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Flow in Enceladus’ Internal Ocean

New thinking about the shape of the ice shell on Enceladus raises the interesting possibility that the ocean believed to lie beneath is churning with currents. That would be a departure from the accepted view that the ocean — thought to be 30 kilometers deep or more, as opposed to Earth’s average of 3.6 kilometers — is relatively homogenous.

Remember, this is a body of water buried beneath perhaps 20 kilometers of ice, locked within a moon that is 500 kilometers in diameter. We know it’s there because a 2014 flyby of the Cassini Saturn orbiter collected data on geyser activity erupting through the striking fissures in the south polar ice. That immediately put Enceladus high on the list of astrobiologically interesting targets along with, of course, Jupiter’s moon Europa.

Image: A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn’s moon Enceladus and its ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. A new theory examines circulation within the ocean beneath the surface, with implications for future searches for life there. Credit: NASA/JPL-Caltech.

The differences from Earth’s oceans are obvious enough, including the fact that the Enceladus ocean is being heated from below at its interface with the core, while being cooled at the top, which argues for a great deal of vertical movement affecting its properties. The paper on this work points out that variations in the thickness of the ice shell would involve separate regions of freezing and melting at the upper ocean/ice interface.

To examine the resulting circulation, Caltech graduate student Ana Lobo, working with colleague Andrew Thompson and JPL’s Steven Vance and Saikiran Tharimena, has applied Thompson’s work on water/ice interactions in Antarctica to the Enceladus environment. The researchers used a computer model to demonstrate the likelihood of ocean currents beyond the expected vertical upwellings. This would be a process that transports heat and introduces a zone of freshwater in the moon’s polar regions, and is partially driven by salinity, a factor that Thompson has found to be affecting ocean circulation near Antarctica.

That’s something to consider as we ponder whether sampling directly from the Enceladus plumes actually reflects the broader conditions found within the global ocean. Cassini has already shown us that the Enceladus ice shell is thinner at the poles than at the equator, a likely indication of areas of melting and freezing that directly affect ocean currents. The reason: Freezing salty water releases salts and causes surrounding water to sink, with the opposite effect happening in regions where the ice is melting. “Knowing the distribution of ice allows us to place constraints on circulation patterns,” according to Lobo.

The researchers’ model implies that ocean circulation on Enceladus connects these areas of freezing and melting in a flow that goes from pole to equator. We will need to take such currents into account as we look into the potential for Enceladus to support life. From the paper:

The circulation will also impact how nutrients from the ocean-mantle boundary are distributed. If nutrients are transported upward primarily by ocean plumes, we would expect detrainment to release these nutrients in the ocean interior. In Earth’s ocean, it has been shown that adiabatic mixing, or stirring along isopycnals [lines connecting points of a specific density], can be an important pathway for nutrient delivery to the surface and modulate productivity there [26].

Thus, the distribution of freshwater fluxes at the ocean-ice interface, by setting global patterns of upwelling, could influence nutrient fluxes and provide insight into which regions of an icy world have resources for life to potentially flourish.

The study of oceans beneath the ice on outer system moons is in its infancy. The authors of the study, which appears in Nature Geoscience, close their paper by noting other ocean worlds where variability in the ice thickness may be large enough to alter the density structure of the ocean. Titan, with an ice shell anywhere from 50 to 100 kilometers thick, is a case in point, and the Dragonfly mission will carry a geophysical package that should yield further clues to the ocean beneath. At Jupiter, we have both Europa Clipper and JUICE to look forward to, helping to constrain variations in the thickness of the shell while probing the properties of the subsurface ocean.

The paper is Lobo et al., “A pole-to-equator ocean overturning circulation on Enceladus,” Nature Geoscience (25 March, 2021). Abstract / Preprint.

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{ 10 comments… add one }
  • Alex Tolley March 26, 2021, 16:22

    The thermal upwelling at the South pole results in meltwater from the surface crust and general circulation with salinity gradients. The low salinity meltwater beneath the South pole ice cap is in some ways similar to the melting Greenland ice sheets that reduce salinity but also slow down the Atlantic circulation. While this may be the condition today, unlike the Earth where the equator is always warmer than the poles, it is unclear to me why the upwelling should be stable at the polar location. Is it not possible that the hot spot can migrate to anywhere on the core’s surface, moving the general circulation model. In addition, while the current polar upwelling may be dominant based on the distribution of the surface ice crust thickness, might there not be other hotspots also resulting in local upwellings, complicating the picture?

    From an operational standpoint of looking for life, the South polar plumes remain the best way to sample the subsurface ocean from orbit. I would not expect the water composition to radically change the presence of any life, although as the authors note, the water sampling may be at variance in composition with the bulk of the ocean.

    If, a big if, there was any motile, complex life in the Enceladan ocean, it might avoid those freshwater zones just below the ice cap unless it was stable enough to allow evolved organisms that are adapted to those conditions. In Clarke’s Odyssey novels, he speculated that “civilizations” appeared around the hotspots in the Europan ocean. However, as the hotspots cooled and stopped, while others started, those civilizations were trapped and died. I would have expected complex life to be able to migrate away from the hotspots and colonize all the ocean’s habitats. Similarly on Enceladus, any complex life around the polar hotspot would be able to live in all the ocean’s environments, from the hostspot itself, to the cold freshwater above, the mixed layers below the ice crust, and the more saline, cold, deep waters everywhere else. One thing I think we can expect is that without sunlight to drive the energy budget of the Enceladan ecosystem, any life will be very sparse, dependent on the chemistry of the interior. Do those compounds accumulate over billions of years, or are there sinks too?

    • David Jernigan March 28, 2021, 1:45

      Do we know whether the hotspots are more likely at the poles due to being tidally induced? If Enceladus’ axis is tilted wrt Saturn, wouldn’t it experience long periods of tidal stress at the poles, a “midnight sun” of tidal pull?

    • David Jernigan March 28, 2021, 1:51

      Nvm, Enceladus has no tilt and is tidally locked, which is to be expected. Though I’m still not certain if that implies a stress differential at the poles vs the equator

      • Michael T March 29, 2021, 5:14

        It seems to me that the stress patterns could be be different at the poles to the equator, but in the case of Europa, the important axis is the one between the moon and its primary, so the fracture system seems centred on the antipodes of the sub-jovian point, if I read things correctly.

  • Henry Cordova March 27, 2021, 12:39

    Astronomy Domine
    by Pink Floyd

    [Verse 1]
    Lime and limpid green, a second scene
    A fight between the blue you once knew
    Floating down, the sound resounds
    Around the icy waters underground
    Jupiter and Saturn, Oberon, Miranda and Titania
    Neptune, Titan, stars can frighten

    [Instrumental Bridge]

    [Verse 2]
    Blinding signs flap
    Flicker, flicker, flicker, blam
    Pow, pow
    Stairway scare Dan Dare who’s there?

    [Verse 3]
    Lime and limpid green, the sound surrounds
    The icy waters under
    Lime and limpid green, the sound surrounds
    The icy waters underground

    …from their first album “The Piper at the Gates of Dawn”.

  • Geoffrey Hillend March 27, 2021, 16:41

    Tidal heating from the gravity of Saturn is the most effect means of heating the surface of Enceladus. Enceladus is much further from the Sun than the Earth keeps the temperature on the surface everywhere well below freezing. We can’t compare Earth’s weather and temperature with Enceladus because Earth has a thick atmosphere which transports the heat of the air from the equator to the poles through different atmospheric cells. The is no air or atmosphere on Enceladus to transport the heat from equator to poles so it’s only the subsurface water currents or an under ice circulation.

  • Robin Datta March 27, 2021, 21:32

    A poverty of energy flows will limit the impetus to develop systems that could support high metabolic rates of neural tissue. Invertebrates are limited in size: the notable exception is those Mollusca that have developed oxygen-carrying blood proteins. Among them are also the most neurologically advanced invertebrates. Sone Arthropods, particularly Crustaceans also have such proteins and achieve sizes rather large for invertebrates.

    Heme itself is found it Annelids as “earthworm hemoglobin”, but in the absence of red blood cells of mammals, its concentration in solution is limited by viscosity.

    Brains require lots of energy. It is surmised that a switch to omnivory by the inclusion of carnivory played an important role in the increase in human brain size. Without pre-existing abundant energy flows and an infrastructure to corral and manage them, the resulting life forms might be less than spectacular.

    • Alex Tolley March 28, 2021, 13:06

      Almost without exception, complex life is primarily aerobic. While I don’t rule out anaerobic complex life, there are issues with developing organs to transport respiratory molecules. For example, insects are size-limited due to the perfusion rate of O2 through the spiracles. More energetic phyla have lungs to force the O2 and CO2 transfers. Aquatic animals have large gill surfaces and either force water over their surfaces (e.g. fish) or swim (e.g. sharks). Enceladus, even more so than Europa, is likely to be a low-energy world, with anaerobic respiration which is far less energy liberating than aerobic respiration. Complex life should any exist, would likely be very slow-moving, possibly even non-motile. If life is sparse too, then such life might rely on currents to bring food to it (e.g. sponges), or drift with currents to interact with food. On Earth, even abyssal fish in the food-scare environment may use lie-in-wait strategies to capture prey, rather than use energy for search.

  • Mike Serfas March 27, 2021, 23:51

    The data they’re trying to explain is this: https://solarsystem.nasa.gov/resources/12631/enceladus-temperature-map/ A golden oldie from 2005, but still a striking surprise. The tiger stripes and the plumes, the thinner ice, and the modelled polar upwelling all tie in with this. But I still don’t understand the paper’s explanation of why the upwelling should be polar, for the same reason as Alex. It seems like an imposition of chicken-and-egg thinking to rival dynamo self-induction.

  • Jeff Wright April 4, 2021, 2:59

    Tiger stripes offer a way down.

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