A recent study published in Nature Astronomy suggests that the methane concentrations measured by Cassini in the plumes of Enceladus could have a biological origin. This methane production could also be the result of non-biological geochemical processes unknown on Earth.
Where does Enceladus’ methane come from?
Enceladus, one of Saturn’s many moons, is just a small ball of ice 500 kilometers in diameter. Despite its small size, the object remains one of the most preferred targets in exobiology. And for good reason, thanks to the American Cassini probe, we know that jets of steam emanate from its depths, suggesting the presence of a global ocean sandwiched between its rocky core and its icy shell.
By flying over and sampling these plumes, Cassini also detected certain molecules associated with terrestrial hydrothermal vents, in particular dihydrogen, methane and carbon dioxide. The quantity of methane was particularly unexpected.
On Earth, hydrothermal activity occurs when cold seawater seeps under the ocean floor and passes near a heat source, such as a magma chamber. The heated water is then spat out through hydrothermal vents.
We know that this hydrothermal activity can then produce methane through microorganisms (archaea) which metabolize molecular hydrogen and carbon dioxide (methanogenesis).
In a recent study, Regis Ferriere and his team at the University of Arizona then wondered if Earth-like microbes known to consume hydrogen and produce methane as a by-product ( biological methanogenesis), could explain the surprisingly large quantity of methane detected by Cassini.
Explain the measurements
Of course, testing this hypothesis in the field is currently impossible. For this work, the researchers therefore relied on mathematical models to calculate the probability that different processes, including biological methanogenesis, that could explain Cassini’s data.
For this work, the researchers assessed which hydrothermal production of dihydrogen would best match Cassini’s observations. They then investigated whether this production could provide enough “food” to support a population of Earth-like methanogenic microorganisms, and whether a given set of chemical conditions (concentration of hydrogen, temperatures, etc.) could provide a suitable environment for the growth of these microbes.
Finally, the researchers examined what effects such a microbial population could have on its environment, in particular on the rates of methane leakage in the plumes.
A possible sign of life
According to these models, even the highest possible estimate of abiotic methane production – without biological aid – based on known hydrothermal chemistry is far from sufficient to explain the concentration of methane measured in the plumes by Cassini. Adding biological methanogenesis to the equation, on the other hand, could produce the amounts of methane measured by the probe.
«We do not conclude that life exists in the ocean of Enceladus ”, immediately warns Régis Ferriere “We just wanted to understand how likely it would be that the Enceladus hydrothermal vents could be habitable by Earth-like microorganisms. According to our models, based on data from Cassini, such a biological environment could be possible ”.
Of course, the methane concentrations measured by the probe could also be explained by geochemical processes that do not occur on Earth.
The methane could for example come from the chemical decomposition of the primordial organic matter which can be present in the core of Enceladus and which could be partially transformed into dihydrogen, methane and carbon dioxide by hydrothermal processes. This hypothesis could hold water if we knew that Enceladus was formed in the first place by the accretion of matter rich in organic matter supplied by comets.