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Gathering the Evidence for Life on Enceladus

With a proposal for an Enceladus Orbilander mission in the works at the Johns Hopkins Applied Physics Laboratory, I continue to mull over the prospects for investigating this interesting moon. Something is producing methane in the ocean under the Enceladus ice shell, analyzed in a 2021 paper from Antonin Affholder (now at the University of Arizona) and colleagues, using Cassini data from passages through the plumes erupting from the southern polar regions. The scientists produced mathematical models and used a Bayesian analysis to weigh the probabilities that the methane is being created by life or through abiotic processes.

The result: The plume data are consistent with both possibilities, although it’s interesting, based on what we know about hydrothermal chemistry on earth, that the amount of methane is higher than would be expected through any abiotic explanation. So we can’t rule out the possibility of some kind of microorganisms under the ice on Enceladus, and clearly need data from a future mission to make the call. I won’t go any further into the 2021 paper (citation below) other than to note that the authors believe their methods may be useful for dealing with future chemical data from exoplanets of a wide variety, and not just icy worlds with an ocean beneath a surface shell.

Now a new paper has been published in The Planetary Science Journal, authored by the same team and addressing the potential of such a future mission. A saltwater ocean outgassing methane is an ideal astrobiological target, and one useful result of the new analysis is that it would not take a landing on Enceladus itself to probe whether or not life exists there. Says co-author Régis Ferrière (University of Arizona):

“Clearly, sending a robot crawling through ice cracks and deep-diving down to the seafloor would not be easy. By simulating the data that a more prepared and advanced orbiting spacecraft would gather from just the plumes alone, our team has now shown that this approach would be enough to confidently determine whether or not there is life within Enceladus’ ocean without actually having to probe the depths of the moon. This is a thrilling perspective.”

Image: This graphic depicts how scientists believe water interacts with rock at the bottom of Enceladus’ ocean to create hydrothermal vent systems. These same chimney-like vents are found along tectonic plate borders in Earth’s oceans, approximately 7000 feet below the surface. Credit: NASA/JPL-Caltech/Southwest Research Institute.

Microbes on Earth – methanogens – find ways to thrive around hydrothermal vents deep below the surface of the oceans, in regions deprived of sunlight but rich in the energy stored in chemical compounds. Indeed, life around ‘white smoker’ vents is rich and not limited to microbes, with dihydrogen and carbon dioxide as an energy source in a process that releases methane as a byproduct. The researchers hypothesize that similar processes are at work on Enceladus, calculating the possible total mass of life there, and the likelihood that cells from that life might be ejected by the plumes.

The team’s model produces a small and sparse biosphere, one amounting to no more than the biomass of a single whale in the moon’s ocean. That’s an interesting finding in itself in contrast to some earlier studies, and it contrasts strongly with the size of the biosphere around Earth’s hydrothermal vents. But the quantity is sufficient to produce enough organic molecules that a future spacecraft could detect them by flying through the plumes. The mission would require multiple plume flybys.

Actual cells are unlikely to be found in the plumes, but detected organic molecules including particular amino acids would support the idea of active biology. Even so, we are probably going to be left without a definitive answer, adds Ferrière:

“Considering that according to the calculations, any life present on Enceladus would be extremely sparse, there still is a good chance that we’ll never find enough organic molecules in the plumes to unambiguously conclude that it is there. So, rather than focusing on the question of how much is enough to prove that life is there, we asked, ‘What is the maximum amount of organic material that could be present in the absence of life?'”

An Enceladus orbiter, in other words, would produce strong evidence of life if its measurements were above the threshold identified here. Back to the JHU/APL Enceladus Orbilander, thoroughly described in a concept study available online. The mission includes both orbital operations as well as a landing on the surface, with thirteen science instruments aboard to probe for life in both venues. The mission would measure pH, temperature, salinity and availability of nutrients in the ocean as well as making radar and seismic measurements to probe the structure of the ice crust.

Image: Artist’s impression of the conceptual Enceladus Orbilander spacecraft on Enceladus’ surface. Credit: Johns Hopkins APL.

Here the chances of finding cell material are much higher than in purely orbital operations, where survival through the outgassing process of plume creation seems unlikely. The lander would target a flat space free of boulders at the moon’s south pole with the aim of collecting plume materials that have fallen back to the surface. The team points out that the largest particles would not reach altitudes high enough for sampling from orbit, making the lander our best chance for a definitive answer.

The paper, indeed, points to this conclusion:

…cell-like abiotic structures (abiotic biomorphs) that may form in hydrothermal environments could cause a high risk of a false positive… Assuming that cells can be identified unambiguously…, we find that the volume of plume material that needs to be collected to confidently sample at least one cell might require a large number of fly-throughs in the plume, or using a lander to collect plume particles falling on Enceladus’s surface (e.g., the Enceladus Orbilander; MacKenzie et al. 2021).

The paper is Affholder et al., “Putative Methanogenic Biosphere in Enceladus’s Deep Ocean: Biomass, Productivity, and Implications for Detection,” Planetary Science Journal Vol. 3, No. 12 (13 December 2022), 270 (full text). The paper on methane on Enceladus is Affholder at al., “Bayesian analysis of Enceladus’s plume data to assess methanogenesis,” Nature Astronomy 5 (07 June 2021), 805-814 (abstract).

{ 14 comments… add one }
  • Andrew Palfreyman January 4, 2023, 9:15

    It’s the Mars rover scenario all over again: whatever you do, avoid microscopy in the search for cellular life. It seems profligate to expend all that resource on getting there, and then not to look for life directly.

    • Alex Tolley January 4, 2023, 13:11

      To be fair, the authors do suggest that cells may not be present in the plume as they are too sparse. An optical microscope wouldn’t be able to identify cellular organelles.
      To do Viking type experiments viable cells need to be captured to detect metabolism and growth.

      Chirality, lipid length, isotopic analysis,and other methods might have to be the life detectors rather than looking for living cells.

  • Larry Kennedy January 4, 2023, 9:40

    If the life opportunity is so rare it would seem to make the odds of life very low, even the “life everywhere” folks might not look hopeful on this one.

    • Antonio January 4, 2023, 14:18

      There are a lot of geocentric assumptions in their calculation. I would not take their probability seriously.

  • Simon January 4, 2023, 10:47

    Many thanks for posting this, I find the search for life on the moons of Jupiter and Saturn absolutely fascinating so I am very curious to see what evidence might be found on the surface of Enceladus next.

  • John walker January 4, 2023, 11:47

    This Affholder study is an entertaining read. As always in similar studies many aspects are based on ill constrained parameters, but the authors make a decent attempt to account for this in their search for practical means of exolife affirmation or rejection.

    The shocking aspect of the paper for me is the infinitesmal total amount of biomass that the authors speculate to exist in the global ocean. Namely total <10 metric tons. Taking the upper bound, this is represents at best approx. one hundred millionth of terrestrial ocean (living)biomass density. Life is exceedingly rare in this analysis.
    From the abstract:
    "We find that although a hypothetical biosphere in Enceladus's ocean could be small (0.1 mL of material needs to be collected. This would require material from more than 100 fly-bys through the plume or using a lander.”
    The Orbilander concept might allow for up to 1000 flybys. By what method a single cell might be detected in that mass of material, I don’t know. Also, it bears consideration that although the samples will be too fresh to be significantly degraded by ionizing radiation following ejection from the ice shell, a large percentage of the cells could be destroyed through simple shock depressurization in the vacuum, reducing the viable cell concentration.

    Additionally, the study addresses amino acid analysis (in particular that of glycine) and there also draws conclusions on the needed instrument detection limits. Thanks to Paul for highlighting this work.

    • Abelard Lindsey January 7, 2023, 13:20

      Put the probe in orbit of this moon and you get all the flybys you need, assuming that such an orbit is stable for a long enough period of time.

  • FrankH January 4, 2023, 15:26

    Enceladus has a thinner ice crust, so it might be easier to find life… BUT one major concern is the age (or recent chaotic history) of some or all of the Saturnian moons, especially the inner ones.

    There was an article here almost 7 years ago:

    Enceladus as it exist now may be a fairly recent construct.

  • Robin Datta January 5, 2023, 10:36

    Should enough material be found and the appropriate instruments be carried aboard to characterize the lineage of origin of any cell(s) found. , that in itself should be a major step in answering the question of abiogenesis vs. panspermia.

  • Mike Serfas January 5, 2023, 15:23

    Supporters of Breakthrough Starshot have described an extraordinarily powerful laser array that can be very tightly focused, and I assume such things are being built for anti-missile and other military applications. Could you simply blast these plumes with a big laser from Earth (while Enceladus is in Saturn’s shadow) to detect fluorescence from any complex carbon compounds? Fluorescence microscopy. :)

  • Robin Datta January 5, 2023, 22:26

    Fluorescence telescopy?

  • Abelard Lindsey January 7, 2023, 13:41

    One whale’s worth of biomass is not a lot to work with. Instead of cells, they should look for the original organics resulting from the energy gradient in those hydrothermal vents that Nick Lane claims was the origin of Earth life. This is when the ATPSynthase molecule first formed. This would at least confirm the hydrothermal vent origin theory of life which, in turn, would suggest that prokaryote life is common to places that are more hospitable than Enceladus. Finding ATPSynthase would be even more dramatic as it would confirm that it is a universal component and requirement of all life in the universe. Confirmation of Nick Lane’s theories would be a very big deal and would most certainly justify the cost of such a probe.

    The theory that Enceladus and the rings are “new” as well as its small size may be the reason for such a small biomass.

  • Abelard Lindsey January 7, 2023, 13:59

    With only one whales worth of biomass, probably the best they can hope for is to find amino acids of clear biological origin or possibly the next higher level molecule. I don’t think they’re going to find anything as complex as ATP Synthase.

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