An image of the surface of Encelad taken from a height of one kilometer. Ice grains containing large and complex molecules of organic matter could gush from the cracks in the ice sheet. They should be the products of chemical reactions between the rocky core of this moon of Saturn and the warm water of its subsurface ocean. Credit: NASA / JPL-Caltech / Space.
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We have written about Enceladus many times in Oslo. Saturn’s small icy moon hides a liquid ocean beneath its frozen surface and a porous rock core hot enough to cause hydrothermal activity, which behaves like a chemical factory. Unlike other ice bodies that jealously guard their secrets, Enceladus has already revealed much about his interior – his carapace is cracked, and geysers of water vapor and ice crystals are gushing through it, carrying samples of seawater into space. Between 2004 and 2017, they were repeatedly examined by the Cassini spacecraft and, in part, by terrestrial telescopes.
Artistic idea of possible hydrothermal activity on the ocean floor at Enceladus. Credit: NASA / JPL-Caltech. Free work.
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It turned out that the geysers are enriched with everything possible. We can find gases (hydrogen, methane, carbon dioxide, nitrogen …), ordinary salt, but also silica particles and organic substances, from the simplest to the most complex. All this shows us that the ocean on Enceladus lacks any important ingredients for the existence of life as we know it from Earth: there is water that does not deviate in any way from the conditions or composition of the Earth, there are all major biogenic elements and heat and energy sources. . But is that life there too? The Cassini spacecraft won’t cut it anymore – on the one hand, it served four years ago, and on the other hand, it was simply not equipped to answer this question. Otherwise, the very ambitiously designed probe did not carry any apparatus that could unambiguously identify the remains of living organisms, even if it collided beautifully with them. Who would have thought thirty years ago that she would be showered by water from an alien sea in Saturn’s orbit, and that she would easily collide there with some of the bacteria she really could? However, another probe will almost certainly not look at Enceladus in the next two decades, so we are unlucky in this regard for the time being.
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A team of researchers from various French institutions has now moved the scales again a little closer to the coveted extraterrestrial life. Let’s see how they came to this and how seriously we can take these clues.
The gas cocktail escaping from Enceladus is a bit like the products of hydrothermal processes here on Earth. The hot rock reacts with water to produce hydrogen gas or methane and other substances. Water saturated with these gases represents a set table for a number of microorganisms, led by methanogens – these creatures diligently capture hydrogen and carbon dioxide and convert them into methane, leaving some energy and organic molecule “behind their nails”, which is quite enough for them to be lucky. .
The French asked themselves the simple question of whether methanogens would like the hot springs on Enceladus. To this end, they simulated the various environments that might exist on Enceladus within the uncertainties of our ignorance. They did not stick to it, they performed 50,000 simulations. In about 1/3 of cases, they came up with a combination of conditions that would indicate terrestrial methanogens. The freaks argue that Enceladian microbes may have preferences a little differently, but let’s put that aside. In any case, this is not a big surprise – older works, such as Taubner et al., Have shown that methanogens would like Enceladus. (2018).
The authors of the current study were primarily interested in whether we know how methanogens work inside and out. An interesting thing turned out here – the simulated Enceladas without life emitted a completely different mixture of gases than those with life. Enceladus the Real clearly sided with life in this regard. The abiotic reactions between water and rock (serpentinization) themselves produce much less methane. It almost seems as if they are receiving biological help in this direction. Some older work has shown that the presence of hydrogen in geysers testifies to life on Enceladus: if it is there, obviously no one has eaten it. These objections turned out to be odd: methanogens, on the other hand, are not so fed up as to be able to pick up everything that has erupted from the hot springs. Are they there, waiting for us to find them? Chances don’t look the worst, but let’s not open the champagne yet.
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Temperature map around the South Pole, where there are several temperature anomalies in the form of so-called tiger stripes. Credit: NASA / JPL / GSFC / SwRI / SSI (free work).
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A skeptic might argue that the Enceladian winds could have enriched some other process with methane. If Enceladus resembles Earth in composition, it can be quite successfully ruled out, but this is, of course, a assumption like thunder. If a reservoir of honestly abiotic organic molecules, such as those in comets and carbonaceous chondrites, exposed to geological heat, lay inside the moon, they could also release methane, confusing scientists. Alternatively, methane could trickle into the forming moon a long time ago, and now simply passively escape. Or the geology of Enceladus completely defies our imagination (after all, this vicious moon often and happily does that).
The authors also flirted with Bayesian probability analysis to assess whether they should prefer an abiotic or a biotic explanation. This modern equivalent of the Delphic Fortune Teller told them, among other things, that if the origin of life is a rare and unlikely event, there will probably be some unknown abiotic process behind the methane, while if life in space is common, it will be the methanogens. And if it doesn’t rain, I won’t get wet. In short, even the most sophisticated probability analysis will not help us if we lack key data. As you can see, we will probably not answer this question without another space mission.
Video: What You Need to Know About Enceladus (NASA)
Resources:
Affholder, A., Guyot, F., Sauterey, B. et al. Bayesian analysis of Enceladus’s plume data to assess methanogenesis. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01372-6
Taubner, R. S., Pappenreiter, P., Zwicker, J., Smrzka, D., Pruckner, C., Kolar, P., … & Simon, K. M. R. (2018). Biological methane production under putative Enceladus-like conditions. Nature communications, 9(1), 1-11.
https://astronomycommunity.nature.com/posts/bayesian-analysis-of-enceladus-plume-data-to-assess-methanogenesis
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