Methane in the plume of the moon of Saturn Enceladus could be evidence of extraterrestrial life,

Methane from Enceladus can be a sign that life is rumbling in the subterranean sea of Saturn's moon, according to a new study.

In 2005, NASA's Cassini Saturn orbiter discovered geysers projecting water ice particles into space from "tiger skate" fractures near the southern pole of Enceladus. That material, which forms a plume that feeds Saturn's E ring (the planet's second-outermost ring), is thought to come from a huge ocean of liquid water that sloshes beneath the moon's icy shell.

And water ice is not the only thing in the plume. During the numerous close flybys of the 313-mile-wide (504 kilometers) Enceladus, Cassini spotted many other compounds as well — for example, dihydrogen (H2) and a variety of carbon-containing organic compounds, including methane (CH4).

Both dihydrogen and methane are particularly intriguing to astrobiologists. The H2 is likely being produced by the interaction of rock and hot water on Enceladus' seafloor, scientists have said, suggesting that the moon has deep-sea hydrothermal vents — the same type of environment that may have been life's cradle here on Earth.

Furthermore, H2 provides energy to certain terrestrial microbes that produce methane from carbon dioxide, in a process known as methanogenesis. Something similar could happen on Enceladus, especially as Cassini has also spotted carbon dioxide, and an astonishing abundance of methane, in the moon plume.

"We wanted to know: Could Earthlike microbes that 'eat' the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?" study co-lead author Régis Ferrière, an associate professor in the University of Arizona's Department of Ecology and Evolutionary Biology, said in a statement.

Thus Ferrière and colleagues constructed a series of mathematical models which assessed the probability that Enceladus methane was produced biologically. These simulations were diverse; the team investigated whether the observed H2 production could sustain a population of Enceladus microbes, for example, and how that population would affect the rate at which H2 and methane escaped into the plume, among other things.

"In summary, not only could we evaluate whether Cassini's observations are compatible with an environment habitable for life, but we could also make quantitative predictions about observations to be expected, should methanogenesis actually occur on Enceladus' seafloor," Ferrière said.

This assessment should encourage those of us who hope that something is swimming in the freezing, dark sea of Enceladus. The team determined that abiotic water thermal chemistry (without the help of life) as we know it on Earth does not explain the methane concentrations observed by Cassini very well. However, the addition of contributions from methanogenic microbes is a good bridge.

To be clear: The new study, which was published last month in the magazine Nature Astronomy, does not claim that life exists on Enceladus. For instance, the icy moon may feature some types of abiotic methane-producing reactions that aren't prevalent here on Earth — perhaps the decay of primordial organic matter left over from the moon's birth, the researchers said. Indeed, this last hypothesis would be perfectly suited if Enceladus were formed from materials rich in organic matter delivered by comets, as some scientists believe.

"Part of it comes down to the probability that we think there are different assumptions out there," Ferrière said. "For example, if we deem the probability of life on Enceladus to be extremely low, then such alternative abiotic mechanisms become much more likely, even if they are very alien compared to what we know here on Earth."

This being said, "biological methanogenesis seems to be consistent with the data," Ferrièr added. "In other words, the assumption of life cannot be ruled out as highly unlikely. To throw out the life hypothesis, we need more data from future missions."

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