Observations from NASA’s Cassini spacecraft established that Enceladus, the sixth-largest of Saturn’s moons, has a global subsurface ocean. An analysis of a plume of ice grains and water vapor ejected into space suggested that hydrothermal vents are present on the moon’s seafloor. On Earth, such deep-sea vents harbor ecosystems rich in methane-producing microorganisms. Now, planetary researchers in the United States and France have constructed mathematical models to calculate the probability that a process called methanogenesis (biotic methane production) might explain the escape rates of molecular hydrogen and methane in Enceladus’s plume, as measured by Cassini.
Enceladus’ tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s south pole and creating Saturn’s mysterious E-ring. Evidence for this has come from NASA’s Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. Tiger stripes are visible in false-color blue. Image credit: NASA / ESA / JPL / SSI / Cassini Imaging Team.
“We wanted to know: could Earthlike microbes that eat the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?” said Dr. Regis Ferriere, a researcher in the Institut de Biologie de l’École Normale Supérieure, the University of Arizona and iGLOBES,
“Searching for such microbes, known as methanogens, at Enceladus’ seafloor would require extremely challenging deep-dive missions that are not in sight for several decades.”
In the new study, Dr. Ferriere and colleagues constructed mathematical models to calculate the probability that different processes, including biological methanogenesis, might explain the Cassini plume data.
They applied new mathematical models that combine geochemistry and microbial ecology to analyze the Cassini data and model the possible processes that would best explain the observations.
Their results suggest that even the highest possible estimate of abiotic methane production based on known hydrothermal chemistry is far from sufficient to explain the methane concentration measured in the 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 Enceladus’ ocean,” Dr. Ferriere said.
“Rather, we wanted to understand how likely it would be that Enceladus’ hydrothermal vents could be habitable to Earthlike microorganisms. Very likely, the Cassini data tell us, according to our models.”
“And biological methanogenesis appears to be compatible with the data. In other words, we can’t discard the ‘life hypothesis’ as highly improbable.”
“To reject the life hypothesis, we need more data from future missions.”
The researchers hope their results provide guidance for studies aimed at better understanding the observations made by Cassini and that they encourage research to elucidate the abiotic processes that could produce enough methane to explain the data.
“For example, methane could come from the chemical breakdown of primordial organic matter that may be present in Enceladus’ core and that could be partially turned into dihydrogen, methane and carbon dioxide through the hydrothermal process,” Dr. Ferriere said.
“This hypothesis is very plausible if it turns out that Enceladus formed through the accretion of organic-rich material supplied by comets.”
The findings were published in the journal Nature Astronomy.
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A. Affholder et al. Bayesian analysis of Enceladus’s plume data to assess methanogenesis. Nat Astron, published online June 7, 2021; doi: 10.1038/s41550-021-01372-6
