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A Role for Comets in Europa’s Ocean?

The role comets may play in the formation of life seems to be much in the news these days. Following our look at interstellar comets as a possibly deliberate way to spread life in the cosmos, I ran across a paper from Evan Carnahan (University of Texas at Austin) and colleagues (at JPL, Williams College as well as UT-Austin) that studies the surface of Europa with an eye toward explaining how impact features may evolve.

Craters could be cometary in origin and need not necessarily penetrate completely through the ice, for the team’s simulations of ice deformation show drainage into the ocean below from much smaller events. Here comets as well as asteroids come into play as impactors, their role being not as carriers of life per se but as mechanisms for mixing already existing materials from the surface into the ocean.

Image: Tyre, a large impact crater on Europa. Credit: NASA/JPL/DLR.

That, of course, gets the attention, for getting surface oxidants produced by solar irradiation through the ice has been a challenge to the idea of a fecund Europan ocean. The matter has been studied before, with observational evidence for processes like subduction, where an ice surface moves below an adjacent sheet, and features that could be interpreted as brine drainage, where melting occurs near the surface, although this requires an energy source that has not yet been determined. But many craters show features suggestive of frozen meltwater and post-impact movement of meltwater beneath the crater.

The Carnahan paper notes, too, how many previous studies have been done on impacts that penetrate the ice shell and directly reach the ocean, which would move astrobiologically interesting materials into it, but while impacts would have been common in the history of the icy moon, the bulk of these may not have been penetrating. Much depends upon the thickness of the ice, and on that score we await data from future missions like Europa Clipper and JUICE to probe more deeply. Current thinking seems to be coalescing around the idea that the ice is tens of kilometers thick.

The authors believe that impacts need not fully penetrate the ice to have interesting effects. Such impacts should produce melt chambers, some of them of considerable size, allowing heated meltwater to then sink through the ice remaining below them. This meltwater mechanism copes with a thick ice shell and does not require that it actually be penetrated to mix surface ingredients with the water below. The observational evidence can support this, not only on Europa but elsewhere. Implicit in the discussion is the idea that the ice surrounding a melt chamber is not rigid. From the paper:

…impacts that generate melt chambers also significantly warm and soften the surrounding ice making it susceptible to viscous deformation. Furthermore, although not explored here, the impact may generate fractures that allow for transport of melts short distances away from the crater melt pond… Importantly, the crater record of icy moons includes craters of varying complexities (Schenk, 2002; Turtle & Pierazzo, 2001) with anomalous features such as collapsed pits, domes, and central Massifs that imply post-impact modifications (Bray et al., 2012; Elder et al., 2012; Korycansky, 2020; Moore et al., 2017; Silber & Johnson, 2017; Steinbrügge et al., 2020). These observed crater features suggest that both impact structures and the generated melts experience significant post-impact evolution that has so far received little attention.

Image: An artist’s concept of a comet or asteroid impact on Jupiter’s moon Europa. Credit: NASA/JPL-Caltech.

The method here is to deploy mathematical simulations to study the evolution of these melt chambers on Europa. The term is ‘foundering,’ which is the movement of meltwater through the ice as it potentially transports oxidants below. If surface ice can be transferred into the ocean in a sustained way, and thus not just through massive impacts but through a range of smaller ones, the chances of developing interesting biology below only increase. The work also implies that Europa’s so-called ‘chaos’ terrain, which some have explained as the result of meltwater near the surface, may have other origins, for in this model most of the meltwater does not remain near the surface. Says Carnahan: “We’re cautioning against the idea that you could maintain very large volumes of melt in the shallow subsurface without it sinking.”

The researchers modeled comet and asteroid impacts using a shock-physics cratering simulation and massaged the output by factoring in both the sinking of dense meltwater and its refreezing within the ice shell. The modeling required analysis of the energies involved as well as the deformation of the surface ice after impact. UT’s Carnahan developed the ice shell convection model that the authors extended to match the geometry of surface impact simulations and subsequent changes in the ice.

The conclusions are striking:

Our simulations show that impacts that generate significant melt chambers lead to substantial post-impact viscous deformation due to the foundering of the impact melts. If the transient cavity depth of the impact exceeds half the ice shell thickness the impact melt drains into the underlying ocean and forms a continuous surface-to-ocean porous column. Foundering of impact melts leads to mixing within the ice shell and the transfer of melt volumes on the order of tens of cubic kilometers from the surface of Europa to the ocean.

Image: A computer-generated simulation of the post-impact melt chamber of Manannan Crater, an impact crater on Europa. The simulation shows the melt water sinking to the ocean within several hundred years after impact. Credit: Carnahan et al.

So we have a way to get surface materials through to the Europan ocean, a method that because it does not require large impacts, has likely been widespread throughout Europa’s history. It’s interesting to speculate on how this process could leave evidence beyond what we’ve already uncovered in the craters visible on the surface and what corroboration in support of the analysis Europa Clipper and JUICE may be able to provide. Other icy worlds come to mind here as well, with the authors mentioning Titan as a place where even an exceedingly thick ice shell may still be susceptible to exchanging material with the surface.

Given how little we know about abiogenesis, it’s conceivable not only that life might develop under Europan ice, but that icy moons elsewhere in the Solar System may hold far more life in the aggregate than exists in what we view as the habitable zone. If that is the case, then the argument that life is ubiquitous in the universe receives strong support, but it will take a lot of hard exploration to find out, a process of discovery whose next steps via Europa Clipper and JUICE will represent only a beginning.

The paper is Carnahan et al., “Surface-To-Ocean Exchange by the Sinking of Impact Generated Melt Chambers on Europa,” Geophysical Research Letters Vol. 49, Issue 24 (28 December 2022). Full text.

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{ 11 comments… add one }
  • Michael Fidler December 21, 2022, 10:54

    With Jupiter being the hotspot in the solar system for interstellar comets, impacts on Europa would put it in the best spot to receive panspermia from beyond. Not only that but a somewhat soft landing may help deliver the package intact plus a warm interior ocean from internal heating! Now octopuses with giant infrared eyes could see the middle Europa world from the illuminating inner core…

    Shades of “EUROPA REPORT”

  • wdk December 21, 2022, 12:32

    This Geophysical Research site is very research and reader friendly, I should note. Cross reference articles on the Europa convection topic are immediately available to the visiting reader, including one tying in Ganymede into the analysis as well. And the article in question includes a feature: Plain Language Summary.

    Considering the evidence ( vs. fact?) that outer planet moons have high H2O ice compositions, whether considered as enriched with solutions or pure as bottled, it must be a conundrum distinguishing which water arrived from where and when. At the very least, the orbital velocity of Europa is an order of magnitude larger than the Moon’s around Earth and there has likely been a lot of debris and unstable minor moons careening around Jupiter – which could come to an end as spectacularly as a comet arriving from the Oort Cloud or Kuiper Belt. And then if the dearly departed jovian minor moon originated in the formative ring, then there would be the issue of distinguishing its composition from that of the outer solar system contributors. So, it would seem that the article demonstrates fluid mechanics for delivery into a subsurface ocean (SSO), but there remains an issue about where the material would have come from.

    Now given a possible means of integrating into the SSO and our own interest in where the material came from ( comets or other small jovian moons), I did a quick check to see if the article gave a rationale for beyond the notion of organic enrichment in the depths below. At least in the Plain Language Summary, there was no discussion about source material. So I come away with the idea that among possible sources, outer solar system regions out to the Oort Cloud could be considered as contributors.

    Even as early as the lunar landings, if I recall correctly, the scientific community was lobbying ardently for attention to comets to look for precursors to life, maybe even at the sacrifice of the Apollo program.

    These days, as we ourselves discuss how to retrace where or how life originated in the solar system, trying to trace such leads back as Europa, comets and jovian moons, I can better appreciate their arguments.

    • Michael Fidler December 21, 2022, 22:50

      Here is some info that may clear up what is going on, I also found something that made my comment that Jupiter was the pivot point for interstellar comets was wrong but then I found that was also wrong. The long period comets from the Oort cloud and interstellar space comets with open orbits come in from all inclinations (angles) but Kuiper belt comets come around near the plan of the solar system. This article about Jupiter-family comets explains why they should be Kuiper Belt objects.
      https://astronomy.swin.edu.au/cosmos/J/Jupiter-family+comets

      These would be mostly left over from the original solar nebula but the odd orbits of objects called Centaurs that orbit beyond Jupiter and very often become Jupiter-family comets. Researchers calculated that 19 of these objects probably have origins outside our solar system. These objects may also impact on Jupiter and Europa. So interstellar comets could bring in life from outside the solar system via Jupiter and the Centaurs.
      High-Inclination Centaur Asteroids Came from Interstellar Space.
      https://earthsky.org/space/19-interstellar-asteroids-in-our-solar-system-centaurs/

      Here is news of a recent very active Centaur comet that may become short period Jupiter Family comets.

      Massive eruption from icy volcanic comet detected in solar system.
      https://www.space.com/comet-29p-erupts-cryovolcanic-ice

      • wdk December 25, 2022, 13:26

        Hello, MF.
        Season’s greetings.

        Your note took some time to ponder. Glad you wrote. For one thing, thought I would have to dust off the history of Centaurs. But despite the nomenclature that blends with others from antiquity, I discovered that it is very recent designation, starting with Kowal’s object of the late 1970s, turning up between Saturn and Uranus. Beside being an object sized eventually to about 210 kilometer diameter, this name or situation introduced the idea of half asteroid – half comet as in half horse – half man. That being the case, some Main Belt asteroids ( e.g., Ceres) are sometime accused being Centaurs as well. It might be arguable, but with the Ceres eccentricity and inclination being very low and semi-major axis near 3.2 AU, the argument likely rests mostly on its volatile reserves and hydrous, salty volcanism.

        This does bring up the issue of when is a comet a comet. Since this is an ancient designation, we could say that it was established before the invention of telescopes when a celestial object developed a tail in the sky, trailing away from the sun. The longer the better.

        But at this point we can see some inadequacies. E.g., are Kuioper Belt objects all comets waiting to be ignited? And how about Oort Cloud objects? Or minor satellites of Outer Planets? Or even Triton?

        It appears to me that categorization nowadays ought to be akin to that used with museum objects based on traceable history:
        :
        (From Wikipedia)Provenance: … chronology of the ownership, custody or location of an historical object. The term was originally mostly used in relation to works of art but is now used in similar senses in a wide range of fields, including archaeology, paleontology, archives, manuscripts, printed books, the circular economy [?], and science and computing….

        Whether absence of astronomy in the above list is simply an omission or not, it looks like it will be needed to account for where or how volatiles reached the Earth from the outer solar system or beyond.

        As to Ceres vs. less suspect Centaurs, the volcanism of the asteroid is distinct from the volcanism of the moon Triton ( H2O vs. N2 and other cryogenic compounds). That might be another way to distinguish flare-ups we generally equate with comets. I suspect that comet 29P above is more likely a cryogenic eruption.
        Yet in the case of Enceladus, the eruption is more like that we would attribute to Ceres ( water vapor). Now for the case of Enceladus, with respect to Saturn, it is about double the distance from the outer edge of its visible ring system. My suspicion is that it is debris from processes long going on around Saturn rather than a captured comet.

        We got into this discussion talking about the role of comets delivering life precursors to enrich Europa’s oceans. I guess my point is that it is hard to pigeonhole what a comet is even in these days or where they came from.

      • Michael Fidler December 25, 2022, 21:43

        I’m wondering what an icey Centauri comet/asteroid impact on Europa would be like? Would it be a much milder and softer impact since a large part of it would be volatile organic icey compounds.

    • Mike Serfas December 26, 2022, 11:31

      Here’s something recent I found in ArXiv, describing many different means of studying isotope ratios. https://arxiv.org/pdf/2203.10863.pdf I’ve scarcely scratched the surface of what they have in that, but they look at things like the difference in the ratio of oxygen isotopes in the CO2 versus the H2O that has condensed on dust grains, and cross-reference by fine spectroscopic details of ALMA images of protoplanetary disks. As usual I’m impressed – nobody beats astronomers at extracting every scrap of data from limited observations. I don’t know how they’ll tackle these sites on Europa, but let’s expect the unexpected. :)

  • Alex Tolley December 21, 2022, 16:19

    Here comets as well as asteroids come into play as impactors, their role being not as carriers of life per se but as mechanisms for mixing already existing materials from the surface into the ocean.That, of course, gets the attention, for getting surface oxidants produced by solar irradiation through the ice has been a challenge to the idea of a fecund Europan ocean.

    The fecundity is perhaps more a question of carbon availability over the very long term. In the short term, the implication is that peroxide is the source of oxygen for aerobic organisms. However, if the extant organisms are anaerobic, e.g. methanogens, then the oxygen will be poisonous to them, rather than supportive. Therefore comets forcing surface oxidants in the subsurface ocean might sterilize the anaerobes.

    But let us suppose that aerobes are present and can use that oxygen drawn down from the surface, the useful free O2 is both limited by the rate of formation and the sinks. As the formation is from Jupiter’s radiation emissions, the rate of formation is quite low, and certainly much lower than the production by terrestrial photosynthesis and probably not higher than the UV photolysis of water on the paleo Earth. During that period, any O2 was combined with iron (Fe 2+) to oxidize it to Fe 3+, with the record suggesting that the O2 in the oceans and atmosphere was very low.
    Perhaps the sinks have been fully saturated on Europa by now, but the rate of oxidant production is still so low that it will not support much aerobic metabolism, with no other source to increase it.

    If the biomass is anaerobic, then a more valuable commentary payload is the CO2, CO, and CH4. Seeping down into the oceans this will provide energy for chemotrophs, whether methanogens using the CO2 and H20 in any hot rocky vents, or chemotrophs feeding on the Ch4 if there is enough oxidant, whether O2 or S to oxidize the carbon. Unless there is a sink for the carbon, this should accumulate in the ocean, providing for a slowly increasing biomass should there be any extant life.

    My guess (and it is only a guess) would be that any Europan life would likely be anaerobic, single-cell, or communal, prokaryote, with the total biomass dependent on the accumulated carbon from Europa and comet impacts, but with a slow metabolism as seen in terrestrial ocean sediment bacteria.

  • Christian G December 22, 2022, 17:19

    Combining this with the previous article gives us the vision of a mission tunneling through Europa’s ice, only to discover a biosphere identifiably descended from Earth life.

    As the comments for that article pointed out, the viability of comets for that kind of panspermia is still up in the air. But I admit I find it compelling, to imagine that given sufficient time, Earth organisms might colonize all the viable environments in the Solar System.

  • Michael December 23, 2022, 9:20

    Interesting though although the impact may sterilise itself and the surrounding area all the rain of particles would form a very nice soft bed of snow for other life bearing particles to possibly land on over a very large area.

  • ljk December 28, 2022, 11:20

    https://arxiv.org/abs/2212.08947

    [Submitted on 17 Dec 2022]

    Sulfur Ion Implantations Into Condensed CO2: Implications for Europa

    D. V. Mifsud, Z. Kaňuchová, P. Herczku, Z. Juhász, S. T. S. Kovács, G. Lakatos, K. K. Rahul, R. Rácz, B. Sulik, S. Biri, I. Rajta, I. Vajda, S. Ioppolo, R. W. McCullough, N. J. Mason

    The ubiquity of sulfur ions within the Jovian magnetosphere has led to suggestions that the implantation of these ions into the surface of Europa may lead to the formation of SO2. However, previous studies on the implantation of sulfur ions into H2O ice (the dominant species on the Europan surface) have failed to detect SO2 formation. Other studies concerned with similar implantations into CO2 ice, which is also known to exist on Europa, have offered seemingly conflicting results.

    In this letter, we describe the results of a study on the implantation of 290 keV S+ ions into condensed CO2 at 20 and 70 K. Our results demonstrate that SO2 is observed after implantation at 20 K, but not at the Europa-relevant temperature of 70 K. We conclude that this process is likely not a reasonable mechanism for SO2 formation on Europa, and that other mechanisms should be explored instead.

    Comments: Published in Geophysical Research Letters

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Space Physics (physics.space-ph)

    Cite as: arXiv:2212.08947 [astro-ph.EP]
    (or arXiv:2212.08947v1 [astro-ph.EP] for this version)

    https://doi.org/10.48550/arXiv.2212.08947

    Journal reference: Geophys. Res. Lett., 2022, 49(24), e2022GL100698

    Related DOI:
    https://doi.org/10.1029/2022GL100698

    Submission history
    From: Duncan V. Mifsud [view email]
    [v1] Sat, 17 Dec 2022 20:38:11 UTC (472 KB)

    https://arxiv.org/ftp/arxiv/papers/2212/2212.08947.pdf

  • ljk December 28, 2022, 11:39

    https://arxiv.org/abs/2206.15325

    [Submitted on 30 Jun 2022]

    Different ice shell geometries on Europa and Enceladus due to their different sizes: impacts of ocean heat transport

    Wanying Kang

    On icy worlds, the ice shell and subsurface ocean form a coupled system — heat and salinity flux from the ice shell induced by the ice thickness gradient drives circulation in the ocean, and in turn, the heat transport by ocean circulation shapes the ice shell.

    Therefore, understanding the dependence of the efficiency of ocean heat transport (OHT) on orbital parameters may allow us to predict the ice shell geometry before direct observation is possible, providing useful information for mission design.

    Inspired by previous works on baroclinic eddies, I first derive scaling laws for the OHT on icy moons, driven by ice topography, and then verify them against high resolution 3D numerical simulations. Using the scaling laws, I am then able to make predictions for the equilibrium ice thickness variation knowing that the ice shell should be close to heat balance.

    Ice shell on small icy moons (e.g., Enceladus) may develop strong thickness variations between the equator and pole driven by the polar-amplified tidal dissipation in the ice, to the contrary, ice shell on large icy moons (e.g., Europa, Ganymede, Callisto etc.) tends to be flat due to the smoothing effects of the efficient OHT.

    These predictions are manifested by the different ice evolution pathways simulated for Enceladus and Europa, considering the ice freezing/melting induced by ice dissipation, conductive heat loss and OHT as well as the mass redistribution by ice flow.

    Comments: arXiv admin note: substantial text overlap with arXiv:2203.16625

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:2206.15325 [astro-ph.EP]
    (or arXiv:2206.15325v1 [astro-ph.EP] for this version)

    https://doi.org/10.48550/arXiv.2206.15325

    Related DOI:
    https://doi.org/10.3847/1538-4357/ac779c

    Submission history
    From: Wanying Kang [view email]
    [v1] Thu, 30 Jun 2022 14:56:16 UTC (10,432 KB)

    https://arxiv.org/pdf/2206.15325.pdf

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