Saturn’s moon Enceladus is unique in our Solar System — with plumes of water vapor and ice perpetually erupting, shooting jets hundreds of miles into space from its global subsurface ocean through cracks —parallel, evenly spaced “stripes” that are some 130 kilometers long and 35 kilometers apart–on Enceladus’s ice-encased surface providing an intriguing glimpse into what the moon’s subsurface ocean might contain, possibly providing conditions favorable to life. The answer, a new study has found, may lie in the plumes.
A Sign of Life? –Unexplained Levels of Methane
“We wanted to know: Could Earth-like microbes that ‘eat’ the hydrogen (H2) and produce methane (CH4)” explained Régis Ferrière, an associate professor at the University of Arizona Department of Ecology and Evolutionary Biology about the surprisingly large amount of methane detected by Cassini in a new study that concludes that known geochemical processes on Enceladus, can’t explain the levels of methane measured by the Cassini spacecraft at Enceladus’ South Pole.
“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,” observes Ferrière.
Many Solar System Objects Contain Methane
“Abiotic mechanisms of methane formation and release are not only possible but likely, and so life may be involved with methane production on Enceladus, but not necessarily so. The two are not mutually exclusive,”astronomer Jeffrey Kargel at the Planetary Science Institute, who was not involved in the study, wrote in an email to The Daily Galaxy. “Many Solar System objects contain methane. Comets commonly have a small amount, and some, like Hayakutaki, have a considerable quantity. More likely than cometary material in Enceladus is a rocky substance known from meteorites as carbonaceous chondrites, which contain variable small amounts of methane, which can be released by gentle crushing and mild heating, or more vigorous means of releasing these meteorites’ trapped gases. This class of meteorites, or condensed rocky material similar to them, is thought to comprise a major building block accreted by planets and their moons, such as Enceadus.”
“However, indications are that Enceladus has been heated inside to temperatures where any original building blocks have been substantially altered, and the original methane would have been released long ago,” observed Kargel in his email. “So something else is probably producing and releasing it freshly. Hydrothermally altered carbonaceous chondrite-like material would have complex, high-mass, low-solubility hydrocarbons– heavier and less volatile and less soluble molecules than methane. This rocky material and its hydrocarbons then may have been heated, altered, and stored somewhere deep inside Enceladus. Continued heating, happening today, then warms the hydrocarbons enough that they undergo thermal decomposition. The metamorphosed organics form and emit methane. This may be what Enceladus is releasing today. A byproduct is nearly pure carbon, which can be in the form of graphite (the stuff of pencil lead). Graphite also can undergo methane formation by hydrothermal processes if hydrogen is supplied. We already know that Enceladus is emitting hydrogen in addition to methane, so it seems that the basic ingredients for abiotic methane production are in place. This would be my bet. But more exciting, and also possible, is biological methanogenesis.”
Unknown Process at Work?
An unknown methane-producing process is likely at work in the hidden ocean beneath the icy shell, 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 (CO2). The amount of methane found in the plumes was particularly unexpected.
Taking a Different Route–New Mathematical Models
Ferrière 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.
Plume Composition–A Deep Dive
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.
Earth-like Serpentinization May Explain Presence of Water and Hydrogen
In an email to The Daily Galaxy, Ferriere wrote: “Serpentinization might explain both H2 and CH4 concentrations in Enceladus’s plume. Serpentinization –the name originating from the similarity of the texture of the rock to that of the skin of a snake–Is well known on Earth, is when with the addition of water, rock is transformed into the crystal structure of the minerals found within the rock. In the case of Enceladus, it involves the whole ocean circulating through the porous, rocky core. As water moves through hot rocks, iron in the rocks is oxidized by water and hydrogen is released. Then this hydrogen can react with CO2 in the ocean water and that produces methane.
Two Alternative Abiotic Processes
“What we show is that serpentinization alone is unlikely to explain the Cassini data,” expalined Ferriere. “One can think of two alternative abiotic processes. First, the core might be outgassing primordial methane. Thus, the methane would be of cometary origin, tracing back to Enceladus formation. Second, the methane could result from a process called “pyrolysis”, which in this case would involve the decomposition of organic molecules (also of cometary origin and thus also tracing back to Enceladus formation) by hot temperature in the core.
“If Enceladus is young (some models conclude that the moon could be as young as a few hundred million years), these processes could account for a large fraction of the methane detected in the plume,” continued Ferriere. “But if Enceladus is old (dating back to the origin of the solar system, like Jupiter’s moon Europa probably is), one may wonder if outgassing of primordial methane or pyrolysis of primordial organics would still yield much methane today. Unfortunately, Cassini was not equipped to make some of the isotopic measurements that could have helped finding out.”
“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,” Ferrière 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,” Ferrière 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.
Methanogenesis Appears Compatible with the Data
“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, Ferrière 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.
Something else is probably producing and releasing it
“However, indications are that Enceladus has been heated inside to temperatures where any original building blocks have been substantially altered, and the original methane would have been released long ago,” observed Kargel in his email to The Daily Galaxy. “So something else is probably producing and releasing it freshly. Hydrothermally altered carbonaceous chondrite-like material would have complex, high-mass, low-solubility hydrocarbons– heavier and less volatile and less soluble molecules than methane. This rocky material and its hydrocarbons then may have been heated, altered, and stored somewhere deep inside Enceladus. Continued heating, happening today, then warms the hydrocarbons enough that they undergo thermal decomposition. The metamorphosed organics form and emit methane. This may be what Enceladus is releasing today. A byproduct is nearly pure carbon, which can be in the form of graphite (the stuff of pencil lead). Graphite also can undergo methane formation by hydrothermal processes if hydrogen is supplied. We already know that Enceladus is emitting hydrogen in addition to methane, so it seems that the basic ingredients for abiotic methane production are in place. This would be my bet. But more exciting, and also possible, is biological methanogenesis.”
Source: “Bayesian analysis of Enceladus’s plume data to assess methanogenesis,” in the June 7 issue of Nature Astronomy.
Image & Caption Credit: NASA / JPL-Caltech