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On Outer System Oceans

Back in the days when I was reading Poul Anderson’s The Snows of Ganymede and thought of the moons of Jupiter as icy wastelands, I never would have dreamed there could be an ocean below their surfaces. But now we have oceans proliferating. Ganymede’s may contain more water than all Earth’s oceans, while Callisto is also in the mix, and we’ve known about Europa for some time now. At Saturn, the case for an ocean inside Titan seems strong, while Enceladus continues to spark mission proposals to study its frequent geysers.

If you’re a Centauri Dreams regular, you know that we’ve talked about Pluto’s oceanic possibilities for some time, now strengthened in new work from Brandon Johnson (Brown University). Johnson and colleagues have modeled an ocean layer on Pluto more than 100 kilometers thick, with a salt content more or less like that of the Dead Sea on Earth. Johnson focused on Sputnik Planum, the 900-kilometer basin that comprises part of the heart-shaped feature we all learned to recognize during last summer’s New Horizons flyby.

The work is helpful because unlike previous studies, it puts constraints on the depth of the ocean and offers information about its composition. Sputnik Planum, as it turns out, sits exactly on the tidal axis linking tidally-locked Charon with Pluto, suggesting that the area has a positive mass anomaly; i.e., there is more mass here than the average for Pluto’s crust.

Thus Charon’s gravity would eventually pull this higher-mass area into the alignment we see. A large impact, like the one that created Sputnik Planum, would eventually be filled with liquid water, and in Johnson’s scenario, nitrogen ice deposited later would tip the scales, creating the positive mass anomaly. “This scenario, says Johnson, “requires a liquid ocean.” And of the various thicknesses of the water layers modeled, 100 kilometers works out best at creating the features we see at Sputnik Planum.

Usefully, the team’s computer models showed that the thickness of the ocean layer could be tied directly to the production of the mass anomaly, which also turns out to be sensitive to the salinity of the water, affecting the water’s density. We can tell a lot from this impact of an object 200 kilometers across or larger and its effects upon an ocean with a salinity of about 30 percent.

“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly —and it appears as though it is — this ocean layer of at least 100 kilometers has to be there,” Johnson says. “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”

The Case for Dione

Now we have word of yet another possible ocean, this one on Saturn’s moon Dione. Mikael Beuthe (Royal Observatory of Belgium) and team have been modeling the icy shells of Enceladus and Dione, using gravity data from Cassini flybys. The models study the shells of both moons as global icebergs immersed in water — here surface peaks are held up by a large mass under the water. The approach has not previously proven effective in modeling Enceladus, for it produces a thick crust for the moon that is inconsistent with our observations.

But isostasy, the equilibrium in parts of the Earth’s crust, can be similarly modeled, as blocks floating on the underlying mantle, which rises and falls as material is added or removed. What the researchers have done is to add a new wrinkle:

“As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms,” says Beuthe. “More stress would break the crust down to pieces.”

This revised model works well at producing an ocean for Enceladus under a thinner crust than previous models, with water close enough to the surface to produce the famous geysers at the south pole. The model is also consistent with the libration of Enceladus in its orbit, oscillations that would be smaller if the moon had a thicker crust. Under the same model, Dione is revealed to have a much thicker crust covering a deep ocean between the crust and the moon’s core.

The Cassini data thus become consistent with an ocean about 100 kilometers below the surface of Dione, an ocean perhaps tens of kilometers deep surrounding a large, rocky core. We may be able to test this prediction with future spacecraft in the Saturn system, which may be able to measure the libration of Dione, an oscillation the researchers believe would be well below the detection capabilities of Cassini, but a useful marker as evidence of the subsurface ocean.

pia18345_modest

Image: Dione with Enceladus in the background. This image was taken by the Cassini spacecraft on 8 September 2015. Credit: NASA/JPL-Caltech/Space Science Institute.

And what of other outer system objects? From the paper:

Beyond Enceladus and Dione, our new take on isostasy is applicable to large icy satellites
with global oceans, such as Europa [Nimmo et al., 2007b], Titan [Nimmo and Bills,2010], and particularly Ganymede whose gravity and shape will be measured by the JUICE mission. According to Park et al. [2016], gravity and shape data from the Dawn mission suggest isostasy on Ceres, but the case is far from clear because compensation does not occur for all gravity components. Finally, isostasy plays a crucial role in understanding the long-wavelength gravity and shape as well as estimating the crust thickness of the planets Mars [Wieczorek and Zuber , 2004], Venus [James et al., 2013], and Mercury [Perry et al., 2015]. Thanks to the simultaneous availability of gravity/shape and libration data, Enceladus’s case constitutes the first validation of planetary-scale isostasy.

The paper is Beuthe et al., “Enceladus’ and Dione’s floating ice shells supported by minimum stress isostasy,” published online by Geophysical Research Letters 28 September 2016 (abstract / preprint). The paper on Pluto’s ocean is Johnson et al., “Formation of the Sputnik Planum basin and the thickness of Pluto’s subsurface ocean,” Geophysical Research Letters 19 September 2016 (abstract).

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Comments on this entry are closed.

  • Tom Mazanec October 5, 2016, 12:35

    If Dione has oceans, how about Iapetus, Rhea and Tethys?

  • Alex Tolley October 5, 2016, 12:47

    I’m having trouble understanding how a moon as small as Dione can maintain liquid water without it freezing solid. If Dione can have a liquid ocean, then why not Ceres which is not that much smaller?

    I’d ;like to see some work that offers explanations for the heat sources or mechanisms that can maintain H2O as water, rather than freezing out like the ice above it, with a model to calculate where the liquid-ice boundary should be.

    • DJ Kaplan October 5, 2016, 15:56

      I think that the unspoken assumption was that of tidal heating, which wouldn’t be expected to occur at Ceres.

    • andy October 5, 2016, 17:33

      There’s also the possibility that Dione (and the other satellites inward of Titan) formed relatively recently (estimated around ~100 Myr), based on predictions of the dynamical evolution of the Saturnian satellite system. See this previous Centauri Dreams article for more details on that.

      • DJ Kaplan October 6, 2016, 12:32

        To my mind, it doesn’t explain why its water would be liquid rather than frozen solid. Are we assuming that it started out liquid and it takes 100 Myr to freeze solid? Any water out there would be in a solid state naturally and would require a heat source to render it liquid.

        • andy October 6, 2016, 13:09

          Heat from accretion?

  • Neil S October 5, 2016, 12:50

    What does he mean by “long-wavelength gravity” in regard to Mars? He’s surely not referring to the gravity waves so long theorized from relativity theory. Since long-wavelength EM waves have less energetic photons does he mean weak gravity? It seems he’d have said so. Is he talking about time or space variation in the strength of gravity? Clarity would be appreciated. Otherwise, very interesting.

    • Paul Gilster October 5, 2016, 13:11

      I’ve written Dr. Beuthe for a summary of the long-wavelength gravity concept, which shows up frequently in the literature on planetary gravitational fields. Will pass along what I hear.

      • Paul Gilster October 5, 2016, 16:58

        Here is Dr. Beuthe’s response:

        “Thank you for your interest. I am so glad to see serious discussions about space science on websites like yours. About isostasy, there was also a nice explanation related to GRAIL a few years ago on Emily Lakdawalla’s blog:
        http://www.planetary.org/blogs/emily-lakdawalla/2012/12110923-grail-results.html

        “Regarding the question raised by your reader, we are talking about variations in space, not in time, and this space is more particularly the surface of the moon. It is probably easier to consider first the example of long-wavelength topography which, by the way, we also use in our analysis. The topography of a moon or planet can be decomposed into contributions at different scales. At the largest scale, long-wavelength topography describes the flattening of the Earth due to rotation. A synchronous moon (such as ours) is also elongated at the same scale because of the permanent tidal deformation. By contrast, short-wavelength topography describes mountains & valleys, hills, pebbles and sand grains ad infinitum. Similarly to topography, the gravity field can be measured everywhere at the surface of a planet or moon (spacecraft do it from a distance, but it boils down to the same thing). The spatial variations of gravity when you move from one point to the other on the surface are called gravity anomalies. These gravity anomalies can be analyzed, in the same way as topography, into long-, medium-, and short scales. In the case of Dione, we study the gravity variations that are similar in scale to the topographic flattening and tidal deformation. Mathematically speaking, the wavelength decomposition on a sphere yields spherical harmonics. I join a picture found on the web showing what they look like at the largest scales (http://www.atmos.albany.edu/daes/atmclasses/atm562/). We use (l,m)=(2,0) and (2,2) for Dione, and also (l,m)=(3,0) for Enceladus.”

        I can’t reproduce the image in the comments, but you can find it on the server at:

        https://www.centauri-dreams.org/wp-content/images/beuthe.jpg

        I’d like to thank Dr. Beuthe for his quick response and helpful explanation.

  • Ashley Baldwin October 5, 2016, 13:47

    Still waiting on the results of Cassini’s close flyby through Enceladus ‘ plumes last October . Molecular hydrogen the key to showing whether the subterranean ocean has contact with the outer mantle and thus a pm energy source via ocean floor “geysers”/ black smokers . ( the result of which may even determine whether “Ocean Worlds ” is picked as the New Frontiers 4 mission choice next year ) Energy that could drive an ecosystem . Provided the inner moons of Saturn are old enough with one recent study suggesting that all moons interior to Titan, and the rings , were only created around or since about the time of the Cretaceous period . So far tidal heating ( as with Io and Europa) hasn’t been shown to offer enough heat to as yet to provide the heat required to create Enceladus ‘ ocean though the new theory offered here offers a promising alternative possibility . Sixty five million years isn’t long for life to develop.

    In terms of “Ocean worlds” the moons of Uranus and especially Ariel ( with a recent formed surface and evidence of ice tectonics like Europa) offers great potential too. All the more reason to explore the system as soon as , though a budget of $2 billion has been calculated as the absolute minimum required for a meaningful science return , more than twice New Frontiers and with a 10-12 year transfer time. Flagship class in effect which will need very high Decadel priority. The SLS could reduce journey time by up to two years . Role on the “ice Giants” . Jupiter gravity assists are out of action due to absence of planetary alignment with the outer solar system from 2019-2030 but can also reduce transfer times significantly , though that takes us up to the 2040s.

    • ljk October 5, 2016, 16:12

      Some details on the Ocean Worlds NF4 mission plan here:

      http://futureplanets.blogspot.com/2016/08/selecting-next-new-frontiers-mission.html

      To quote:

      The Ocean Worlds mission theme is focused on the search for signs of extant life and/or characterizing the potential habitability of Titan and/or Enceladus. For Enceladus, the science objectives (listed without priority) of this mission theme are:

      • Assess the habitability of Enceladus’ ocean; and
      • Search for signs of biosignatures and/or evidence of extant life.

      For Titan, the science objectives (listed without priority) of the Ocean Worlds mission theme are:

      • Understand the organic and methanogenic cycle on Titan, especially as it relates to prebiotic chemistry; and
      • Investigate the subsurface ocean and/or liquid reservoirs, particularly their evolution and possible interaction with the surface.

  • Peter Hook October 5, 2016, 13:55

    And Triton?

    • Adam October 5, 2016, 17:20

      Javier Ruiz has computed a relatively shallow ocean might be present under Triton’s N2/CO ices due to their poor thermal conductivity.

      • ljk October 6, 2016, 8:51

        Those multiple geysers first seen by Voyager 2 in 1989 are being fed from somewhere underground. It doesn’t have to be a whole liquid ocean, but it also has to be more than a few isolated puddles. I am sure that Neptune’s mass is contributing to their existence as well, just like Jupiter with Io and its volcanoes.

  • ljk October 5, 2016, 14:05

    Before New Horizons flew through the Pluto system in July of 2015 with some of Clyde Tombaugh in tow, did anyone else think about what Pluto and Charon might do to each other being gravitationally locked over one spot? It is impressive how much some people can deduce from the evidence even from brief flyby data.

    Back in 1979, just days before Voyager 1 did its flyby through the Jupiter system, a couple of smart scientists conjectured that the giant planet’s moon Io might be volcanically active due to all the tugging it receives between Jupiter and the other Galilean moons. Turned out they were so very right.

    Same thing happened with Europa and its global ocean of liquid water:

    https://www.math.washington.edu/~greenber/EuropaHistory.html

  • ljk October 5, 2016, 14:07
    • FrankH October 5, 2016, 15:49

      Also, right here!
      https://www.centauri-dreams.org/?p=35300

      Saturn’s moon and ring system are strange, which (I think) points to recent chaos:
      Ring system
      Hyperion in a chaotic rotation/tumble
      Enceladus (more internal heat than expected)
      Iapetus
      Titan’s atmosphere

      So the sub surface oceans on Saturn’s moons may be interesting – but probably lifeless.

      • DJ Kaplan October 6, 2016, 12:38

        The article said that Titan is probably as old as Saturn, however if it is indeed losing its finite supply of methane into space (and still be around to gaze upon), it may not be that old either.

  • Amelia Díaz October 5, 2016, 14:33

    What about Tethys’ case in having a deep subglacial ocean of salty wáter?

  • RobFlores October 5, 2016, 14:37

    As previously mentioned, Red dwarves of the milky way are where the bulk of these Icy worlds are likely to exist. More likely as planets rather than
    Moons.
    Is there enough ‘data’ about icy planetoids to give a general formula as to
    how thick an ice crust the Earth would have if the sun were a M5 star, given a similar orbit. ?

    How deep is the probable ocean under ice likely to be in that case.?

    If we find that there is even ONE of the icy bodies harbors microbial or even small animal life (Tardigrade for ex.) in our solar system, then these type worlds would be main fount of life in the universe, due to their vast
    numbers compared to habitable zone worlds.

    • DJ Kaplan October 6, 2016, 12:41

      If more than one of our solar system moons have something that we decide to recognize as life, then we may be able to compare them and derive some modulus of variability.

  • ljk October 5, 2016, 14:46

    I wondered about Neptune’s big moon Triton having a subsurface ocean, especially in light of the Pluto revelation, which some scientists were saying was a model for Pluto in the Voyager 2 days, partly in compensation for Pluto being removed from the Grand Tour target list.

    Triton definitely has active geysers, which I recall was a big surprise in 1989. The material for those features certainly has to come from somewhere underground.

    http://www.space.com/17470-neptune-moon-triton-subsurface-ocean.html

  • andy October 5, 2016, 17:39

    Regarding the possibility of even more distant oceans, there was a recent paper posted to arXiv that seems to have identified a possible negative feedback mechanism for atmospheric carbon dioxide on extrasolar ocean planets. This is in contrast to previous results which tended to find that the carbon dioxide feedbacks in the absence of silicate weathering would be positive, leading to either runaway greenhouse or snowball glaciation.

    Levi, Sasselov & Podolak (arXiv:1609.08185 [astro-ph.EP]) “The Abundance of Atmospheric CO₂ in Ocean Exoplanets: A Novel CO₂ Deposition Mechanism

    • Adam October 6, 2016, 5:32

      Andy, it’s well known that a hydrogen greenhouse can keep terrestrials warm enough to maintain open-sky water oceans out way beyond the Ice Moons. With Earth levels of silicates providing radioactive warmth an Ocean Planet could persist in interstellar space. Given enough hydrogen.

      • andy October 6, 2016, 13:08

        Adam, the study I linked is about planets which do not have substantial hydrogen atmospheres. The answer to the question of whether you can have an open-sky ocean planet with a carbon-dioxide greenhouse is not well known.

  • Phil October 6, 2016, 3:54

    Is that very young age for Saturn’s moons really consistent with the saturation cratering seen on their surfaces?

    P

    • andy October 6, 2016, 13:23

      As noted in the paper in question: “This scenario implies that most craters on the moons interior to Titan must have been formed by planetocentric impactors.”

      There is a brief discussion of the craters on pages 46/47 of the arXiv paper, not much detail but it does at least seem plausible that planetocentric impactors can work to put the craters on the moons, though further research is needed (from the paper’s conclusion: “We hope that further work on crater and impactor
      population modeling will be able to put stronger constraints on impactor sources.”)

    • FrankH October 6, 2016, 15:32

      If the moons formed from the disruption of a larger moon, there’s going to be a lot of debris flying through the system in orbits similar to the new moons. You would expect a lot of cratering.

    • Adam October 6, 2016, 15:44

      Think that one through Phil. An event that made the rings and the moons would mean *lots* of cratering…

      • ljk October 6, 2016, 17:22

        The novelization of 2001: A Space Odyssey had an answer….

  • david lewis October 8, 2016, 14:32

    An idle thought on my part, but can those oceans provide a source of energy for any bases placed on them, the same as geothermal energy can here on earth?

    You have liquid water, and above ice, providing a temperature differential.

  • Alex Tolley October 10, 2016, 16:00

    Thermal differences are small, so that means their Carnot Cycle efficiency will also be very low. It is one of the reasons that OTEC plants on Earth are not cost effective.

    If the oceans are still rather than moving, the salinity differences might possibly be exploited using osmotic pressure.If there are currents, then turbines to extract the mechanical energy of the flow might work.

    I would think that the best bet is still solar power from the surface or nuclear.

  • ljk November 3, 2016, 9:17

    First global geologic map of Jupiter’s largest moon Ganymede details an icy world

    November 2, 2016

    More than 400 years after its discovery by Galileo, the largest moon in the solar system has finally claimed a spot on the map.

    A team of scientists led by Wes Patterson of the Johns Hopkins Applied Physics Laboratory (APL), Laurel, Md., and Geoffrey Collins of Wheaton College, Norton, Mass., has produced the first global geologic map of Ganymede, a Galilean moon of Jupiter. Published by the U.S. Geological Survey, the map technically illustrates the varied geologic character of Ganymede’s surface, and is the first complete global geologic map of an icy, outer-planet moon.

    Patterson, Collins and colleagues used images from NASA’s Voyager and Galileo missions to create the map. It’s only the fourth of its kind covering a planetary satellite; similar maps exist for Earth’s moon as well as Jupiter’s moons Io and Callisto.

    “By mapping all of Ganymede’s surface, we can more accurately address scientific questions regarding the formation and evolution of this truly unique moon,” says Patterson, a planetary scientist.

    Full article here:

    http://military-technologies.net/2016/11/02/first-global-geologic-map-of-jupiters-largest-moon-ganymede-details-an-icy-world/

    The map is available for download at http://pubs.usgs.gov/sim/3237/

  • ljk February 2, 2017, 19:19
  • ljk March 1, 2017, 15:48

    Saturn’s ‘Death Star’ moon may not conceal an ocean after all

    BY THOMAS SUMNER

    2:07 PM, FEBRUARY 28, 2017

    An ocean of liquid water probably doesn’t lurk beneath the icy surface of Mimas, Saturn’s smallest major moon, new calculations suggest. Scientists had proposed the ocean in 2014 to help explain an odd wobble in the moon’s orbit.

    Other ocean-harboring moons, such as Jupiter’s Europa and Saturn’s Enceladus, are crisscrossed by fractures opened by strong tides that cause their oceans to bulge outward. Mimas, though freckled with craters, lacks any such cracks.

    Planetary scientist Alyssa Rhoden of Arizona State University in Tempe and colleagues calculated whether Mimas’ icy shell could withstand the stress of a subsurface ocean pushing outward. Taking into account the moon’s elongated orbit, the researchers estimate that a subsurface ocean would produce tidal stresses larger than those on crack-riddled Europa. Mimas therefore probably doesn’t have an ocean, the researchers conclude February 24 in the Journal of Geophysical Research: Planets.

    Full article here:

    https://www.sciencenews.org/blog/science-ticker/saturn-death-star-moon-may-not-conceal-ocean-after-all

    Nevertheless we still need to land there and find out one way or the other for certain.

  • ljk March 8, 2017, 12:10
  • ljk March 11, 2017, 17:46

    Exploring Titan with Aerial Platforms

    Article Updated: 11 March 2017

    by Matt Williams

    Last week, from Monday Feb. 27th to Wednesday March 1st, NASA hosted the “Planetary Science Vision 2050 Workshop” at their headquarters in Washington, DC. During the course of the many presentations, speeches and addresses that made up the workshop, NASA and its affiliates shared their many proposals for the future of Solar System exploration.

    A very popular theme during the workshop was the exploration of Titan. In addition to being the only other body in the Solar System with a nitrogen-rich atmosphere and visible liquid on its surface, it also has an environment rich in organic chemistry. For this reason, a team led by Michael Pauken (from NASA’s Jet Propulsion Laboratory) held a presentation detailing the many ways it can be explored using aerial vehicles.

    The presentation, which was titled “Science at a Variety of Scientific Regions at Titan using Aerial Platforms“, was also chaired by members of the aerospace industry – such as AeroVironment and Global Aerospace from Monrovia, California, and Thin Red Line Aerospace from Chilliwack, BC. Together, they reviewed the various aerial platform concepts that have been proposed for Titan since 2004.

    Full article here:

    http://www.universetoday.com/134274/exploring-titan-aerial-platforms/

  • ljk March 14, 2017, 11:24

    Enceladus’ south pole is warm under the frost

    March 14, 2017

    Over the past decade, the international Cassini mission has revealed intense activity at the southern pole of Saturn’s icy moon, Enceladus, with warm fractures venting water-rich jets that hint at an underground sea. A new study, based on microwave observations of this region, shows that the moon is warmer than expected just a few metres below its icy surface. This suggests that heat is produced over a broad area in this polar region and transported under the crust, and that Enceladus’ reservoir of liquid water might be lurking only a few kilometres beneath.

    Full article here:

    https://phys.org/news/2017-03-enceladus-south-pole-frost.html

    To quote:

    Many of Cassini’s flybys of Enceladus have been dedicated to understanding the structure of the interior of this fascinating body and its potentially habitable water reservoir. Now, a study based on data collected during a close flyby in 2011 indicates that the moon’s hidden sea might be closer to the surface than previously thought.

    “During this flyby, we obtained the first and, unfortunately, only high-resolution observations of Enceladus’ south pole at microwave wavelengths,” says Alice Le Gall from Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), and Université Versailles Saint-Quentin (UVSQ), France. Alice is an associate member of the Cassini RADAR instrument team and the lead scientist of the new study, published today in Nature Astronomy.

    “These observations provide a unique insight into what is going on beneath the surface. They show that the first few metres below the surface of the area that we investigated, although at a glacial 50-60 K, are much warmer than we had expected: likely up to 20 K warmer in some places,” she adds.

    “This cannot be explained only as a result of the Sun’s illumination and, to a lesser extent, Saturn’s heating so there must be an additional source of heat.”

    The detected heat appears to be lying under a much colder layer of frost, as no similar anomaly was found in infrared observations of the same region – these probe the temperature of the surface but are not sensitive to what is underneath.

    The observations used by Alice and her collaborators cover a narrow, arc-shaped swathe of the southern polar region, about 500 km long and 25 km wide, and located just 30 km to 50 km north of the tiger-stripe fractures. Because of operational constraints of the 2011 flyby, it was not possible to obtain microwave observations of the active fractures themselves. This had the benefit of allowing the scientists to observe that the thermally anomalous terrains of Enceladus extend well beyond the tiger stripes.

    “The thermal anomaly we see at microwave wavelengths is especially pronounced over three fractures that are not unlike the tiger stripes, except that they don’t seem to be the source of jets at the moment,” Alice says.
    These seemingly dormant fractures lying above the warm, underground sea point to a dynamic character of Enceladus’ geology: the moon may have experienced several episodes of activity at different locations during its past history.

    Even if the observations cover only a small patch of the southern polar terrains, it is likely that the entire region is warm underneath and Enceladus’ ocean could be a mere 2 km under the icy surface. The finding agrees well with the results of a recent study, led by Ondrej Cadek and published in 2016, which estimated the thickness of the crust on Enceladus. With an average depth of 18–22 km, the ice shell appears to reduce to less than 5 km at the south pole.

    Alice and her collaborators think that the underground heating source is linked to the tidal cycle of the moon along its eccentric orbit around Saturn. This induces stress compressions and deformations on the crust, leading to the formation of faults and fractures while at the same time heating up the sub-surface layers. In this scenario, the thinner icy crust in the south pole region is subject to a larger tidal deformation that, in turn, releases more heat and contributes to maintaining the underground water in liquid form.

    “This discovery opens new perspectives to investigate the emergence of habitable conditions on the icy moons of the gas giant planets,” says Nicolas Altobelli, ESA’s Project Scientist for Cassini–Huygens.

    “If Enceladus’ underground sea is really as close to the surface as this study indicates, then a future mission to this moon carrying an ice-penetrating radar sounding instrument might be able to detect it.”

    More information:

    A. Le Gall et al. Thermally anomalous features in the subsurface of Enceladus’s south polar terrain, Nature Astronomy (2017). DOI: 10.1038/s41550-017-0063

    http://www.nature.com/articles/s41550-017-0063