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Europa’s Double Ridges: Implications for a Habitable Ocean

I’m always interested in studies that cut across conventional boundaries, capturing new insights by applying data from what had appeared, at first glance, to be unrelated disciplines. Thus the news that the ice shell of Europa may turn out to be far more dynamic than we have previously considered is interesting in itself, given the implications for life in the Jovian moon’s ocean, but also compelling because it draws on a study that focused on Greenland and originally sought to measure climate change.

The background here is that the Galileo mission that gave us our best views of Europa’s surface so far showed us that there are ‘double ridges’ on the moon. In fact, these ridge pairs flanked by a trough running between them are among the most common landforms on a surface packed with troughs, bands and chaos terrain. The researchers, led by Stanford PhD student Riley Culberg, found them oddly familiar. Culberg, whose field is electrical engineering (that multidisciplinary effect again) found an analog in a similar double ridge in Greenland, which had turned up in ice-penetrating radar data.

Image: This is Figure 1 from the paper. Caption: a Europan double ridge in a panchromatic image from the Galileo mission (image PIA00589). The ground sample distance is 20 m/pixel. b Greenland double ridge in an orthorectified panchromatic image from the WorldView-3 satellite taken in July 2018 (© 2018, Maxar). The ground sample distance is ~0.31 m/pixel. Signatures of flexure are visible along the ridge flanks, consistent with previous models for double ridges underlain by shallow sills. Credit: Culberg et al.

The feature in Greenland’s northwestern ice sheet has an ‘M’-shaped crest, possibly a version in miniature of the double ridges we see on Europa. The climate change work used airborne instrumentation producing topographical and ice-penetrating radar data via NASA’s Operation IceBridge, which studies the behavior of polar ice sheets over time and their contribution to sea level rise. Where this gets particularly interesting is that flowing ice sheets produce such things as lakes beneath glaciers, drainage conduits and surface melt ponds. Figuring out how and when these occur becomes a necessary part of working with the dynamics of ice sheets.

The mechanism in play, analyzed in the paper, involves ice fracturing around a pocket of pressurized liquid water that was refreezing inside the ice sheet, creating the distinctive twin peak shape. Culberg notes that the link between Greenland and Europa came as a surprise:

“We were working on something totally different related to climate change and its impact on the surface of Greenland when we saw these tiny double ridges – and we were able to see the ridges go from ‘not formed’ to ‘formed… In Greenland, this double ridge formed in a place where water from surface lakes and streams frequently drains into the near-surface and refreezes. One way that similar shallow water pockets could form on Europa might be through water from the subsurface ocean being forced up into the ice shell through fractures – and that would suggest there could be a reasonable amount of exchange happening inside of the ice shell.”

Image: This artist’s conception shows how double ridges on the surface of Jupiter’s moon Europa may form over shallow, refreezing water pockets within the ice shell. This mechanism is based on the study of an analogous double ridge feature found on Earth’s Greenland Ice Sheet. Credit: Justice Blaine Wainwright.

The double ridges on Europa can be dramatic, reaching nearly 300 meters at their crests, with valleys a kilometer wide between them. The idea of a dynamic ice shell is supported by evidence of water plumes erupting to the surface. Thinking about the shell as a place where geological and hydrological processes are regular events, we can see that exchanges between the subsurface ocean and the possible nutrients accumulating on the surface may occur. The mechanism, say the researchers, is complex, but the Greenland example provides the model, an analog that illuminates what may be happening far from home. It also provides a radar signature that future spacecraft should be able to search for.

From the paper:

Altogether, our observations provide a mechanism for subsurface water control of double ridge formation that is broadly consistent with the current understanding of Europa’s ice-shell dynamics and double ridge morphology. If this mechanism controls double ridge formation at Europa, the ubiquity of double ridges on the surface implies that liquid water is and has been a pervasive feature within the brittle lid of the ice shell, suggesting that shallow water processes may be even more dominant in shaping Europa’s dynamics, surface morphology, and habitability than previously thought.

So we have a terrestrial analog of a pervasive Europan feature, providing us with a hypothesis we can investigate with instruments aboard both Europa Clipper and the ESA’s JUICE mission (Jupiter Icy Moons Explorer), launching in 2024 and 2023 respectively. Confirming this mechanism on Europa would go a long way toward moving the Jovian moon still further up our list of potential life-bearing worlds.

The paper is Culberg et al., “Double ridge formation over shallow water sills on Jupiter’s moon Europa,” Nature Communications 13, 2007 (2022). Full text.

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{ 29 comments… add one }
  • Alex Tolley April 22, 2022, 10:41

    In the Nature article, Figure 4 showing the 4 stages of double ridge formation is perhaps the best explanation.

    However, there may be some sleight of hand going on here. The Greenland ridges are far shorter than the Europan ridges. The length of the ridge is determined by the size of the water body below the surface, implying that in Europa, this subsurface “lake” is perhaps 100+ km in size. Is this plausible as the lake must form from the subsurface ocean venting to the surface and appear as a geyser. Why doesn’t the refreezing vent water form a plug with a circular ridge around it?

    For long double ridges, it would seem that there must be water running along a fracture line beneath the surface and that the refrozen plug must be either linear for the length of the ridge, or that a subsurface hotspot is moving relative to the surface ice.

    Skimming the paper, I didn’t read anything about single ridges either in Greenland or Europa. In Greenland, they presumably appear well away from the meltwater sinks where no plug forms. In Europa, something similar should occur well away from the vent. Do double ridges merge into single ridges, or are they disjoint features?

    The most useful idea from this model is that the likelihood of a subsurface “lake” increases. This would indicate that any drilling of the Europan surface may only need to be relatively shallow, a fraction of the depth of the ice shell-sea interface. Even better, any organisms would eventually be found frozen within the lake preserving them. This would make sample collection bother harder and easier depending on what is being sampled. For any motile, complex organisms, being frozen makes capture easier if they can be detected. Whether this is easier than using terrestrial capture techniques for abyssal organisms IDK. What this model does suggest is that subsurface mapping using the techniques used in Greenland should be used on Europa to locate these lakes if they exist.

    Lastly, if these double ridges appear on Europa, they might also appear on other icy moons like Enceladus. Do we have evidence for this? [The only mention of Enceladus in the paper is a reference to an experimental model which suggests that water can erupt from the surface of Enceladus, but is less likely to do so from Europa. This may mean that ice plugs in the Enceladan ice crust do not freeze and produce these double ridges, although, AFAICS, that does not preclude subsurface lakes from forming in the Enceladan ice crust.]

  • Gary Wilson April 22, 2022, 14:41

    That moon is just intriguing. The colours, which may be in part at least due to organic compounds like tholins, the induced magnetic field, the density, the likely presence of a metal core, the relatively smooth but continuously re-shaped surface… It goes on and on. Who wouldn’t love to have a decent sized lander there?

  • Geoffrey Hillend April 22, 2022, 16:50

    An interesting theory, but the scale of the two photos or area viewed in each of them is not the same in size. The Europa photo area is fifty times larger than the Greenland one as shown in the scale bar. How do we know that the photo of the double ridges on Europa are not just cracks or fracturing of the ice shell caused by the tidal forces of Jupiter and “eccentric orbit” of Europa around Jupiter? A. McManus, Cracking Europa’s mysterious surface, ABC Science.

    There certainly could be underwater flow in many places on Europa. The melting would be caused by tidal heating of course. There may be double ridges caused by subsurface water flow. I don’t think such an event is shown in the Europa photo though because of the faulting shown in it. In large plates of ice, there can be movement of the water in the direction of flow which is frozen or locked into the ice when frozen. In the photo we can clearly see in the bottom or south of the ridge, the streaks or creases are at an odd angle, but on the north side of the double ridge they are completely vertical or perpendicular to the ridge which suggests the ridges were formed by cracking or the movement of two different plates of ice. The Europa ridges are also much straighter than the Greenland one’s another suggestion of a difference in how they were formed?

  • Michael Fidler April 22, 2022, 21:32

    Could this be similar to ocean ridge spreading in plate tectonics but in different form on ice worlds. The upwelling of mixed water ice cryovolcanoes like Pluto but in a ocean ridge fashion, the magma chamber being a water chamber.

    https://www.researchgate.net/figure/A-schematic-illustration-of-crustal-formation-beneath-fast-spreading-mid-ocean-ridges_fig1_323738499

    http://www.explorevolcanoes.com/Volcano%20Glossary%20MidOcean%20Ridge.html

    • Alex Tolley April 23, 2022, 8:29

      If they were like ocean ridges, they would have to be be very extensive and there would have to be subduction zones too to keep the ice crust area constant. I would expect to see some evidence of the material on either side of the ridge showing progressive aging, perhaps increasing peroxide concentration.

      • Michael Fidler April 24, 2022, 9:25

        Take a look at the first image above of Europa, see all the old ridges beside the large new one and notice how they intersect in all different directions. Each time the new ridges form it pushes the old ridges down until they become heated enough and merge. Like a giant 3 dimensional jigsaw puzzle, the old ridges probable help form the new ridge by bringing enough fractures and cracks in the area below. The second picture is very simplistic but think what a jumbled mess it would look like underneath with old ridges. Subduction would cool the water and slow the ridging.

        https://scx2.b-cdn.net/gfx/news/hires/2014/usplanstoans.png

  • Adam Crowl April 22, 2022, 21:46

    Hi Paul
    Such sills might form melt caverns, possibly filled by gases from the water. Given the oxidants present in the ice, it could be well oxygenated, though there might be a certain fraction of hydrogen peroxide.

  • Geoffrey Hillend April 23, 2022, 15:41

    The generally accepted view it that Europa has ice plates like ours on Earth except they are ice. The smaller groves in the Europe photo above which extend away from the larger ridge probably formed at different angles due to different pressure points or strengths in the ice which stronger or weaker in different places and cracked along the weaker points like cleavage in fractures in crystal?

    It still would be interesting if Europa has any flow of subsurface water. The melting of any subsurface ice would be near the vents or cracks where there are cryovolcanoes? The NASA Clipper mission has all the instruments to prove the existence of water and cryovolcanoes including an infra red spectrometer. It also has a mass spectrometer which uses the mass of the atoms to identify the chemical elements. From what I recall reading, the heavier atoms have more mass, so they travel further in the spectrometer, the distance traveled indicating their exact mass and chemical element.

  • Geoffrey Hillend April 23, 2022, 18:05

    With a mass spectrometer, the atoms are made ions by knocking off one or more electrons. 2. The atoms are accelerated with the same kinetic energy. 3. They are deflected by a magnetic field and the lighter atoms are deflected more than the heavier one’s. 5. They are electrically detected.

    https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Instrumental_Analysis/Mass_Spectrometry/How_the_Mass_Spectrometer_Works

  • Peter Nielsen April 23, 2022, 19:41

    Europa’s double ridges are due to tidal opening and closing of cracks in the ice surface causing freezing, with this new soft ice being forced upwards by closure forces. Upwards in a curling way, a cylindrical curling away from the crack, until two cylindrical edges are formed :
    a double ridge. Huge and fully formed on Europa because the water there is very salty, with very low gravity, “0.029 compared to the Earth”. http://vendetustangas.com/…/europa-gravity-compared-to… I know this from seeing similar, 1/3-circle-curved beginnings of half-metre high double ridges on sea-ice off Davis station 1973-75.
    I have also seen how ice on the very salty salt lakes of the Vestfold Hills there is doughy, formable, NOT brittle like seawater and fresh water ices, strongly indicating that double ridges as big as those observed would be strong and tough enough to be so produced in such a low gravity environment.
    I explained all this online at my http://www.nodrift.com website in the early days of the Internet 20+ years ago.

  • Peter Nielsen April 23, 2022, 20:14

    “I have also seen how ice on the very salty salt lakes of the Vestfold Hills there is doughy, formable, NOT brittle like seawater and fresh water ices” The details here reinforce my argument by proving how very tough Europa’s surface ice could be : I accompanied nearly everyone going walking/snowcruising in the Vestfolds, mostly by helping the two biologists with their separate field-working.
    We were the first expedition to start studying the Vestfolds many lakes, from freshwater to salt saturated. One time I was with Dick Williams on a half-saturated salt lake, NOT saturated like Deep Lake.
    It was very much like a trampoline to walk on. We were both immediately struck by this, how it felt so safe. When we got to the middle, Dick raised his crowbar and dropped it vertically, in his usual way for low salty lake ice, only to see it disappear, with the ice showing almost NO RESISTANCE.
    A shock to both of us. Dick then hit the ice with a sideways swipe of his long-handled shovel. Another Big Shock, its immediately loading with a 15 x15 cm2 complete chunk of ice, showing that we had been walking on water beneath only 2-3 cm thick ice, what ice-skaters would generally regard as dangerously thin, while we knew that air and water temperatures were ~-20 deg C, cold enough to kill us within a minute or two.
    Also showing how doughy and TOUGH is such half-saturated salty ice, our soon relaxing, and doing our usual work there for half an hour, taking temperatures and water samples at various depths and so on. knowing that 2-3 cm of such ice truly is as safe as a trampoline to walk on.
    Before present-day Occupational Health and Safety (OH&S) madness of course, all of this.

  • David Herne April 23, 2022, 21:16

    For me, the ‘go to’ book on Europa is Richard Greenberg’s 2008 book ‘Unmasking Europa’. He touches on the topic of double ridges often. I would need to re-visit his work to enable useful further commentary, however, he discusses the possibility of water approaching the surface episodically and even has a chapter – ‘The Biosphere’ on the possibility of life in these rilles. I suspect that his work has been somewhat or even largely, ignored, which would be sad if true. A Europa surface mission must be as much as possible a high-priority destination for future exploration.

  • Peter Nielsen April 24, 2022, 1:05

    Found the diagram for that 20+year old explanation in the attic. The RHS column, going from top to bottom, shows how those “Ice petals” I saw in Antarctica seemed to be produced by tidal action across water freezing in cracks between ~1m thick fast ice. I propose that Europan double ridges are similarly produced. See it at https://www.facebook.com/photo.php?fbid=5139074139502114&set=p.5139074139502114&type=3

  • Geoffrey Hillend April 24, 2022, 16:34

    The double ridge idea with subsurface water flow is an interesting theory, but examples from our Earth’s ice might not always apply to Europa’s surface because it is a different environment. Consequently, I don’t think it supports observations considering the following physical principles; 1. The ice shell on Europe is 10 to 15 miles thick and I don’t think there will be found a lot of soft ice on the surface or liquid water running beneath a thin surface because of the average temperature is much colder than Earth’s due to Europa being outside the life belt and much further away from the Sun than our Earth. The cracks and ice fragments are very large. 2. Heat can only be near the area around the postulated cryovolcano openings because there is no atmosphere around Europa and therefore no vapor pressure which is necessary for liquid water to exist and liquid water can’t form even under the ice because the temperature is too cold. Hot water could certainly come up through the cryovolcano vents, but how far can it exist away from them which is not very far away. It becomes steam or water vapor upon reaching the surface. Maybe the infra red spectrometer of the Europa clipper will allow us to see how the heat can be away from the cryovolcano vents. It also depends on how large the vents are. The infra red images should tell us. Maybe the mass spectrometer can detect sodium chloride and salt water vapor?

  • charlie April 24, 2022, 18:13

    Do we even know if life is even possible in the absence of photosyn.; which seemingly leads to (possible?) living cells – which can lead to many other type of living cells with alternative oxidation-reduction chemical schemes or pathways ? Little if no sunlight out there

    • Alex Tolley April 24, 2022, 21:30

      Chemotrophs existed on Earth for at least a billion years before photosynthesis occurred. While photosynthesis allows for the capture of more energy and therefore a richer biota, life can exist without it as long as there is a food source that can generate net energy. Archaea and bacteria living in hot vents become the base of the food chain for other, complex life. Animals living in sealed caves live on the bacteria that can thrive there. Archaean methanogens gain energy when the rocks have liberated the needed hydrogen to allow them to reduce CO2. Other bacteria feed on methane. There are other favorable redox reactions that produce net Gibbs free energy for life to exploit.

      Any Europan life living in the subsurface ocean will be very sparse compared to terrestrial life, just as terrestrial life in the abyssal depths is very sparse compared to much of the surface life where nutrients and water are not limiting factors.

      • Mike Serfas April 25, 2022, 10:55

        It is true that photosynthesis as formally defined arose rather late in evolution, and made a remarkable impact by poisoning the atmosphere with then-toxic oxygen gas. However, harvesting of light for energy, at least to the extent of photoheterotrophy, likely occurred much earlier. There is a wide range of proteins loosely categorized as “microbial rhodopsins” that harvest light for energy, though these can date back at absolute maximum to the origins of protein synthesis.

        Because all known organisms use proteins, we can’t go back further except by speculation… still, I can’t help but speculate about molecules such as riboflavin, which is a universal and essential cofactor for respiration, yet also has a bad reputation in humans as a photosensitizer, collecting solar energy and releasing it randomly. Riboflavin is a nucleotide, produced from GTP (which is essential for an RNA genome), so it is something that should have been available to life from the very beginning. Curiously enough, carotenoids – another ancient vitamin intimately entwined with photosynthesis – are said to help deal with this unwanted energy, though I haven’t seen the details: https://pubmed.ncbi.nlm.nih.gov/22406738/

        Now I can’t say anything of this type really happened, but I surely wouldn’t assume that nothing similar did. I’d imagine life was using sunlight for energy in very early times, possibly before the evolution of a cell membrane or even a genetic code.

        • charlie April 25, 2022, 13:46

          “However, harvesting of light for energy, at least to the extent of photoheterotrophy, likely occurred much earlier. ..”

          Pretty much my pt.; its possible that sunlight energy could more easily energize primordial ‘soup’ rather then pure heat. The cells thus made could then go to vents to live. Dont forget lightening as well

          • Alex Tolley April 26, 2022, 6:55

            The sunlight on the early Earth was insufficiently filtered against UV. Any molecules able to harvest light energy would also be broken up. AFAIK, most origin-of-life-on-the-surface seem to face the problem that large molecules are unstable, preventing macromolecules from falling apart. [You may recall in my last post on CD the figures that show that some hot vent types allow large molecules to remain stable, a piece of evidence supporting this locale for the origin of life.]

            High energy sources, UV, lightning, do support the formation of small organic molecules such as amino acids. This is why we see such molecules almost wherever we look for them e.g. on comets, in dust clouds around other stars. But running Miller-Urey type experiments does not produce proteins (or the polymers of nucleic acid bases). This needs other conditions.

            • Mike Serfas April 26, 2022, 16:47

              Early RNA life would have had two problems: ribozymes are poor catalysts, involved in few reactions; and RNA is unstable in UV light. Perhaps these two issues could cancel out? There’s some discussion of UV-assisted catalysis here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7003795/ Was it really just an unfortunate accident that RNA (hence DNA) is a potent dye in the UV range?

        • Alex Tolley April 25, 2022, 17:23

          There is also anoxygenic photosynthesis practiced by some bacteria. However, without sunlight, this would not be useful under the Europan ice. So the question becomes what is the energy source in that ocean. Reduced compounds would be the source, whether methane or hydrogen (see my recent post on this). So think of anaerobic respiration of these compounds as the energy source for life. Because peroxides are created on the surface of Europa, there is the possibility that they could reach the ocean allowing for some aerobic respiration, although I doubt there is enough for much respiration. IMO, terrestrial chemotrophic anaerobe ecosystems would be the best model for life where photosynthesis is not possible.

          • Geoffrey Hillend April 26, 2022, 15:33

            The problem with the hydrogen peroxides is the lack of vapor pressure, so that hydrogen peroxide and also water could only be a vapor or solid frozen on the surface. There would be no mixing with any water. The Radiolysis of water into hydrogen and oxygen by radioactive isotopes at the crust ocean floor to ocean is possible.

  • Gary Wilson April 24, 2022, 22:54

    The earliest forms of life on Earth are thought to have been anaerobic bacteria Charlie so we know that life can arise without photosynthesis. Current evidence indicates photosynthetic organisms arose much later.

  • FrankH April 25, 2022, 14:19

    Could the Europa cracks imply a slushy area under the ice crust? Rather than just thick ice and liquid water. The slush would help transport liquid water towards the ice.

    In my opinion, Europa should be #1 on the list of potential life bearing worlds in our solar system (after Earth, of course). It’s very unlikely that Mars has extant life – I don’t see how the extremely heavy bombardment Mars was subjected to could have allowed life to and find a foothold (cold dry periods punctuated by massive impacts that produced mega floods and short-lived lakes.)
    Enceladus is interesting, but some of the current Saturnian moons may be too young for life to have had time to form. (https://www.centauri-dreams.org/2016/03/28/saturns-moons-a-question-of-age/)

    Pluto and the other icy worlds are also interesting, but too far out for quick access.

  • Geoffrey Hillend April 25, 2022, 15:46

    Life may have still needed sunlight to start to create enzymes. https://www.science.org/content/article/how-sunlight-might-have-jump-started-life-earth. The hydrothermal origin idea is weak since cells had to form first in order to have membranes to adapt to that environment, i.e., there is nothing in it that shows how the cells formed, but only the adaptation of them to that undersea environment after they already formed.

    Europa is also a target of the infra red spectrometer of the JWST. Maybe we can get confirmation of the cyrovolcanos before the Europe Clipper.

  • Gary Wilson April 25, 2022, 18:14

    I think we humans might consider ourselves to be “photocentric” as we consider the current state of life on earth. However in the deep past when conditions on Earth were much more hostile the likelihood is that primitive organisms used a variety of energy sources including those that lived (and still live) deep beneath the surface. I think the same might be true of more hostile environments further out in the solar system. The further away from the sun, the less solar energy will be a factor possibly. If life exists in places like the subsurface ocean of Europa and elsewhere it probably didn’t evolve to use photosynthesis.

  • Geoffrey Hillend April 26, 2022, 15:25

    I am not debating the idea that life started without photosynthesis which is clearly not debatable, but is fact. I am only pointing out that the hydrothermal origin of life idea only explains the adaptation to the hydrothermal environment, but not how the cells could have formed there. I think that lakes and ponds have much more chemical mixing and higher chemical concentration than the undersea environment with more water which is more diluted. This is only a hypothesis and does not rule out life on Europa. More evidence or lack of it is needed for that.

  • ljk May 12, 2022, 14:56

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