlight and so no photosynthesis, but might use hydrogen to synthesize methane from carbon dioxide — a process evolved by the Earth microbes known as methanogens. The methane could even be recycled if geological processes raise its temperature above 500 °C.
“The fact we see a lot of methane makes it interesting,” says McKay.
Ronald Oremland, a microbiologist with the US Geological Survey in Menlo Park, California, believes an even better food source would be acetylene, which exists in comets and there are hints it might be present in the Enceladus jets and serve as a “fast food” source for microbes, Oremland says.
Acetylene-eating microorganisms on Earth convert the molecule into ethanol and acetate. Their biochemistry is much simpler than that of methanogens, suggesting that acetylene might be the food of choice for the most primitive organisms.
But acetylene, methane, ethanol and acetate aren’t biomarkers. But if the methane coming from Enceladus is relatively rich in carbon-12, it’s a strong sign of biological activity. Similarly, if the plume is emitting ethanol and acetate rich in light carbon, acetylene-eating bugs may be at work.
Amino acids might also reveal life, especially if they reproduce, or reverse, Earth life’s overwhelming preference for ‘L’ isomers over their mirror image ‘D’ isomers, a skew not seen in abiotic processes. Any such bias would be “persuasive evidence for a biological origin,” says McKay.
Heat output from the south polar region of Saturn’s moon Enceladus is far greater than was previously thought possible, according to a new analysis of data collected by NASA’s Cassini spacecraft, published in the Journal of Geophysical Research on March 4. Data from Cassini’s composite infrared spectrometer of Enceladus’ south polar terrain, which is marked by linear fissures, indicate that the internal heat-generated power is about 15.8 gigawatts, approximately 2.6 times the power output of all the hot springs in the Yellowstone region, or comparable to 20 coal-fueled power stations.
The moon’s heat is more than an order of magnitude higher than scientists had anticipated, according to Carly Howett, the lead author of study, at Southwest Research Institute in Boulder, Colo., and a composite infrared spectrometer science team member.
“The mechanism capable of producing the much higher observed internal power remains a mystery and challenges the currently proposed models of long-term heat production,” said Howett.
It has been known since 2005 that Enceladus’ south polar terrain is geologically active and the activity is centered on four roughly parallel linear trenches, 130 kilometers (80 miles) long and about 2 kilometers (1 mile) wide, informally known as the “tiger stripes.”
Cassini also found that these fissures eject great plumes of ice particles and water vapor continually into space. These trenches have elevated temperatures due to heat leaking out of Enceladus’ interior.
A 2007 study predicted the internal heat of Enceladus, if principally generated by tidal forces arising from the orbital resonance between Enceladus and another moon, Dione, could be no greater than 1.1 gigawatts averaged over the long term. Heating from natural radioactivity inside Enceladus would add another 0.3 gigawatts.