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Europa’s Oxygen and Aerobic Life

Few destinations in the Solar System have excited the imagination as much as Europa. Could a deep ocean beneath the ice support a biosphere utterly unlike our own? If so, we could be looking at a second emergence of life unrelated to anything on Earth, with implications for the likelihood of life throughout the cosmos. But so much depends on what happens as Europa’s surface and ocean interact. Alex Tolley, a fixture here on Centauri Dreams, today looks at new work suggesting the deeply problematic nature of Europa’s ocean from the standpoint of astrobiology. He also offers an entertaining glimpse at what Europa might become.

by Alex Tolley

Image: Plume on Europa’s Surface. Credit: NASA

With the abundance of newly discovered exoplanets, a fraction of them being both rocky and in their habitable zones (HZ), the excitement at finding life on such worlds is increasing. Given the ambiguous results of the attempt to detect life on Mars with the Viking experiments in 1976 and the subsequent NASA missions to look for proxies rather than direct detection, it is only to be expected that astrobiologists turned their attention to these exoplanets.

We tend to think of life primarily in terms of metazoa rather than unicellular organisms, so the search for life generally focuses on finding evidence of free oxygen (O2), even though aerobic metazoa only appeared on Earth within the last billion years. But detecting proxies for life is not the same as studying it directly. Therefore a search for metazoa in our own solar system would offer the opportunity to sample extraterrestrial life well before we get such an opportunity from an exoplanet.

The discovery of ecosystems in the near lightless abyssal depths of Earth’s oceans around “hot smokers” has stimulated new hypotheses concerning abiogenesis and extended the known environments for extremophile life. Anaerobic bacteria feeding on the chemical brew from the vents become the primary food of aerobic metazoans living around these smokers. On Earth, all but a few metazoa are aerobic [5], as the higher energy from this mode of respiration allows faster growth and reproduction, as well as active behaviors. With the discovery that some icy moons around Jupiter and Saturn have subsurface oceans and active geologies, it seemed possible that these moons might harbor life too, and therefore offer a local, solar system destination to discover life and return samples. Because terrestrial metazoans are aerobes, the presence of oxygen in the icy moons would be a positive indication that there may be metazoan life forms as well as microbes.

Of the icy moons that might have oxidizing oceans Europa is the clear favorite.

Fictional Interlude 1

The Europa Oxygen and Life Surveyor (EOLiS) probe swung by Europa. Earthside Mission Control had done all it could to ensure the craft had successfully reached its target for orbital insertion. At 13:31:07 UTC the lander released itself from the orbiter, deployed its own magnetic radiation shield, and fired its braking rockets. Now the lander’s onboard AI became fully autonomous as it guided the craft towards the preselected surface destination, fusing its sensor data, radar and vision, to locate its surface landing point. The mother craft would remain in orbit and release 4 more communication relay satellites to maintain uninterrupted communications with Earth. The lander quickly reached the surface, mere meters from its preferred landing spot, and in the smoothest terrain within its target radius.

One of the mission objectives was to determine the depth profile of oxidants in the surface ice, a key variable for the oxygen levels in the subsurface ocean, and a factor for the evolution of metazoa. A nuclear fission-heated probe slowly melted its way down through the ice. Measurements of free O2, H2O2, and other oxidizing molecules were continually taken. Initial readouts indicated that the very high oxygen levels on the surface were not maintained below a few meters of the surface.

After 60 orbits around Jupiter, the lander had transmitted its findings back to mission control. Oxygen was mostly in the top meter of ice and snow, but in much lower concentrations down to at least the 3 kilometers its probe had penetrated. Despite this, the oxygen levels were still of the order of grams per cubic meter of ice. The planetologists inferred that the Europan ocean was likely still anoxic. The astrobiologists had to content themselves that maybe anaerobic microbes were still possible. The lander had performed well in its primary mission, so the extended mission included boring down to the base of the surface ice and into the ocean below.


Europa’s surface ice is subjected to about 0.125 W/m2 of ion radiation, radiolytically producing oxygen on the surface and a very thin atmosphere (the mechanism shown in figure 1).

Figure 1. A simplified sequence for radiolytic production and destruction of water, H2O2, and molecular oxygen. Some H2O2 and O2 become sequestered below the heavily processed surface. UV, ultraviolet. [Hand et al – [3]]

Hand estimated up to 7.6% oxidant contaminants in the surface ice, and up to 53% of the surface in the form of clathrate cages containing oxygen and other oxidants [3]. Most important for life is this trapping of oxygen that might find its way to the subsurface ocean. The relatively young surface of Europa is criss-crossed by ridges due to liquid or slush being pushed up from below, freezing and overlaying the surface. These clathrates would descend and oxygenate the subsurface oceans due to the resurfacing. The base of the ice crust melts into the ocean, a cycle that takes between 20 and 500 million years.

Hand’s estimates placed the rate of production of O2 at 2E-7 to 8E-7 kg/m2/yr.

Greenberg was very much more positive about ocean oxygenation, suggesting that the oceans could reach Earth levels of O2 saturation (around 10 mg/L) well within the rapid resurfacing rate times for the clathrates to reach the oceans, in about 10 million years [6].

In 2016, Vance and Hand continued to use Hand’s earlier O2 production rates of 3E-7 to 3E-4 kg/ m2/ yr.

These estimates were based on assumptions about how the tenuous atmosphere was maintained. If it was primarily due to radiolysis, then subsurface radiolysis to produce O2 would result in trapping of the O2 for transport to the ocean. This would support the calculations by Vance of a rapidly oxidizing ocean that could support aerobic life, if not a rich as Earth’s, then at least within striking distance of abyssal life density.

These estimates may have been optimistic.

In recent papers [1,2], R E Johnson and A Oza call into question this model. They simulate the atmosphere and find that the best explanation for the atmosphere is thermal release of the O2 from the surface ice by desorption. This implies that far less O2 is trapped in the ice grains that can be subducted to the oceans.


This assumes that Europa’s ice regolith is permeated with trapped O2, which could also affect our understanding of the suggestion that the radiolytic products in Europa’s regolith might be a source of oxidants for its underground ocean.

While the O2 is produced within the top meter of ice, gas diffusion prevents loss of O2, and regolith subduction and mixing draw down the O2 into the lower depths. Gardening only allows mixing to about 10 meters, but resurfacing due to upwelling at the ridges results in the O2 to be drawn down to the base of the ice sheet and enter the oceans below on timescales of tens to hundreds of billions of years.

Johnson et al:

Although direct diffusion to the depth of the ocean is likely problematic, geologic mixing and subduction of oxygen rich ice has been suggested as a possible source of oxidants for putative ocean biology.

Oza and Johnson’s previous paper [2] estimated production of O2 on Europa was just 0.1-100 kg/s, or about 3E6 to 3E9 kg O2 /yr (Earth year) or 1E-7 to 1E-4 kg/m2/yr. Their mechanism is explained in figure 3 below. They argue that thermally desorbed O2 from the ice best explains the atmospheric dynamics over a Europan day, and therefore the O2 at depth is less than previous estimates and models suggest.

Figure 2. Schematic diagram of O2 trapping and thermal desorption: 1) Primary origin of O2 (and H2) is magnetospheric ion radiolysis. 2) Due to preferential loss of H2, the regolith becomes oxygen rich enhancing the production of O2. Formed and returning O2 can become trapped at incomplete (dangling) H bonds (shown) as well as in voids (as shown and observed by Spencer & Calvin 2002). 3) The accumulated O2 can then be thermally desorbed from the weak dangling bonds due to solar heating, maintaining a quasi vapor pressure equilibrium (Oza et al. 2018a), with a smaller gas-phase contribution from direct sputtering of O2. A fraction of the trapped O2. is likely subducted. [Johnson et al – [1]]

Fictional Interlude 2

The probe ran the samples from the bore hole through a battery of molecule and life detectors. While the usual mix of carbon compounds that could be found on any icy body, including comets and asteroids were present, none registered anything definitive for life. Asymmetry in organic molecules’ chirality was absent, as were odd lipid chain lengths. None of the growth experiments registered any change. Like the previous disappointment with Mars, the hopes of the early 21st century astrobiologists to find life in the icy moons were frustrated. Europa had so far proven sterile. Neither was there any unambiguous evidence of prebiotic chemistry.

The top kilometer of ocean below the ice crust proved still rather anoxic compared to the ice above it. The sheer volume of the ocean, plus the reducing nature of the vent emissions kept the oxygen levels well below that of the terrestrial oceans. Coupled with the absence of any signatures of microbial life, it was clear that there could not be multicellular life in that ocean.

While disappointing to the biologists, this finding indicated that there would be no violation of a putative “Prime Directive” should colonization be attempted.


Whatever the amount of O2 trapped in the ice, it is the production rate of O2 that determines the steady state in a biosphere, even if accumulation can create highly oxic conditions in the oceans suitable for aerobic life to exist.

Therefore the key question is just how much O2 is produced by radiolysis? Let me put that in perspective, given the earlier conclusions, especially the optimistic ones of Greenberg.

On Earth, photolytic O2 production is insignificant compared to that from photosynthesis. Earth’s environment was largely anoxic for billions of years, with aerobic, multicellular life only appearing in the fossil record less than a billion years ago and flowering in the Cambrian as O2 levels increased. This was a result of the evolution of photosynthesis, which is the dominant source of Earth’s O2.

On Earth, net primary production (carbon fixation by photosynthesis minus plant respiration) creates about 3E14 kg O2/yr, or about 0.65 kg /M2/yr averaged over the total Earth’s surface. It is about a tenth as much if all respiration from heterotrophs and saprophytes is included.[7].

Therefore the rate of O2 production on Europa is 3-6 orders of magnitude lower than Earth’s net primary production of released O2. The difference between Earth’s and Europa’s O2 production is somewhat larger than Vance’s suggestion that Europa’s O2 production is about 1% of Earth’s. Therefore, even without any other sinks, Europa’s O2 production is 1/1000th that of Earth, at best, on an area based comparison, and possibly just a millionth at worst. Johnson’s analysis of likely lower O2 concentrations in the surface would further reduce the subduction rate of O2.

The implication for life in Europa is that the production of oxygen via radiolysis is clearly insufficient to replace photosynthetic organisms that produce the oxygen in quantities to support aerobic life on Earth, even those most adapted to low concentrations, such as sessile invertebrates.

While Greenberg has suggested that photosynthetic life might reach just below the surface to add primary production to the oceanic organisms below the surface, it is more likely that if life exists at all, it is going to be anaerobic bacteria, like those of the Archaean in Earth’s history. If that is correct, any ocean vents may have bacteria, but aerobic metazoa will not be present around them as they are on Earth now.

How does this impact the search for life in Europa? If life is either absent or anaerobic, the fanciful suggestion by Freeman Dyson that we might look for fish remains ejected from the ocean is likely futile. As all but a few terrestrial metazoa are aerobic, the lack of significant O2 production seems to diminish any likelihood that Europa hosts large animals as suggested by Clarke [9]:

Suddenly, a vast bulk broke through the surface of the ocean and arched into the sky. For a moment, the whole monstrous shape was suspended between air and water.

The familiar can be as shocking as the strange – when it is in the wrong place. Both captain and doctor exclaimed simultaneously: ‘It’s a shark!’

Image: Europa deep ocean vent visited by a robotic submersible. Credit: NASA/JPL

However, there is the possibility that without a sink via consumption, free O2 could just accumulate over the eons and possibly jumpstart the greening of Europa’s oceans.

The O2 level in Earth’s oceans is saturated around 10 mg/L at 0 degrees C and declines with rising temperatures. Active vertebrates like fish need around 4 mg/L, much more than the 1% saturation required by sessile invertebrates like sponges.

Using Oza and Johnson’s estimated range of 0.1 – 100 kg/s O2 production on Europa by radiolysis, and ignoring issues of reduced surface concentration levels, and other sinks for O2, Europa’s ocean would reach saturation at 10 mg/L in 3 million to 3 billion years. The time required is due to the immense volume of the estimated 100 km deep ocean, 2-3x as great as Earth’s oceans.

However, the rich O2 levels in the ice might range from 0.01 to 10 kg/m3. Melted, this ice would provide for a more than adequate level of oxygen saturation for terrestrial fish. Adding terrestrial life to such lakes would quickly deplete the O2 levels. New oxygen would have to be added by either maintaining a rate of ice melting or adding photosynthetic organisms.

If this analysis is correct, while it seems to rule out a rich aerobic ecology today, it does not preclude one tomorrow, if the production rates of O2 could be enhanced.

Fictional Interlude 3

“Sub One operating nominally,” intoned a somewhat bored Thomas Roberts. He was lead eco-engineer at Nagata base on Callisto, well clear of the intense, deadly radiation from Jupiter that was the key to the greening of Europa. Roberts team was monitoring the newly created subsurface lake christened Dodon Lake, known more colloquially as “dee-el” by his co-workers. It was situated in the Conamara Chaos and radar imaging had indicated it was now about 1 kilometer long and half a kilometer wide, with a maximum depth of 10 meters, laying just 20 meters below Europa’s surface near what appeared to be an old plume vent. The shallow depth of the lake beneath Europa’s surface ensured both sufficient radiation shielding as well as relatively easy access via the fractured ice in the vent.

The lake had started out as a natural fracture below the surface. Nuclear generators had melted the ice at the base of the fracture, creating a freshwater lake that was saturated with oxygen, and with more than enough extra oxygen to fill the void above it with breathable air. Preliminary tests indicated the water column was now mostly freshwater, not unlike that of L. Vostok in Antarctica, although with more dissolved CO2 and SO4. The dissolved O2 was at saturation. All that was missing was enough nitrogen and phosphorus, as well as trace minerals, to make this a living lake like those in Northern Canada, albeit without the summer mosquitoes. It was as dark as any subterranean cave on Earth, although that would soon change. 10 submersibles with high intensity LED lamps had been lowered down from the surface and had swarmed out across the surface of the lake, guided by the swarm intelligence of their onboard AI. The juice needed to power the motors and lights came from small nuclear reactors which fed waste heat to the lake bottom to increase the O2 release.

When the first sub powered up its lights, for the first time since its formation, the lake became a wonderland, illuminated with purple light, whose red and blue wavelengths were suited to maximize the photosynthesis that was to come. A cocktail of single cell algae originally sourced from subsurface Antarctic and Greenland lakes and cryogenically stored during transit, was released from the subs and soon began to photosynthesize and reproduce near the lights.

After a week, the crystal clear water started to become faintly cloudy as the density of algae increased to become the needed food for the large variety of invertebrates that followed. After a month, Thomas was certain that there would be no need to tweak the nutrients that had been added to the lake. The relatively simple starter ecosystem was on the predicted growth path that would reach its stable state cycle in 2 years. Within a decade, it was expected that a stable ecology would be established with sufficient oxygen production to maintain the first seeding of fish. But that was a job for the next crew of engineers to baby. The first steps to the greening of Europa had begun.


1. Johnson, R.E. et al (2019) “The Origin and Fate of O2 in Europa’s Ice: An Atmospheric Perspective,” Space Sci Rev (2019) 215:20 DOI 10.1007/s11214-019-0582-1

2. Oza A P et al (2019) “Dusk Over Dawn O2 Asymmetry in Europa’s Near-Surface Atmosphere,” Planetary and Space Science 167 23-32

3. Hand, K. P., Chyba, C. F., Carlson, R. W., & Cooper, J. F. (2006). “Clathrate Hydrates of Oxidants in the Ice Shell of Europa,” Astrobiology, 6(3), 463–482. doi:10.1089/ast.2006.6.463; Davis, J C, (1975) “Minimal Dissolved Oxygen Requirements of Aquatic Life with Emphasis on Canadian Species: a Review,” J. Fish Res. Bd. Can. Vol. 32(12)

4. Danovaro, R., Dell’Anno, A., Pusceddu, A., Gambi, C., Heiner, I., & Kristensen, R. M. (2010). “The first metazoa living in permanently anoxic conditions,” BMC Biology, 8(1), 30. doi:10.1186/1741-7007-8-30

5. Greenberg, R., (2010) “Transport rates of radiolytic substances into Europa’s ocean – Implications for the potential origin and maintenance of life,” Astrogiology Vol. 10, Number 3, 2010. DOI: 10.1089/ast.2009.0386

6. Huang J, et all (2018) “The global oxygen budget and its future projection,” Science Bull.. v63:18 pp1180-1186 https://doi.org/10.1016/j.scib.2018.07.023

7. Vance, S. D., K. P. Hand, and R. T. Pappalardo (2016), “Geophysical controls of chemical disequilibria in Europa,” Geophys. Res. Lett., 43, 4871–4879, doi:10.1002/2016GL068547.

8. Clarke, A C. 2061: Odyssey 3. Ballantine Books, 1987.

{ 55 comments… add one }
  • Jesus Olmo May 10, 2019, 14:23

    (“A Darkling Sea”, a sci-fi novel by James L. Cambias)

  • Gary Wilson May 10, 2019, 15:06

    I’m betting on anaerobic unicellular life in Europa’s ocean Alex. I hope I live long enough to get an answer to this intriguing question. Do you write SF?

    • Alex T. May 11, 2019, 3:40

      I’m betting that there is no any life on/in Europa, but me too want to live long enough to get an direct answer to this question …

    • Alex Tolley May 11, 2019, 11:48

      @ Gary and Alex T. I too hope to get an answer in my lifetime. The discovery of life would be most exciting, but I suspect it may be absent. I still hold out some hope for life in Mars simply due to its history and proximity to Earth.

      • Alex T. May 12, 2019, 2:39

        Me too , expect at least some news about finding on Mars life signs at least signs of Mars’s life existence in the far past.

  • Supernaut May 10, 2019, 15:10

    Thanks for the Science. However, your fictional interludes remind me a bit of KS Robinson’s Mars Trilogy: you should write an equivalent for Europa (Icy/Green/Blue Europa?): I’d buy the book(s)!

    • Alex Tolley May 11, 2019, 11:52

      @Gary, Supernaut. No, I don’t write any fiction at all. I don’t have the skill, patience, and dedication to write beyond very short pieces. A snippet of fiction is all I can manage.

      • ljk May 13, 2019, 9:22

        That is why you need a co-author. :^)

        I will get you started….

        White Europa: A frozen wasteland of a moon tackled by a few brave and hardy pioneers from Earth, some with questionable pasts. Their struggles, their passions, their political battles with a home world no longer theirs that still wants to control them and every other world in the Sol system. And far below, in the uncharted depths of the alien satellite they call home, hints that they are not alone….

        Green Europa: The brave and hardy settlers, having successfully repelled a hostile takeover of their new home by Earth authorities, now plan to truly make Europa their world with an aggressive terraforming scheme. Yet there are some who oppose this effort in the name of the native beings who live in the deep global ocean depths – and may even be intelligent.

        A brave young couple, whose families are on either side of the terraforming Europa divide, risk everything – including their growing love – to make contact with the mysterious Europans, who may hold the key to the survival of every living being on this alien moon.

        The storm clouds of an ultimate confrontation between the two worlds and two species looms on their metaphorical horizon.

        Blue Europa: The conflict between those who would turn Europa into another Earth and those who would keep Europa for the Europans finally comes to a head. Who will master this strange alien satellite, or will their greed and aggression destroy it? What will become of our young couple who now lives among the peaceful and enlightened Europans, who know the secret of their new alien friends that could save not only all those who call this moon home, but the cosmic destinies of every living being in the Sol system and beyond!

        Even more importantly, will their epic story get a nice, juicy Hollywood contract?!

        See? Easy. Now for when you write about that other life-promising moon with the water and geysers….

        White Enceladus: A frozen wasteland of a moon tackled by a few hardy pioneers from Earth. Their struggles, their passions, their political battles with a home world no longer theirs that still wants to control them and every other world in the Sol system. And far below, in the uncharted depths of the alien satellite they call home, hints that they are not alone….

        • Alex Tolley May 13, 2019, 13:30

          LOL. Have you tried pitching this to KSR wannabes? ;)
          Now that I think about it, very few stories have been set on Europa… What about a story from the POV of the Europans – more James Tiptree Jr, maybe even Cordwainer Smith?

          • ljk May 13, 2019, 15:11

            I have indeed considered a story set on Europa from the perspective of the natives. I think it will be rather short…

            The intelligent beings of the alien moon called Europa have existed for eons in peace and harmony with their environment, a massive global ocean of liquid water many miles deep beneath the icy surface of the radiation-shrouded Galilean satellite.

            Then one day an expedition of humans from the planet Earth arrives at their world and encounters the aquatic Europans. These strange bipedal creatures share their entire history using a medium the Europans are eventually able to decipher.

            The Europans subsequently construct a giant black monolith and place it on the surface of their moon, where it broadcasts on a loop that their world is permanently off-limits to any and all talking primates hailing from Sol 3.

            They also launch some fish-type animals from their ocean into orbit around the moon to distract the humans long enough so they can find an even deeper level to hide.

            • Supernaut May 14, 2019, 17:03

              Nicely done!

  • Coacervate Droplet May 10, 2019, 18:15

    Excellent reading, thanks Alex. Regarding those ruddy red pigments that accompany the cracks on Europa, do we know if that is the same organic-rich material called tholins that occurs on many other deep space/ancient objects?

    • Alex Tolley May 11, 2019, 11:55

      It would be interesting if the red material was like the tholins on comets. However, it seems it is more likely irradiated salts. This is why it is so important that we get samples from the plumes and eventually the surface to analyze.

    • Steven May 13, 2019, 21:58

      Regarding the reddish brown lines on Europa, I remember reading an article about the possibility that microbes were responsible.

      Here is the link, a bit dated but still interesting.



  • Robin Datta May 10, 2019, 19:14

    A thoughtful and thought-provoking piece. Thank you, Alex – and our host Paul.

    If biological life as we know it is in consideration, then it may have a fastidiousness at its abiogenesis that precludes its appearance in austere environments. Energy would seem to be a critical issue, varying from too much too close to the host star to too little at a great remove. Whether or not forms of life that are less demanding at abiogenesis could exist, remains among the unknowns. Extending our biology to other parts of the solar system would be in consonance with its growth imperative.

    • Alex Tolley May 11, 2019, 12:04

      Unless one is of the belief that terrestrial life (especially humans) is a cancer, then spreading our life to other worlds (and habitats) is a worthy goal, IMO. If we take a very long view, seeding the galaxy (and the universe?) with terrestrial life should give rise to incredible diversity as life evolves on different worlds in different ways. Our distant descendants could live in a galaxy with a huge diversity of life forms with varying levels of intelligence. Whether they could determine if it meant abiogenesis was common, or it was due to [directed] panspermia would be an interesting question, one that we may face ourselves when we start to obtain samples of life from exoplanets.

  • Michael May 11, 2019, 6:45

    Just wondering if they used an angled surface for impingement and sputtering of ions as surfaces are rarely flat in the atomic world. Angled sputtering yields higher numbers of secondary particles, the surface of Europa is expected to also be very rough with sharp structures.

  • Alex Tolley May 11, 2019, 11:45

    A few points on my thoughts for Europan life:

    1. The ice is rich in oxidants, although there seems to be a range of several orders of magnitude over the production rate, and uncertainly as to the migration to the subsurface ocean.
    2. The depth and volume of the ocean is such that the time to reach saturation could be billions of years.
    3. Venting of hydrogenated gases like Ch4 from the vents partially offsets the excess oxidants.

    Therefore the surface ice is likely much richer in oxidants than the ocean

    1. The rate of oxidant production on Europa is orders of magnitude lower than on Earth. This means that there cannot be a much aerobic organism biomass living in the ocean. It would be a relative desert for aerobes.
    2. Anaerobes could exist around the vents, feeding off the hydrogen emissions. However, the oxidants in the ocean might be so toxic that this confines them to niche environments.
    3. On Earth, aerobic respiration releases 19x more energy than anaerobic .While we very rarely see anaerobic metazoa, it is possible that this is due to the period when metazoa evolved. It is also likely that aerobic organisms would have rapidly outcompeted anaerobic ones.

    Therefore, while it is possible that relatively sessile anaerobic metazoa could exist in Europa, more likely if there is any life it remains unicellular and located around deep ocean vents. For the fiction, I assumed Europa was sterile to obviate any “Prime Directive” or planetary protection issues and to allow a free hand for aerobic terrestrial organisms to be seeded using the rich O2 sources in the ice supplemented by photosynthesis. This is a precursor for a greater “galaxy greening”.

    • Michael May 11, 2019, 17:25

      There should be plenty of nutrients via asteroid impacts scattered on and beneath the surface, maybe little islands of life.

  • Michael T May 12, 2019, 1:43

    Interesting article.

    I’m wondering if life couldn’t arise/survive in pockets of water nearer to the surface and O2 supplies, much like the lake in the last Fiction section.
    For one thing, there would be higher energy supplies due to irradiation, similar to what is postulated for abiogenesis on Earth.

    • Alex Tolley May 12, 2019, 9:27

      The surface is so cold (average 110K) that any pockets of water quickly freeze solid. Any pockets would have to be at depth where the pressure would allow a liquid state to exist. If the base of the ice crust becomes liquid as it is pushed down by the added material on the surface, then any pocket of liquid will move down to the base and melt. Its survival period is therefore at maximum the turnover time of the ice crust, and most probably only a tiny fraction of that time as it forms near the base, is pushed down to the base and merges with the ocean below.

      • Michael T May 12, 2019, 11:48

        Thanks. Yes, fair enough. One does hear people speak of small lakes at intermediate depth in the ice sometimes, which is what I was thinking of.

        I’d given up much hope of Europan fish, nice though the idea is.
        But even extremophilic anearobic bacteria (in the ocean) would be something.


        • Alex Tolley May 12, 2019, 12:53

          While metazoan forms are always interesting, it is the basic molecular machinery that is perhaps more interesting from a fundamental perspective. Europan anaerobes would tell us:
          1. Do they use DNA, some variant, the same genetic code, etc.?
          2. Are the pathways the same as terrestrial ones, or have different solutions been found?
          3. Is there evidence for a local abiogenesis, or are these organisms similar to terrestrial one?
          A host of other questions could also be answered by acquiring living samples of such organisms. Those answers would shed some light on life in the galaxy, answers that cannot be obtained by remote sensing of living exoplanets.

      • Michael May 12, 2019, 12:54

        There is the distinct possibility of magnesium salts been present on the surface and at depth on europa, this salt has ice melting benefit.


  • wdk May 12, 2019, 17:22

    Alex. T.,
    Thank you here as well for a deciphering of the Ionian environment with regards to aerobic biological traces.

    Considering the picture you paint, it just occurs to me that something to be taken into account, if not inherently already, is the nature of Europa in the past. Io’s icy surface loses its features faster than Callisto and Ganymede, but that doesn’t necessarily mean that the surface of Europa hasn’t been entirely capped for a very long time. Billions of years perhaps.

    But whether it’s been aeons or say millions of years since the last liquid surface episode of any consequence, the environment for life could have been a little more inviting. If we take a very early state, the whole Jovian neighborhood could have been a lot hotter with heats of formation, and the water content of all four Galilean moons would have been higher. Maybe even significant fractions of mass. Boil-off might have been high enough to allow atmospheres to reach partial pressures, say on Europa, and there would in effect be partially boiling seas.
    Maybe somewhere in the literature there are already accounts of such or models.

    So that would take us back, so to speak, to Freeman Dyson’s proposition of looking for traces of life amid the geysers. Not recent but remains from another time.

    Maybe a lump or two of coal?

    Best regards,


    • Alex Tolley May 12, 2019, 23:31

      AFAIK, there is no evidence that the environment out at Jupiter was ever much hotter. The orbit is out past the snow line where water was always in a frozen state. If the sun was much hotter during formation, then Ceres would have been a more likely water world once. Jupiter’s mass is well below that needed for a brown dwarf, so that it was not going to produce much more energy to keep its moons liquid. All that is left is radioactive decay heat in the rocky cores of the moons. Is there any model that shows this is sufficient to have ensured the Galilean icy moons were initially liquid? That mechanism might also have applied to other icy moons, e.g. Enceladus and Titan around Saturn, and Pluto and Charon. So the current idea that it is the flexing of Europa’s interior that provides the internal heat to keep its oceans liquid is the dominant source of energy seems to apply throughout its history since formation. Don’t forget that coal beds on Earth only started to form in the Carboniferous period, a mere 360 mya when the forests first appeared. That is less that 10% of of Earth’s history, and required land above sea level. Europa, in contrast, would be a water world with all that implies about evolutionary trajectories even assuming life emerged there in some more clement conditions.

      If you have some knowledge that Europa could have been hotter and possibly relatively ice free, please post a reference.

  • Geoffrey Hillend May 13, 2019, 14:57

    Interesting idea. Oxygen is not life though, and how can one replace photosynthetic organisms which produce oxygen since the aerobic life evolved from them.

    • Alex Tolley May 14, 2019, 0:23

      I’m not quite sure what you are trying to say here. If there was a thin biosphere supported by radiolytic O2 production, then that is the source of O2, rather than photosynthesis. But it would be very low biomass based on production rates.

      But note that the ice itself is O2 rich. Think of it like a solid bubble. It is conceivable that some organisms, even multicellular ones, could live in that ice. It would not support them for very long, so those ecologies might be transient, but also potentially much closer to the surface.

      So consider this as a speculative life cycle. The eggs are only hatchable in the ocean where the O2 is low but above some level, but liquid water exists. Once hatched the organism burrows into the base of the ice crust where the O2 supply is richer and predates on other organisms with a chemotroph at the base of the food chain. When teh O2 level falls in its location, it lays its eggs which lay dormant until they migrate to the ocean and eventually hatch if the oceanic O2 is at a certain level. Given O2 depletion by aerobes in the ocean, it may take up to millions of years for conditions to become suitable for the eggs to hatch and the life cycle and ecosystem in the ice crust to become active again. Life adapting to the O2 rich ice crust, but low O2 production rates at the surface.

  • Gary Wilson May 13, 2019, 15:00

    Does anyone know if the Europa Clipper is still confirmed as a future mission? It looks very interesting and I hope it hasn’t been cancelled as missions to Jupiter take so long to come to fruition based on travel time alone.

  • Geoffrey Hillend May 13, 2019, 15:09

    In other words, one still needs sun light, but If we can find life that evolved without it, I will certainly be surprised, so it is worthwhile to look there.

  • Spaceman May 13, 2019, 15:27

    Hi Alex,

    Great piece! Under the scenario that no independently evolved life is detected elsewhere in the solar system after, say, centuries more of thorough exploration, what type of bound would this perhaps disappointing revelation place on the Fl term (abiogenesis) in the Drake equation? What would no evidence of a second abiogenesis event in our solar system indicate about our place in the Universe in your estimation?

    • Alex Tolley May 13, 2019, 23:04


      That is the $64K question. I tend to be conservative regarding life elsewhere in teh solar system. I still think early Mars was the best alternative site for abiogenesis.

      I think if we found life in different environments on different worlds in our system, and it was shown to be from separate geneses, then I think that would bode well for increasing that parameter. However, if life is confined to Earth, then we are still stuck with whether Earth life is unique, rare, or common on Earth-analog worlds. On this matter, I think we will have our answer relatively soon [a few decades], based on telescope observations of exoplanets. Data, rather than speculation is what we need.

      Having said that, if we find life from separate abiogenesis on different worlds in our system, whether that abiogenesis was local or the result of panspermia, it would be a huge boost for [astro]biologists to study very different basic biologies. It would inform us of a wider range of possibilities as well as the more prosaic commercial possibilities.

      • Spaceman May 14, 2019, 14:33

        Thanks for answering my question. To follow up: When it comes to the remote detection of life on earth-like exoplanets in the next few decades, how certain can we be any life detected would have come from an independent genesis vs panspermia from our own planet? In other words, how likely is interstellar panspermia to stock the galaxy’s earth-analog planets with life from our planet?

  • wdk May 13, 2019, 16:52

    Alex T.,

    Here is what I have been able to find so far: an abstract. I don’t have immediate access to the specific details we would be interested in, but perhaps there is an indication early thermal conditions that would
    give better resolution. E.g., varied convective history vs. a fixed “upwelling”. A couple of other paragraphs in the paper leave open a door, perhaps, but it would be better to get the full story.

    Volume 218, Issue 2, April 2012, Pages 1006-1019
    “A whole-moon thermal history model of Europa: Impact of hydrothermal circulation and salt transport”


    A whole-moon numerical model of Europa is developed to simulate its thermal history. The thermal evolution covers three phases: (i) an initial, roughly 0.5 Gyr-long period of radiogenic heating and differentiation, (ii) a long period from 0.5 Gyr to 4 Gyr with continuing radiogenic heating but no tidal dissipative heating (TDH), and (iii) a final period covering the last 0.5 Gyr until the present, during which TDH is active. Hydrothermal plumes develop after the initial period of heating and differentiation and transport heat and salt from Europa’s silicate mantle to its ice shell. We find that, even without TDH, vigorous hydrothermal convection in the rocky mantle can sustain flow in an ocean layer throughout Europa’s history. When TDH becomes active, the ice shell melts quickly to a thickness of about 20 km, leaving an ocean 80 km or more deep. Parameterized convection in the ice shell is non-uniform spatially, changes over time, and is tied to the deeper ocean–mantle dynamics. We also find that the dynamics are affected by salt concentrations. An initially non-uniform salt distribution retards plume penetration, but is homogenized over time by turbulent diffusion and time-dependent flow driven by initial thermal gradients. After homogenization, the uniformly distributed salt concentrations are no longer a major factor in controlling plume transport. Salt transport leads to the formation of a heterogeneous brine layer and salt inclusions at the bottom of the ice shell; the presence of salt in the ice shell could strongly influence convection in that layer.

    ► Hydrothermal convection alone can maintain an ocean in Europa. ► Hydrothermal convection plays a major role in Europa’s thermal history. ► Hydrothermal plumes transport heat and salt from Europa’s mantle to its ice shell. ► Europa’s ocean is likely of uniform salinity due to turbulent mixing.

    • Robin Datta May 13, 2019, 22:08
      • wdk May 14, 2019, 14:44


        It worked. Thank you!


    • Alex Tolley May 13, 2019, 22:51

      I skimmed the paper. Reagrding the eraly thermal history, they say this:

      Hydrothermal convection in phase II exhibits two stages, an initial short burst of energy transport, followed by a weaker,
      quasi-equilibrium stage. During the initial burst, a warm ocean
      forms for a period of a few Myr, during which the ice layer thins
      dramatically to only a few kms. During the second, long-lasting
      stage, the ocean cools to a few degrees below 0 C (possible
      because of dissolved salt), and the ice shell slowly thickens to
      about 80 km.

      From the graphs, I see no stage in its history where Europa has a surface temperature > 0C. The various heating stages seem to all incur an ice shell, just with varying thickness.

      The authors also say this, although this just supports the current interest about life in the Europan ocean:

      Hydrothermal circulation in the outer mantle could create a
      haven for primitive life, providing thermal energy and a rich
      chemical environment, similar to mid-ocean ridge systems on
      Earth (Irwin and Schulze-Makuch, 2003).

  • Rob Flores May 13, 2019, 19:07

    Animal life on Earth is incredibly energy intensive.
    I don’t see anything multicelluar on Europa or Titan or Enceladous
    beyond colonies, that could develop in these worlds.

    I should posit that this assumes that single celled life was slow
    to arise and sluggish to evolve on the icy worlds.
    WHY WOULD THAT BE. some reasons below

    As mentioned before complex unicellular life requires nutrients,
    which dont arrive very often to these ocean worlds.

    Cold settings even on Earth grow slow. and that implies slow evolution

    Very Limited mutation rates due to the thickness of ice and water collum. Thus the source events needed for evoultion are scarce.

    Maybe a couple of Billion years more are required before these worlds
    create animal life.

    • Alex Tolley May 13, 2019, 22:32


      I think you are implying that mutations need external events to drive it. However, I would suggest that for bacteria, it is copy errors and natural selection that drive evolution. Copy errors in prokaryotes are about 10x higher than in eukaryotes. Replication and competition for resources then operates via Darwinian selection.

      I do think that the cold environment that reduces growth and replication rates is the more important factor you mention to slow evolution.

  • wdk May 15, 2019, 0:51

    From years back I do remember some pictures depicting a liquid surface with many icebergs on Europa with Jupiter on the horizon. It might have been simply a fantasy inserted into an astronomy article. But given that Jupiter would have been hotter to some degree (sic) eons ago, its satellites would have been too. We can add the temperatures from various heat sources in this case and see what we come up with.

    In these comments we talked about early thermal history of Europa up above and came up with a backward extrapolation that its temperature likely was not above the ice temperature based on internal heat sources.

    And it was noted that Jupiter was not a brown dwarf – so that it did not have a period where deuterium powered fusion illuminated. Still it had a heat of formation – and it still radiates about twice the amount that it receives from the sun. We can assume that it radiated more heat than it does now in that same distant past. The question is, how much?
    Looking at the literature that has a lot of uncertainties too. The amount of internal heat initially available is easier to discern than the curve over time describing its escape. Jupiter is simply a complicated object, as data from Galileo and Juno attest.

    Europa’s orbital radius from Jupiter is about 670,900 km. Jupiter’s radius itself is about 71,492 km ( equatorial, less for polar). So you could say that Europa is roughly ten jovian radii away. Now let’s consider Jupiter as a radiating object:

    Based on black body calculations for the sun: with a surface temperature of 5800 deg Kelvin, its control volume temperature ratio at Earth’s orbit to this surface temperature is about 0.0684 or the square root of its radius to Earth’s orbital radius.

    That would be about 400 degrees Kelvin out our way. We spend our days around 300 Kelvin due to half a sphere irradiated and then its reflectance properties – but let’s just use 400 degree criterion for Europa at 10 radii away from Jupiter. How hot would Jupiter’s effective surface temperature have to be? About square root of 10 times hotter than 400 Kelvin, about 1265 degrees K.

    It could been a bigger ball in the old days and have shrunk since then.
    Say it were twice as wide; then 895 degrees K would have done it. But there are limits to how wide we can make Jupiter before it engulfs or rips up existing moons.

    Well the sun does not cause Jupiter’s current temperature entirely. It is said that Jupiter radiates about twice the energy it receives. Roughly, that would indicate an increase of effective temperature by a factor of 1.2 based on the quartic root. So it evidently has considerable remaining internal heat. As an object 5.2 times as far from the sun as Earth, that solar control volume radius temperature ought to be about 174 degrees K. If 3/4 that temperature is still a good guess for equilibrium, that would be about 130 degrees K. I believe that some of the Galilean satellites are about that, if not Jupiter. At the same radius, the temperature ratio is 7.23, constituting a heat flux 2730 times greater. Could that have been sustained for long? No. But would such an episode have been significant in Europa’s geological history?

    A remaining area to look for possible increased temperature would be reflectance from other objects surrounding Europa and Jupiter. There must have been a lot of dust and debris in the old days.

    But in the final analysis everything would have to work very hard and row toward the same objective to get any aerobic life before the temporal oasis shut down.

  • Alex Tolley May 15, 2019, 14:23

    Some nice calculations. What I question is this. If the local Jupiter radiation emissions were that high, wouldn’t that make the accretion of Europa’s water more difficult as it might well dissipate rather than aggregate as ice? If Jupiter was larger, would the tidal flexing energy be greater and sufficient to keep the surface relatively ice free?

    It is interesting to speculate an early Europa as being more hospitable for life, like an early Mars. Both planets have lost their surface water. Both have large amounts of ice – an ice shell on Europa, glaciers below the surface on Mars. Europa maintains liquid water due to internal heat and tidal flexing, and possibly Mars still has enough residual heat, possibly in pockets below volcanoes, to maintain pockets of water. I hope the issue of Mars’ internal heat profile will be determined by the InSight lander currently in Mars. This may help answer that question.

  • wdk May 15, 2019, 20:48

    Good question or objection.

    Maybe I set the equilibrium temperature a little too high, considering that the sun itself has probably increased in luminosity by about a half magnitude. Don’t have a quick answer on that one, but suggests something in the 300 degree kelvin range. 350? 375? vs. the 400 K used.

    Another consideration is comparing the four Galilean satellites which provide forensic evidence themselves. I question whether all successive levels of dessication (Io, Europa, Ganymede, Callisto…) is related to
    tidal dissipation). Looking at our own case, here we have a moon that came from somewhere, usually attributed to an early breakoff from the Earth. The moon has near complete absence of volatiles vs. the Earth and the Earth sat where wrt the snow line. Moreover, moon light in the early days for Earth must have been pretty hot itself…

    But if I pursue all these various lines of argument, I’m probably just stirring up dust rather than addressing an issue of whether aerobic life could crop up on Europa. I don’t have a definite mechanism; just possible factors that might explain ex post facto any LGMs we might someday be able to find.

  • Andrew R Brown May 16, 2019, 16:13

    Problem I have with potential life in any subsurface water on Earopa, is the contant build up of acids and salts. That water will be toxic, with perhaps a PH similar to battery acid.

    If there are indeed volcanoes on the ocen floor, the sulpuric acid buildup will have been tremendous over the past 4.56 GY. Also the neighbouring, active volcanic moon Io, dumps sulphur and sulphuric acid on the surface.

    We will not know for sure until we go there and look with a crybot, etc. The upcoming ESA JUICE and NASA Europa Clipper missions will reveal vastly more for sure in the meantime.


  • wdk May 16, 2019, 22:27

    Another thought regarding the varying volatile or water content of each Galilean satellite. At a period when there was a “circum jovian” disk from which these objects probably formed, there were probably a lot of spare parts remaining. In addition, Jupiter was supposedly instrumental in sweeping the solar system clear of debris as well, according to some observers, making this world safe for life and democracy. So given that there are two populations of debris and the Galilean satellites are tightly bound, each satellite of the satellite has a characteristic collision energy associated with encounters. If you have co orbiting objects with inclination differences of, say, ten degrees, the hits are hardest at Io, then in descending order, Europa, Ganymede, Callisto. The same with in-falling matter going around the sun. The energy of impacts among the Galilean satellites are very high. Such impacts could be significant contributors to the initial internal heat in addition to that associated with coalescence out of a model circum-Jupiter disk. Europa orbits Jupiter at about 16.9 km/sec. Jupiter around the sun appears to be about 13 km/sec.

    But a comet crossing the path of Galilean satellites would be a terrific impact. Lots of Chicxulub like events – but getting worse as they migrate inward from satellite to satellite with the same unit mass.

    • Rob Flores May 17, 2019, 18:56

      If that is the case, then the icy crust of Europa maybe quite thin. maybe only 3-5 Km. My SWAG below

      There must have been a liquid ocean for quite a while on Europa early
      in its history. The heat from impacts would take a lot of time to dissipate because it would have an atmosphere of Water Vapor and H2/HE remants.

      Eventually a balance occurred wherein a very thin atmosphere allowed
      colder water layer on top of the ocean, This transitioned into a icy
      crust. Its thickness determined by stored heat from impacts plus
      vulcanism from the ocean floor, and lastly while Europa became resonant and tidaly locked more heat was added to the heat budget.

      I think its a fair guess that Europa’s Icy surface has been getting deeper in the last billion year.

  • Bruce D. Mayfield May 18, 2019, 2:19

    Perhaps I missed something but in all this discussion re internal heat budgets in the Jovian system I’ve seen no mention of heat given off by radioactive elements. Both short lived and long lived isotopes have and continue to have a role to play in providing an additional heat source inside rocky bodies. Sure these are largely gas (Jupiter) and water (excepting Io) worlds but they all also have rocky cores. Therefore they could have had a somewhat longer time with ice free surface waters than the above conversation implies.

    • Alex Tolley May 18, 2019, 7:15

      The reference cited by wdk upthread does include radiogenic heating of Europa in their model. It still is insufficient to increase surface temperatures above 0C for any time since Europa’s formation. Their model may be incomplete or even wrong, but it does imply Europa was always in a “snowball state” with no free surface water except for temporary cracks in the icy surface.

    • Bruce D. Mayfield May 18, 2019, 11:24

      My apologies to Alex Tolley, for you did mention radiogenic heating in one of your comments, even requesting input if anyone had anything to add about it. From what I can find the current consensus is that Europa’s heat budget could have an about 1% radiogenic contribution, with the vast remainder from tidal flexing. But I guess we need to probe that ocean to find out;) I would expect however that in the very distant past closer to the time of Europa’s formation it’s heat budget would have been much higher than it is today. It would have had leftover heat from gravitational contraction and impacts, even more tidal flexing from non synchronous rotation, and a much greater contribution from radiologic sources. Some of all that Mg no doubt came from the decay of 26Al.

  • ljk May 21, 2019, 13:06

    Meanwhile, Europa’s neighbor is undergoing a significant change…


  • ljk May 29, 2019, 14:17

    Is Europa Clipper funding in jeopardy?


    To quote:

    “Despite robust early-stage funding, a series of significant developmental and personnel resource challenges place the Clipper’s current mission cost estimates and planned 2023 target launch at risk. Specifically, NASA’s aggressive development schedule, a stringent conflict of interest process during instrument selection, and an insufficient evaluation of cost and schedule estimates has increased project integration challenges and led the Agency to accept instrument cost proposals subsequently found to be far too optimistic. Moreover, Clipper has had to compete with at least four other major JPL-managed projects for critical personnel resources, causing concern that the project may not have a sufficient workforce with the required skills at critical periods in its development cycle. … In addition, although Congress directed NASA to use the SLS to launch the Clipper, it is unlikely to be available by the congressionally mandated 2023 date and therefore the Agency continues to maintain spacecraft capabilities to accommodate both the SLS and two commercial launch vehicles, the Delta IV Heavy and Falcon Heavy. … We also believe that requiring the Agency to pursue a Lander mission at the same time it is developing the Clipper mission is inconsistent with the NRC’s recommended science exploration priorities.”

  • ljk June 10, 2019, 14:48

    June 10, 2019

    New study dramatically narrows the search for advanced life in the universe

    by University of California – Riverside




    The team’s work, published today in The Astrophysical Journal, shows that accounting for predicted levels of certain toxic gases narrows the safe zone for complex life by at least half—and in some instances eliminates it altogether.




    To quote:



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