Conditions for life in the ocean on Saturn’s moon are getting better

Enceladus, a satellite of Saturn, and not Mars, astrobiologists now consider the most promising place to search for life. This is because there is liquid water. Although it is located under a multi-kilometer ice shell, but according to calculations, there should be quite a lot of it there. For the first time, the existence of a subsurface ocean was confirmed using the Cassini spacecraft, which in 2005 captured powerful water geysers near the south pole of the satellite. Cassini even managed to fly directly over one of these geysers at an altitude of less than 50 km, thanks to which we now know the approximate chemical composition of the subglacial reservoir. And in terms of the possibility of the existence of life, it is very good. It contains hydrogen, which can be used as an energy source by organisms like terrestrial methanogen bacteria. It at least has simple organics: methane, propane and formaldehyde, as well as carbon dioxide, which can act as carbon sources. There are also sulfur-containing compounds and ammonia as a source of nitrogen. But what could not be fixed in the geysers was traces of phosphorus. Although phosphorus is one of the most important elements for life, it is part of the DNA and RNA molecules, and in general, it is difficult to imagine cellular biochemistry without it.

The absence of traces of phosphorus in the geyser “exhaust” cooled the ardor of astrobiologists a little. Indeed, without this element, there is very little chance that the chemistry of the subsurface ocean will be able to get the prefix “bio”. There is no available phosphorus or there is very little of it – there is no life or the rate of its evolution is so low that it simply does not have time to be born during the existence of the reservoir. However, as the researchers write in PNAS, phosphorus in the subsurface ocean should be abundant. How did they come to that conclusion when all current observations so far suggest otherwise?

To do this, scientists built a complex model of the chemical evolution of the enceladic ocean, trying to take into account all the possible ways in which phosphorus can become soluble (and therefore available for biochemical processes), as well as the ways in which phosphorus can, on the contrary, precipitate and become biologically inaccessible. The amount of phosphorus dissolved in water is also affected by the acidity of the water as a whole, and the presence of other dissolved substances and ions. For example, a large amount of fluoride ions contributes to the formation of poorly soluble fluorapatites. Combining all currently known data on the estimated composition of the subsurface ocean with our knowledge of the solubility of a wide variety of phosphorus compounds and other chemicals, the researchers concluded that the dissolved phosphorus in the subglacial waters of Enceladus should be about one ten millionth to one hundredth of a percent. Such a large corridor is associated with a large uncertainty in the initial parameters of the calculation. However, the minimum concentration of phosphorus turns out to be 3–4 orders of magnitude higher than previously thought, and only ten times less than the average concentration of phosphorus in the waters of the World Ocean on Earth. How true such calculations are, it will be possible to check only when an apparatus is sent to Enceladus, capable of at least more accurately determining the composition of water geysers than Cassini did. Judging by the plans of the space agencies, this will happen no earlier than the 2030s, and maybe later.

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