An unknown process of methane production is likely at work in the ocean hidden under the icy shell of Saturn’s moon Enceladus, suggests a new study published in Nature astronomy by scientists from the University of Arizona and the University of Paris Sciences & Lettres.
The giant plumes of water emerging from Enceladus have long fascinated scientists and the public, inspiring research and speculation about the vast ocean that would be sandwiched between the moon’s rocky core and its icy shell. By hovering over the plumes and sampling their chemical composition, the Cassini spacecraft detected a relatively high concentration of certain molecules associated with hydrothermal vents at the bottom of Earth’s oceans, particularly dihydrogen, methane, and carbon dioxide. The amount of methane found in the plumes was particularly unexpected.
“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 Regis Ferriere, associate professor in the Department of Ecology and Evolutionary Biology at the University of Arizona and one of the two lead authors of the study. “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. “
Ferriere and his team took a different and easier route: They built mathematical models to calculate the likelihood that different processes, including biological methanogenesis, could explain Cassini’s data.
The authors applied new mathematical models that combine geochemistry and microbial ecology to analyze the data from the Cassini plume and model the possible processes that would best explain the observations. They conclude that Cassini’s data is consistent either with the activity of microbial hydrothermal vents or with processes that do not involve life forms but are different from those known on Earth.
On Earth, hydrothermal activity occurs when cold seawater seeps into the ocean floor, flows through the underlying rock, and passes near a heat source, such as a magma chamber, before spitting back into the water through hydrothermal vents. On Earth, methane can be produced by hydrothermal activity, but at a slow rate. Most of the production is from microorganisms that harness the chemical imbalance of hydrothermally produced hydrogen as an energy source and produce methane from carbon dioxide in a process called methanogenesis.
The team examined the composition of the Enceladus plume as the end result of several chemical and physical processes taking place inside the moon. First, the researchers assessed which hydrothermal hydrogen production would best match Cassini’s observations, and whether that production could provide enough “food” to support an Earth-like population of hydrogenotrophic methanogens. To do this, they developed a model for the population dynamics of a hypothetical hydrogenotrophic methanogen, whose thermal and energy niche was modeled on known strains of Earth.
The authors then ran the model to see if a given set of chemical conditions, such as the concentration of hydrogen in the hydrothermal fluid, and temperature would provide an environment for these microbes to grow. They also examined what effect a hypothetical microbial population would have on its environment, for example on the rates of hydrogen and methane leakage in the plume.
“In summary, not only could we assess whether Cassini’s observations are compatible with a habitable environment for life, but we could also make quantitative predictions about the observations to be expected, if methanogenesis actually occurred on the seabed of Enceladus, ”Ferriere explained.
The results suggest that even the highest possible estimate of abiotic methane production – or methane production without biological aid – based on known hydrothermal chemistry is far from sufficient to explain the methane concentration measured in the cells. plumes. Adding biological methanogenesis to the mix, however, could produce enough methane to match Cassini’s observations.
“Obviously, we are not concluding that life exists in the ocean of Enceladus,” Ferriere said. “On the contrary, we wanted to understand how likely it would be that the Enceladus hydrothermal vents could be habitable by Earth-like microorganisms. Most likely, the data from Cassini tell us, according to our models.
“And biological methanogenesis seems 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, ”he added.
The authors hope their paper provides guidance for studies to better understand the observations made by Cassini, and that it encourages research to elucidate abiotic processes that might produce enough methane to explain the data.
For example, methane could come from the chemical decomposition of primordial organic matter which may be present in the core of Enceladus and which could be partially transformed into dihydrogen, methane and carbon dioxide by the hydrothermal process. This hypothesis is very plausible if it turns out that Enceladus was formed by accretion of material rich in organic matter supplied by comets, Ferrière explained.
“Part of it comes down to the likelihood that we think different assumptions are at the start,” he 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. “
According to the authors, a very promising advance of the article lies in its methodology, as it is not limited to specific systems such as the interior oceans of icy moons and opens the way for the processing of chemical data from planets outside the solar system. as they become available in the decades to come.
Antonin Affholder et al, Bayesian analysis of data from the Enceladus plume to assess methanogenesis, Nature astronomy (2021). DOI: 10.1038 / s41550-021-01372-6
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