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Revisiting Enceladus’ Ocean

As we saw yesterday, there is a case to be made that the ocean beneath Pluto’s ice is still liquid, based on phase changes in ice under varying pressures and temperatures. Today we turn to another world with interesting oceanic possibilities, Enceladus. Here the data are problematic and contradictory. Flybys by the Cassini Saturn orbiter detected tiny deviations in the spacecraft’s trajectory that could be used to measure the gravity of the Saturnian moon.

Weighing these perturbations in the spacecraft’s motion against the known topography of Enceladus, scientists could draw tentative conclusions about the moon’s internal structure. Enceladus appeared to be internally asymmetric, with an ice shell between 30 and 40 kilometers thick in the southern hemisphere, perhaps thickening to 60 kilometers at the equator. Moreover, the Cassini data were not sufficient to conclude anything about the extent of the ocean. Did it extend beneath the entire shell, or was it confined largely to the southern polar regions?

Work out of Cornell University in 2015 charted some 5800 surface features at different angles and times, bringing dynamical modeling and statistical analysis to bear on Enceladus’ interior. Peter Thomas and team found that they could use a wobble in the moon’s rotation — a libration — to show that the surface of Enceladus was not solidly connected with its core. The libration, then, told us that the ocean was global. It also pointed to thinner ice, with a mean thickness of about 20 kilometers, a result inconsistent with the Cassini gravity studies.

To resolve the contradiction, a new model from a team of international researchers now proposes an asymmetric model with a strong heat source. The work sees a rocky core with radius of 185 kilometers, an internal ocean about 45 kilometers deep, and an ice shell with a mean thickness of around 20 kilometers except at the south pole, where it appears to be less than 5 kilometers thick. The ocean in this model comprises about 40 percent of the total volume of Enceladus, and the salt content is estimated to be similar to that of Earth’s oceans.

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Image: The thickness of Enceladus’s ice shell, which reaches 35 kilometers in the cratered equatorial regions (shown in yellow) and less than 5 kilometers in the active south polar region (shown in blue). Credit: LPG-CNRS-U. Nantes/U. Charles, Prague.

Relatively thin ice in the south polar regions is promising for future exploration. But while this model gives us a way to reconcile previous results, it also offers another question. A thinner ice shell holds less heat, which means tidal stresses caused by Saturn can no longer explain the heat flow affecting the south polar region. We must be dealing with strong heat production deep inside Enceladus that may be driving hydrothermal venting on the ocean floor.

And yes, complex organic molecules have been found in the plumes from Enceladus, making for an astrobiologically interesting scenario when coupled with a warm ocean. But we’ll need a closer look to find out whether the model put forth by Ondřej Čadek (Charles University, Prague) and his colleagues at the Royal Observatory of Belgium and the Laboratoire de Planétologie Géodynamique de Nantes holds up. Thin ice at the south pole could offer us a window into the ocean, a study relevant not just to Enceladus but many icy objects in the outer system.

The paper is Čadek et al, “Enceladus’s internal ocean and ice shell constrained from Cassini gravity, shape, and libration data,” published online by Geophysical Research Letters 11 June 2016 (abstract).

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Comments on this entry are closed.

  • Hop David June 23, 2016, 14:32

    And there was a recent article giving more evidence of a Ganymede subsurface ocean: http://www.dailygalaxy.com/my_weblog/2016/06/jupiters-ganymede-opens-exciting-possibilities-for-life-harbors-a-buried-ocean-100-kilometers-deep-w-1.html

    Many of our gas giant moons seem to have internal liquid oceans. Liquid water suggests temperatures hospitable to humans. These strata also have human friendly pressures. There would certainly be lots of in situ water and organic compounds. I am becoming more interested in icey moons as potential homes for humans.

    Earlier I had written an article suggesting a system of moons about a gas giant could provide a science fiction setting with fast pacing. Launch windows between moons can be days or weeks apart instead of years. Trip times between moons can also be days. Originally I was suggesting a gas giant’s system of moons in a star’s Goldilocks zone.

    But I no longer see a Goldilocks as a requirement. Jupiter’s and Saturn’s moons could provide such a setting. Maybe the moon systems of Uranus and Neptune.

    I write about such moon systems here:
    http://hopsblog-hop.blogspot.com/2013/01/mini-solar-systems.html

    Another possibility with tide locked, coplanar moons are elevators passing through the planet-moon L1 and L2 regions. With such elevators it is possible to travel from one moon to another using almost zero propellent. Most of our gas giant moons are tide-locked and most hug the gas giant’s equatorial plane.

    • Michael June 23, 2016, 17:24

      Unfortunately gas giants have nasty magnetic fields that whip particles up to near light speeds. If we live near them we have to tame the particles flying around there or we are going to get a large dose of radiation. Might be better to dismantle them and form a larger moon further out where there is less radiation freeing up the inner area as a natural particle accelerator.

      As for Enceladus I am not sure Saturn’s magnetic field can induce high enough ohmic heating to be a contender for an energy source, but Enceladus might just have the right type of ions and numbers for it to be a factor.

      • Hop David June 23, 2016, 22:14

        Near light speeds? Like Galactic Cosmic Rays? (Googling….) No, the ions in Jupiter’s belt are traveling a small fraction of light speed.

        Jupiter has some nasty Van Allen belts. Saturn, Uranus and Neptune have less extreme radiation.

        And any temperate layers would be under layers of ice and protected from radiation.

        • Michael June 24, 2016, 10:42

          Heavy ions can have MeV energies but the electron ions can be bounced from pole to pole in a magnetic mirror effect and they can become highly relativistic. Although living under the ice would be ok moving between the moons would be dangerous, the ions will have to be absorbed and new ones stopped from getting up to high speeds (not impossible).

          http://www.ifpan.edu.pl/firststep/aw-works/fsIV/mahjouri/mahjouri.pdf

  • RobFlores June 23, 2016, 15:34

    Great work within the limited data available:
    It’s still an unresolved matter about what the source of the Heating of
    South Pole is.

    I was not convinced it was tidally related, since Mimas, which isclose to the same size to Enceladous. displays no geysers or asymmetric geophysical feature between It’s poles. And it’s something like 25% even closer to Saturn “surface”

    Eliminating Tidal heating leaves,

    Asteroid with high content of Radioactive compounds, Or compounds that cause ongoing chemical reaction. Slightly acidic asteroid compound meets slightly basic ocean, slow but constant, and temporary, it would have to be a relatively recent impact.

    And of course, Chemical heating due to biological chemistry, (ie respiration and breakdown of materials for growth/replication. This biological life must be relatively recent (less than a billion year?), as the base materials would get used up over several billion of years since the formation of Solar System.

  • Ashley Baldwin June 23, 2016, 19:00

    Fantastic article and posts too. So much to find out , but how mad when as ever given budgetary restrictions not least due to the Congress driven Europa mission .( generously supported to date , but politicians change ) .The big key in the first instance is the analysis of data obtained from the last close Cassini flybys in October and December 2015. Right through the plumes and as close as 30 miles in October. The Ocean’s ph is already known to be strongly alkaline , but not so much as to impede life in its own right .

    The mass spectrometer is nowhere near sensitive enough to pick up all but the most simple organic molecules but critically can detect molecular hydrogen. Why so exciting ? Its presence in significant amounts gives evidence of geothermal interaction between a warm core and the surrounding ocean ( serpentisation) . Possibly what is seen in the deep ocean “black smokers” ( undersea geysers in essence) of the deep oceans on Earth which have been found to create and support ecosystems through chemical energy rather than photosynthesis. Not indicative of life in itseelf and well worth investigating and probably essential for life regardless of the moon’s age ,. With the ideal opportunity offered by the upcoming New Frontiers 4 call of opportunity ,which has a recently and controversially added an Enceladus/ Titan “Ocean Worlds ” theme on its “long list” for a mid 2020s mission.

    The budget for this should allow a much more modern payload than Cassini such as a mass spectrometer an order of magnitude or two more sensitive than the previous mission’s INMS instrument . The JET , THEO and ELF Titan/Enceldaus concepts have already been worked up to a significant level of detail and also include a high resolution thermal ( IR) imaging camera / telescope specifically for Enceldaus “Tiger stripes” as well as far greater precision mapping of Titan than is possible with Cassini’s radar. Given the large amount of recent outer solar system work, on comet and asteroid organise in particular, any New Frontiers payload could be larger than usual given mature technology for just what is required and better still, cheap back up instruments such as the Mass Spectrometer from Rosetta.

    The main lead on this is Jonathan Lunine who is already preparing a bid for New Frontiers 4. As part of the mission profile , Nasa are providing a lot of additional “free” technology such as the NEXT ion drive , optical/laser communications ( with a $30 million incentive for use thrown in) and the new ( very larger) enhanced ROSAT solar arrays that can produce kilowatt levels of energy even at Saturns 9.5 AU. Combining this last with an ion drive produces the mooted “Holy Grail” of future solar system space travel , solar electric power . Ten times more efficient than traditional chemical power it also saves on critical launch mass providing a shorter transfer time . Some Enceladus concept transfer times were near ten years without a Jupiter gravity assist for the ten years from 2019 onwards- solar electric could bring this down to less than seven and with just a single Earth flyby rather than involving Venus too with the added hassle and cost of heat protection .

    The key is the reliability of the new system and whether Nasa offer a further financial “incentive “to use it ( Pu239 RTGs are available but are not free and come at a cost of $100-165 million plus handling , all of which eats into the limited mission budget rather than siting outside it as with solar power ). Cheap, mature instrumentation and possibly an extra $ 60 million extra over budget gives a near flagship mission and detailed characterisation of Enceladus ocean via deliberately slowed down flybys without recourse to an expensive lander as required for Europa .( an organic molecule busting 10 km/s for Cassini down to a ” slow ” 2 km/s that should allow intact analysis ) . Something not so easy at Europa given its still as yet unsubstantiated plumes.

    The big deal breaker is whether the whole system is old enough . Recent ( unsubstantiated ) evidence has hinted that Saturn’s rings and inner moons ( though not as far out as primordial Titan ) are less than a 100 million yeras old , far to short for “life as we know it ” to have developed .

    Whatever, we will know if there will indeed be a Cassini follow up mission sometime next year or early 2018. If there is , transfer allowing it should directly follow on from the Europa mission , making for more than a decade of ground breaking planetary science and who knows what will be found .

    Anything ( or everything ) goes !

  • Michael June 24, 2016, 1:11

    ‘Enceladus ocean via deliberately slowed down flybys without recourse to an expensive lander as required for Europa .( an organic molecule busting 10 km/s for Cassini down to a ” slow ” 2 km/s that should allow intact analysis ).’

    I am for a lander even if it is a reflectance plate, it lands and when suitable instruments are pointed at it there will be a much greater resolution of the material between it and the orbiter. It could also have off angle reflectance to reflect instrument light from the orbiter then onto Earth for more information. Not sure if it could act as a lens to study material on the surface using the above orbiter as the energy source and scanner though, perhaps simple sensors on the disc could sniff out certain molecules.

    • Ashley Baldwin June 25, 2016, 5:49

      A lander would be great . Nasa have even worked up concepts based around an Orbiter but despite not unreasonable costs, this is where the limitations of the excellent New Frontiers programme become obvious as opposed to even a smaller flagship mission. Despite the far more benign environment at Enceladus as opposed to Europa , even a small orbiter supporter lander based around say the Mars pathfinder would be far too much for a sub $billion budget.

      However , between Rosetta ,New Horizons and Europa Multi flyby there are numerous already mature payload instruments available with little if any extra development costs or better still actual spare devices ( such as the STEAM mass Spectrometer from Rosetta) . The JET Discovery proposal included this with a thermal ( IR) imaging camera which could accomplish substantial science within a year’s mission. With so much preformultion work having been done on Enceladus allowing diversion of funds to a mission proper , along with possible financial incentives for using optical communications and solar electric power , it may be that a larger New Frontiers mission could carry up to 5 or even six proven payload instruments .
      The dust analysing instrument slated for Europa , SUDA, for instance would be perfect for assessing Enceladus’ plumes . Along with that mission’s mapping spectrometer to supplement JETs thermal imaging camera , Tiger, Huygen’s level mapping of both Titan and Enceladus could be achieved (especially the “Tiger stripes” of the latter) .to a degree far in excess of Cassini’s radar , even down to landing site mapping resolution for any future lander .
      An enhanced New Horizons style LORRI imager is already being developed for Europa and is perfect for flyby imaging so it would be great if it could be included too given its now established heritage and subsequent lower cost . All of this allows a payload approaching the level of a small flagship , and taking advantage of plumes and very close multiple flybys to achieve almost as much as a lander but at far lower cost and risk ( the New Frontiers “free” engineering ironically includes technology for landing on solid bodies but for Enceladus that could only be for a lander only that transmitted direct to Earth without the added high expense of a relay satellite . Certainly possible ( though power would be a big issue and require an RTG for any sort of extended life ) but assuming a landing site near the “Tiger stripes” of the southern pole , Earth drops below the Enceladus horizon at this latitude for twenty years from the late 2020s thus militating against such a lander given it couldn’t arrive in time even assuming a 2024 launch and relatively quick transfer time .
      New Frontiers missions come with free but “reasonable” operations costs , which we’ve seen with similar style Juno to be about two years . Unlike Juno however any Saturn mission wouldn’t be subject to such a hostile radiation environment and perhaps be extended given the huge success of the numerous Cassini extensions . It can’t be stated enough that all of a New Frontiers’ 4 payload instruments would be orders of magnitude more sensitive than the late 90s technology of Cassini making it a new mission in effect rather than just than a repeat Cassini, as has been mooted on some space exploration forums by those irritated by Ocean World’s admittedly late inclusion as a mission thene for the programme . Fingers crossed for “Ocean Worlds” from me though !

  • Harry R Ray June 24, 2016, 9:31

    A recent study came to the conclusion that Teyths and Dione are BOTH LESS THAN 100 MILLION YEARS OLD! It now appeasrs that Enceladus may be MUCH YOUNGER THAN THAT and that the heat source is simply LEFTOVER HEAT FROM ITS VERY RECENT(in astronomical terms)FORMATION!

  • Coacervate June 25, 2016, 1:04

    I’m not qualified in this area so maybe I can say something really stupid with a measure of impunity. It seems to me that there are more than one possible modes of tidal heating. It is easy to picture the frictional forces caused by deformation of a solid mass. But imagine a frozen outer shell covering a liquid ocean over a small, solid rocky core. I’m picturing the motion of this core relative to the ocean as being similar to a large ball bearing inside a water balloon. As the whole body rotates in a gravity field, the liquid tries to redistribute just as it does in Earth’s oceans. But on Enceladus the water must rapidly move around the bearing. Trapped between a rock and hard place so to speak. That could generate some frictional heat perhaps?

    • Michael June 25, 2016, 10:15

      Unfortunately the moon is tidally locked in that it faces Saturn permanently and therefore there is no movement. However the moon has other moons moving around Saturn to cause liberation of the moon -a small oscillation. Now due to the inertia of the rocky core it is not as mobile to rotate as the icy mantle. What may be happening is that the mantle has frozen from the outside and the freezing material at the lower part of the mantle is grinding at the ice/rock interface generating heat.

  • david lewis June 26, 2016, 10:47

    The answer is simple – there’re aliens on Enceladus, and they’ve warmed up a portion of the place as a cheap source of energy.

    We on earth use the equivalent of 92Gt of tnt each year. By using multiple thermonuclear explosions in the range of 100Gt to 1000Gt of tnt, the aliens have warmed up parts of Enceladus creating an energy source that will last for thousands of years; when it does run down they simply repeat the process. It’s fusion the simple way.

    Or maybe not.

    • Harry R Ray June 26, 2016, 16:48

      If there is STILL THAT MUCH HEAT, there must also be leftover RADIATION hanging around as well, and Casswini would have been able to DETECT IT! ALSO: IF this is a REPEDITIVE PROCESS, each time, water should have BREACHED THE SURFACE, like it did at Conomara Chaos on EUROPA, and we would be seeing ICEBERGS instead of Tiger Stripes and there would be NO GUYSER ACTIVITY AT ALL!

  • Harry R Ray June 29, 2016, 10:32

    UPDATE: I have some good news and some bad news! First the good news: The mysterious “heat source” is now almost CERTAINLY “serpentization”, which is a CHEMICAL process NOT REQUIRING ANY TIDAL FRICTION OR ABNORMALLY HIGH ABUNDANCE OF RADIOACTIVE MATERIALS! Now the bad news. The chemicals REQUIRED for the serpentization process would not be available TODAY if they formed BILLIONS OF YEARS AGO! To be AVAILABLE today, they MUST HAVE FORMED RECENTLY(IN ASTRONOMICAL TERMS). This BOLSTERS the new hypothesis that Saturn’s INNER MOONS are less than 100 million years old and may not have ENOUGH TIME to have formed any kind of life EXCEPT the most primitive kind(IF EVEN THAT). Alas, the “whale-analogs” of my previous comment ALMOST CERTAINLY DO NOT EXIST!

  • ljk October 4, 2016, 12:53
  • ljk November 21, 2016, 14:07

    Waiting on Enceladus

    Posted on 2016-11-17

    by Marc Kaufman

    Of all the possible life-beyond-Earth questions hanging fire, few are quite so intriguing as those surrounding the now famous plumes of the moon Enceladus: what telltale molecules are in the constantly escaping jets of water vapor, and what dynamics inside the moon are pushing them out?

    Seldom, if ever before, have scientists been given such an opportunity to investigate the insides of a potentially habitable celestial body from the outside.

    The Cassini mission to Saturn made its closest to the surface (and last) plume fly-through a year ago, taking measurements that the team initially said they would report on within a few weeks.

    That was later updated by NASA to include this guidance: Given the important astrobiology implications of these observations, the scientists caution that it will be several months before they are ready to present their detailed findings.

    The reference to “important astrobiology implications” certainly could cover some incremental advance, but it does seem to at least hint of something more.

    I recently contacted the Jet Propulsion Lab for an update on the fly-through results and learned that a paper has been submitted to the journal Nature and that it will hopefully be accepted and made public in the not-too-distant future.

    Full article here:

    http://www.manyworlds.space/index.php/2016/11/17/waiting-on-enceladus/