Saturn’s moon Enceladus: are methane emissions an indication of life?

Saturn's moon Enceladus: are methane emissions an indication of life?

A few weeks ago, the French astrobiologists Antonin Affholder and Régis Ferrière from the Université de recherche Paris Sciences et Lettres aroused tremendous media coverage with a publication in the journal Nature Astronomy. After all, it was about possible traces of extraterrestrial life on Saturn’s ice moon Enceladus. But have scientists really found evidence of the existence of aliens?

The ice moons of the gas giants Jupiter and Saturn are currently the most promising candidates for extraterrestrial life in our solar system. The Saturn moon Enceladus is particularly interesting: NASA’s Cassini space probe discovered eruptions of water at the south pole of the ice moon. Through cracks in the ice – the so-called “tiger stripes” – huge tidal fountains of ice and steam hurl up into the atmosphere of the little moon. Under the icy surface there is apparently a liquid alien ocean full of fascinating elements and connections as well as sufficient energy.

The space probe then flew through these fountains in 2015 during several orbits around the moon and the Cosmic Dust Sampler (CDA) developed in Germany was able to analyze the composition with its mass spectrometer. The collected “dust” particles then turned out to be ice grains with an unusual composition. Overall, Cassini and CDA found a relatively high concentration of hydrogen in the form of ice, steam, and liquid water. In addition, there were methane, carbon dioxide and a whole range of salts, including sodium chloride – our terrestrial table salt. The discovery of silicon compounds, which must have come from the solid rock core of the moon, was also unusual. The most likely explanation for the fountains and their composition is currently hydrothermal systems at the bottom of the ocean, as they also exist on Earth.

Enceladus is completely icy – up to 35 kilometers thick at the equator and less than 5 kilometers at the South Pole. A global ocean stretches between the ice and the solid core, the floor of which is around 70 kilometers below the surface. Due to the large temperature gradient in the ocean and at the boundary layer to the ice surface, there are likely to be strong thermal upheavals. The silicon particles are probably generated by hydrothermal activity at more than 90 ° C in the porous rock core of the moon, then come close to the surface due to thermal upheavals and are finally thrown out by the fountains.

The methane discovery had turned the small ice moon with a diameter of only 500 kilometers from an uninteresting ice ball to one of the hottest candidates for astrobiological exploration in our solar system! Methane is so interesting for the search for extraterrestrial life because it can arise on earth through both non-biological and biological processes. The crucial question now is: Was the methane in the Enceladus Ocean produced by living things or was it abiotic?

The direct exploration of such a habitat deep beneath the surface of another celestial body is likely to be impossible for decades to come, explains the Main author of the current study, Antonin Affholder. That is why he and his colleagues tried out a mathematical method here: Bayesian statistics. “We constructed mathematical models for the explanation of the Cassini data in order to quantify the probability for the different explanatory approaches.”

The researchers therefore considered which of the methane formation could best explain the Cassini data and developed a geophysical model from this. On this basis, they then calculated the probability for the two selected hypotheses, one through life forms and one without. The abiotic methane production takes place in terrestrial oceans under high pressures and temperatures in particularly water-containing greenish rocks by the so-called serpentinization, for example at hydrothermal springs. However, only to a small extent and slowly, according to Affholder. Much more and faster, microorganisms produce the colorless gas in a process called methanogenesis.

The amount of methane measured by Cassini cannot be explained by the abiotic formation of methane and is therefore the less likely of the two hypotheses. “Should we conclude from this that Earth-like microbes populate the depths of the Enceladus Ocean? Absolutely not, although it would be fascinating“says Affholder.”Rather, our study tells a different story, namely that of a negative result! “

This was about testing the likelihood of different hypotheses. Rejecting the less probable hypothesis does not mean that the other, somewhat less improbable explanation then automatically has to be correct. Rather, the facts are simply not enough at the moment to answer the question of possible life in the Enceladus Ocean. So it remains the task of future ice moon missions to further research the still unknown methane sources and to discover their origin. Bayesian statistics have in any case proven themselves as a model for weighing up probabilities in the search for extraterrestrial traces of life.

The researchers therefore by no means believe in methane-producing aliens in Saturn’s orbit, but rather that the available data are not yet sufficient for a plausible justification. Even if this is a rather dry mathematical work, the topic, namely the search for life outside the earth, has once again reliably excited the media and the public. The methane traces on Mars keep making headlines for the same reason. Even if, as in the current ESA announcement, only that the methane traces measured by the trace gas orbiter are lower than initially assumed.


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