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The subsurface water oceans of Jovian moons (Europa, Enceladus) are of great interests in the search for extraterrestrial life. I'm wondering what current work has been done to predict the types of lifeforms that might exist there, given their unique environments.

The current best theory would probably be chemotrophs feeding off hydrothermal vents along the ocean floor. Are there research being done evaluating the possibility of other types of lifeforms, and the biological makeup of these potential creatures?

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    $\begingroup$ Even though this is an interesting question, it will probably get closed because it calls for speculation. Check out the tour page. "Don't ask about... Anything not directly related to space exploration, Questions that are primarily opinion-based, Questions with too many possible answers or that would require an extremely long answer..." $\endgroup$
    – phil1008
    Mar 13 at 6:05
  • $\begingroup$ The question is not what would life look like, but what could life look like. That's really guesswork at this point. $\endgroup$
    – GdD
    Mar 13 at 13:35
  • $\begingroup$ Unanswerable. We can make some assumptions about the chemistry of Europan life (if any) but what physical form they would take is pure speculation. $\endgroup$ Mar 13 at 13:36
  • $\begingroup$ Welcome to Stack Exchange! This is of course a very interesting topic, but in Stack Exchange questions need to be answerable with facts and supported by authoritative sources. If you'd like to reopen this one, you could change it to "What theoretical work has been done to try to predict what types of life might exist in the water oceans of Jovian moons?" and add the reference-request tag. $\endgroup$
    – uhoh
    Mar 13 at 20:58
  • $\begingroup$ @uhoh thank you for your suggestion. Have edited my question. Hopefully it would present itself to be more conducive for discourse now. $\endgroup$
    – Shawn Lim
    Mar 14 at 6:28

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The answer to what type of lifeforms are expected might be "microorganisms," since these have been the most considered. Microorganisms appeared on Earth about 3.7 billion years ago, but it wasn't until the last 0.6 billion years that large multicellular life formed. This suggests that there is a large (entropic?) barrier to developing large multicellular life and perhaps the formation of an atmosphere with sizable concentration of oxygen (due to the action of photosynthetic organisms) was needed.

Most peer-reviewed studies have focused on the potential biochemistry and metabolisms of life in the subsurface oceans of Europa and Enceladus. Here's a nice review. To quote the abstract, "The most often hypothesized metabolisms are methanogenesis for Enceladus and methane oxidation/sulfate reduction on Europa."

Photosynthesis is not expected since the surface is too cold and too little light reaches the subsurface ocean. Hence, the source of free energy must be chemical, much like organisms (chemotrophs) on Earth that live near hydrothermal vents in the deep ocean. Molecular oxygen and hydrogen peroxide likely form from irradiation of the ice sheet and have been detected. These might allow for aerobic respiration.

Large organisms on Earth that are not photosynthetic require large concentrations of O$_2$, which may not be available in sufficient quantities without other photosynthetic organisms. This lack of O$_2$ could be a significant challenge for the evolution of large organisms similar to animals on Earth.

Since pH is a very important determiner for the types of organisms on Earth and their metabolism, it is expected to be for Europa and Enceladus as well. Based on spectra from plumes measured by spacecraft, the pH of Europa is expected to be acidic (4–6), while that of Enceladus is expected to be basic (8–11). However, the pH of Europa's ocean is not well constrained by the data.

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  • $\begingroup$ Very interesting reads. Slightly off-topic: I suppose Europa Clipper would set out trying to better constrain Europa's ocean characteristics (pH levels, chemical composition etc.). Wondering how it's intending to do that, especially considering Europa's relative lack of plume activity. $\endgroup$
    – Shawn Lim
    Mar 15 at 2:55

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