An unknown methane-producing process is likely at work in the hidden ocean
beneath the icy shell of Saturn's moon Enceladus, suggests a new study
published in Nature Astronomy by scientists at the University of Arizona and
Paris Sciences & Lettres University.
Giant water plumes erupting from Enceladus have long fascinated scientists
and the public alike, inspiring research and speculation about the vast
ocean that is believed to be sandwiched between the moon's rocky core and
its icy shell. Flying through the plumes and sampling their chemical makeup,
the Cassini spacecraft detected a relatively high concentration of certain
molecules associated with hydrothermal vents on the bottom of Earth's
oceans, specifically dihydrogen, methane and carbon dioxide. The amount of
methane found in the plumes was particularly unexpected.
"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 Regis Ferriere, an associate professor in the University of
Arizona Department of Ecology and Evolutionary Biology and one of the
study's two lead authors. "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."
Ferriere and his team took a different, easier route: They constructed
mathematical models to calculate the probability that different processes,
including biological methanogenesis, might explain the Cassini data.
The authors applied new mathematical models that combine geochemistry and
microbial ecology to analyze Cassini plume data and model the possible
processes that would best explain the observations. They conclude that
Cassini's data are consistent either with microbial hydrothermal vent
activity, or with processes that don't involve life forms but are different
from the ones known to occur on Earth.
On Earth, hydrothermal activity occurs when cold seawater seeps into the
ocean floor, circulates through the underlying rock and passes close by a
heat source, such as a magma chamber, before spewing out into the water
again through hydrothermal vents. On Earth, methane can be produced through
hydrothermal activity, but at a slow rate. Most of the production is due to
microorganisms that harness the chemical disequilibrium of hydrothermally
produced dihydrogen as a source of energy, and produce methane from carbon
dioxide in a process called methanogenesis.
The team looked at Enceladus' plume composition as the end result of several
chemical and physical processes taking place in the moon's interior. First,
the researchers assessed what hydrothermal production of dihydrogen would
best fit Cassini's observations, and whether this production could provide
enough "food" to sustain a population of Earthlike hydrogenotrophic
methanogens. To do that, they developed a model for the population dynamics
of a hypothetical hydrogenotrophic methanogen, whose thermal and energetic
niche was modeled after known strains from Earth.
The authors then ran the model to see whether a given set of chemical
conditions, such as the dihydrogen concentration in the hydrothermal fluid,
and temperature would provide a suitable environment for these microbes to
grow. They also looked at what effect a hypothetical microbe population
would have on its environment -- for example, on the escape rates of
dihydrogen and methane in the plume.
"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 at Enceladus' seafloor," 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 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,"
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," he
added.
The authors hope their paper provides guidance for studies aimed at better
understanding the observations made by Cassini and that it encourages
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. This hypothesis is very plausible if it turns out that
Enceladus formed through the accretion of organic-rich material supplied by
comets, Ferriere explained.
"It partly boils down to how probable we believe different hypotheses are to
begin with," he said. "For example, if we deem the probability of life in
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."
According to the authors, a very promising advance of the paper lies in its
methodology, as it is not limited to specific systems such as interior
oceans of icy moons and paves the way to deal with chemical data from
planets outside the solar system as they become available in the coming
decades.
Reference:
Antonin Affholder, François Guyot, Boris Sauterey, Régis Ferrière, Stéphane
Mazevet. Bayesian analysis of Enceladus’s plume data to assess
methanogenesis. Nature Astronomy, 2021; DOI:
10.1038/s41550-021-01372-6
Tags:
Space & Astrophysics