Particular readings from Saturn’s moon Enceladus prompted researchers to probe the possibility that life may exist on the bizarre world of the outer solar system.
In 2005, NASA’s Cassini Saturn orbiter took images of geysers hurling water ice particles into space from fractures near the moon’s south pole. The sighting has led to speculation that there could be a vast ocean nestled between the moon’s core and its icy shell.
By flying over these plumes, Cassini was able to sample their chemical composition. The orbiter detected hydrogen, methane and carbon dioxide at unexpected levels. Most notably, the amount of methane detected in the plumes raised eyebrows, as methane production can be a sign of life as we know it.
The observation of a particular quantity of methane – combined with an observation of dihydrogen (H2) and water ice – have left astrobiologists puzzled as to whether Enceladus is a habitable place for life. But methane, hydrogen, and water ice are not a smoking weapon: methane can be produced by natural chemical reactions, and its existence alone is not necessarily an indicator of life.
In order to understand why Enceladus spat out methane, researchers at the University of Arizona and the University of Paris Sciences et Lettres in France built mathematical models to calculate the probability that different natural processes could explain Cassini’s data.
“We wanted to know: Could Earth-like microbes that ‘eat’ hydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini? Said Régis Ferrière, associate professor in the department of ecology and evolutionary biology at the University of Arizona and one of the two main authors of the study published in Nature Astronomy in July. “The search for these microbes, known as methanogens, on the seabed of Enceladus would require extremely difficult deep diving missions that have not been in sight for several decades.”
Methanogens, or methane-producing bacteria, exist en masse on Earth. They live underwater, in swamps and marshes, and in the bowels of most animals, including humans.
However, there are natural geological processes unrelated to life that can produce methane. On Earth, methane is often produced at a slow rate via hydrothermal activity. This happens when cold seawater seeps down the ocean floor and travels through rocks to a nearby heat source, then spits back into the water through a hydrothermal vent.
Yet much of Earth’s methane is produced during methanogenesis, the biological process by which methanogenic bacteria consume carbon dioxide and spit out methane. Using a statistical model, researchers in the aforementioned study concluded that biological methanogenesis is the most likely cause of Enceladus’ abundant methane production.
“Obviously, we are not concluding that life exists in the ocean of Enceladus,” Ferrière said. “And biological methanogenesis appears to be compatible with the data. In other words, we cannot dismiss the life hypothesis as highly improbable. To reject the life hypothesis, we need more data from future missions. “
Indeed, it is possible that the methane of Enceladus is the result of a chemical decomposition of the primordial organic matter which could be present in the core of Enceladus. Thanks to the hydrothermal process, it is also possible that dihydrogen, methane and carbon dioxide are present in the plumes, as Enceladus formed through the accumulation of material rich in organic matter provided by comets.
“Part of it boils down to the likelihood that we think different assumptions are at the start,” Ferrière said. “For example, if we consider the probability of life in Enceladus to be extremely low, then such alternative abiotic mechanisms become much more likely, even though they are very foreign to what we know here on Earth.”
However, some members of the astronomical community are not convinced by this article.
“We know that water ice exists on Enceladus and can break down to produce hydrogen (H2), “said Avi Loeb, Harvard physicist and author of” Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. “” And there are many chemical pathways to produce lifeless methane. “
Loeb added that the combination of methane, hydrogen and water ice “is certainly not sufficient proof of the chemistry of life as we know it.”
“Complex organic molecules would be a better indication,” Loeb added.
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