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An Encouraging Formation Scenario for Icy Moons

It makes sense that planets in other stellar systems would have moons, but so far it has been difficult to find them. That’s why Kepler-1625b, about 8,000 light years out in the direction of Cygnus, is so interesting. As we noted last month, David Kipping and graduate student Alex Teachey have compiled interesting evidence of a moon around this gas giant, which is itself either close to or within the habitable zone of its star. The massive candidate exomoon is the size of Neptune, and if confirmed, would mark the first exomoon detection in our catalog.

As the examination of Kepler-1625b and its transit timing variations continues, we have new work out of the University of Zürich, ETH Zürich and NCCR PlanetS that adds weight to the assumption that moons around large planets should be ubiquitous. Using computer simulations run at the Swiss National Supercomputing Centre (CSCS) in Lugano, a team of researchers led by Judit Szulágyi (University of Zurich and ETH Zurich) has determined that both gas giants and ice giants like Neptune and Uranus will produce moon-bearing circumplanetary disks.

Image: One of the computer simulations on the formation of moons (white bodies) around Neptune (blue sphere). Credit: Judit Szulágyi.

The issue is given point by the difference between Neptune and Uranus when it comes to moons. The five major moons of Uranus do not seem out of place when compared to what we see around Jupiter and Saturn. But we seem to see a different formation history at Neptune, whose solitary major moon, Triton, may well have been captured from the Kuiper Belt.

Szulágyi and team wondered whether the moons of Uranus were not themselves outliers, perhaps formed through a collision in the early days of the Solar System. Our own Moon is thought to have been the result of just such an ancient catastrophe. But the simulations the researchers ran pointed to both Uranus and Neptune originally having their own moon-forming disk of gas and dust. In each case, the simulations produced icy moons. This is a useful result as it has been widely believed that the two ice giants were too light to form such a disk.

The implication: Neptune was itself once orbited by a system of icy moons much like that of Uranus, one that would have been disrupted during the capture of the massive moon Triton. Bear in mind that Triton contains 99 percent of the mass of Neptune’s entire satellite system. The authors point to an earlier study showing that the capture of Triton would only have been possible if Neptune originally had a moon system with the mass of the Uranian moons.

From the paper:

We investigated CPD [circumplanetary disk]- and moon-formation around Uranus and Neptune with combining radiative hydrodynamical simulations with satellite population synthesis. We found that both Uranus and Neptune could form a gaseous disk at the end of their formation, when their surface temperature dropped below 500 K. These disks are able to form satellites in them within a few hundred thousand years. The masses of such satellite-systems for both planets were often similar to the current one around Uranus. All the formed moons must be icy in composition, given that they formed in a CPD that has a temperature below water freezing-point.

All of this has implications for the exomoon hunt, in that the formation of moons seems to be likely across the entire range from gas giant to ice giant. Bear in mind how often we’ve found Neptune-class planets among the population of exoplanet candidates. If such worlds are producing exomoons, then such moons form a larger population than we had realized. Our studies of the Jovian and Saturnian moons have shown how interesting they are in terms of astrobiological possibilities, a realm that now widens as we expand the discovery space.

Says Szulágyi: “[A] a much larger population of icy moons in the Universe means more potentially habitable worlds out there than it was imagined so far. They will be excellent targets to search for life outside the Solar System.”

The paper is Szulágyi, Cilibrasi and Mayer, “In situ formation of icy moons of Uranus and Neptune,” Astrophysical Journal Letters 868 (2018), L13. Abstract / Preprint. Among the papers on Triton as a disrupter of Neptune’s early system of moons, see in particular Rufu & Canup, “Triton’s Evolution with a Primordial Neptunian Satellite System,” Astronomical Journal Vol. 154, No. 5 (2017). Abstract.

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{ 14 comments… add one }
  • jonW November 21, 2018, 18:52

    What’s the latest on how likely a gas giant’s moon is to lie within or outside of the planet’s radiation belt? And what’s the latest on how much of a deal-breaker the presence of a radiation belt would pose for life?

    • Ashley Baldwin November 22, 2018, 14:49

      It’s not so much the radiation belt that would pose a threat to surface based life on a massive “terrestrial” moon “, as the tidal heating arising from the host planet’s gravitational pull. ( enough to warm a subsurface ocean even at Ganymede’s orbital distance from Jupiter and causing extensive vulcanism on more proximal Io) Even an extensive radiation belt like Jupiter’s lies well within the radius of this gravitational effect . It would cause a runaway greenhouse effect in the terrestrial atmosphere of any planet sufficiently massive to hold onto it until a certain semi major axis dependent on the host planet’s mass. ( and likely outside of the planet’s “protective” magnetosphere.)

    • jonW November 23, 2018, 11:56

      On second thought now I’m confused: could a Neptune-sized exomoon in a star’s habitable zone have a rocky surface at all? Or are all Neptune-sized bodies ice giants, wherever they lie in their solar system?

      • wdk November 23, 2018, 20:30

        Following the investigation of solar system satellites with internal heating, several examples of geothermal activity are Io, Europa and Triton. Yet on the other hand, I don’t think there is much activity associated with Titan. Or for that matter, our moon.

        In the case of the Galilean Satellites, they are all Jupiter equatorial and near zero eccentricity but they are of significant mass themselves when compared with our moon and they perturb each other in tightly bound orbits.

        In the case of Triton, it is at near zero eccentricity retrograde in orbit around Neptune, an inclination angle of about 25 or 30 degrees.
        Neptune, since it rotates rapidly is oblate like the other gas giants and Triton cuts across it. Just coincidentally (?) Neptune’s zonal winds near the equator are East West or counter to its rotation and about synchronous with Triton’s period. So you have to wonder if large moons in effect drag anchors when they churn tides on their gas giant primaries.

        Of course, when Voyager got to Neptune and Triton could be observed, there was some expectation of geothermal history on account of its arrival in a near circular orbit. But the evidence that it continues – I don’t know if there ever was a consensus on what I just said. Initially, when geysers were observed, a number of atmospheric scientists offered the “dust devil” explanation.
        But at the very least there is a celestial dynamics argument for what was provided above.

        So, in the case of exo=planets and exo-moons, some perturbation might be desirable to establish geodynamics promotional to life such as convection and plate tectonics similar to what occurs on Earth. And then the local magnetosphere of the primary vs. the flare events of the particular star… ?

        • wdk November 23, 2018, 20:36

          I should add: With the discussion of Triton and its geothermal nature, would have expected that the New Horizons mission Pluto flyby would have resolved why Triton had geysers. Consequently, given the argument above, had expected an object more like Callisto, but it sure wasn’t. For me, Pluto raised more questions than it resolved, if any related to Triton.

        • jonW November 25, 2018, 12:34

          Thanks for all those details… I was actually implying a more basic question, which is: for a *moon* with mass comparable to Neptune’s (as discussed in the first paragraph of Paul’s post), where the host planet orbits within a star’s HZ where water can be liquid, will the moon still be an ice giant, with nothing corresponding to what we think of as a surface, by virtue of its sheer mass? Or because it’s within the HZ will a moon of this size potentially have a surface where the geodynamical processes you describe, comparable to rocky moons in our own outer solar system, will have any chance to happen?

  • Alex Tolley November 21, 2018, 19:34

    much larger population of icy moons in the Universe means more potentially habitable worlds out there

    We have no idea whether icy moons harbor life and are, therefore “potentially habitable”. At a minimum, those that are must have liquid subsurface oceans, which is not the case for most icy moons.

    What might have been more interesting is whether rocky moons of any size that could hold an atmosphere could form around gas or ice giants in the HZ. This work suggests that any moons will be small. If the moons need water and other volatiles then they must form outside the snow line, or they will be even smaller.

    If any of our icy moons prove to have life, even prokaryotes, then I might get excited over an abundance of icy moons in the search for life.

    However, if humanity does reach the stars, icy moons may prove very useful as sources of materials, especially water, for habitats and colonies. Possibly even better than asteroids and comets.

  • Robin Datta November 21, 2018, 21:33

    If we are considering very cold environments for the emergence of life, the issue arises about how cold it can get and still permit the transition from prebiotic chemistry. The constraints in this case be different from those for the adaptation of fully-formed life to very cold environments.

    Habitability may not necessarily permit emergence.

  • James Franklin November 22, 2018, 3:55

    Interesting comments, Alex, I believe you are forgetting that it is currently believed that as well as Europa, Callisto, Ganymede, Titan, Triton and even Pluto have subsurface brine layers. Gravitational heating caused by the parent body and other moon’s, or, in the case of Pluto, a substantial moon, cause tidal heating and thus have cores of significant temperature, sustaining the brine layer.

    The extent of the brine layer for each body is, as yet, unknown, although Ganymede is the only moon with a global magnetic field which implies more of an ocean beneath the surface than a simple layer.

    Could there be simple biospheres in such an environment? We don’t know, but if such bodies are ubiquitous throughout the Universe, as is the implication based solely on our system, then it is reasonable to accept that, at least on some, life must arise from time to time.

    Now we have a gas giant orbited, possibly, by an ice giant. It’s likely that there are no other moon’s around the gas giant due to gravitational influence, but depending on the exact nature of the origins of the ice giant, it is possible, though unlikely, to have moon’s itself… depending on the distance between the major components of the system and how they came together.

    Could there be life within an ice giant, who knows, we know so little about life’s origins. All we can conclude is that any life as may have arose in the brine layer/ocean of an ice moon or in the atmosphere of an ice giant is likely to be reasonably simple and very unlikely to have developed technology.

    • Alex Tolley November 22, 2018, 13:38

      Nasa’s mantra “follow the water” has morphed into a belief that water [almost] implies life. Water is necessary, but not sufficient for life as we know it. The pretty pictures of Europan hot vents with life mimicking that of our deep oceans presume that such vents exist and that life could have emerged at such vents. The latter is becoming an attractive idea in contrast to Darwin’s “warm pond”, but that may change. Earth’s vents are above magma due to having a very hot core. Earth’s moon has extensive tidal stretching, but does not have a hot core. Why assume icy moons have them? Maybe all that tidal energy just results in enough heat to keep some ices forming over the rocky core.
      So I am very cautious about life in icy moons. I would welcome its discovery and want samples taken back to Earth to study it. I am far more sanguine that Mars may have life in its lithosphere as the early Mars conditions were much more like that of Earth.

      Sheer numbers doesn’t impress me either. That approach is used to posit that ET must be other somewhere in the universe of hundr4eds of billions of galaxies, and in our galaxy of hundreds of billions of stars. I hope it is, but again, we just do not know. I hope that we will get indications via proxies that life exists on some exoplanets, but these planets will all be terrestrial analogs, as we cannot use such proxies for any life in subsurface oceans.

      Speculation about life on other worlds seems to follow cycles, swinging from one extreme to the other. The logics of such speculations make for interesting historical reading.

      I would prefer that we temper our hopes with evidence. The wild speculation over Tabby’s Star is largely over, especially now that similar stars are being found, suggesting more prosaic natural explanations. The current speculation over the nature of ‘Ouamua are similar, based on flimsy evidence. Mars was thought to have life, if not a dying civilization, then lichens and mosses growing in what appeared to be “greenish” areas. Now we know better.

      Humans have always brought back wild stories of strange creatures and people in remote parts of the planet. Occasionally they were correct, but mostly not. The remoteness of the planets in our system seems to be inviting the same sort of speculation. If a Europa or Enceladus plume mission brings back evidence of life, I would be very happy, but I expect that at best there will be more ambiguous data on organics that could be chemistry or biology. A lander or submarine might get similarly ambiguous results. Or maybe proof. I hope for teh latter, but I don’t expect it.

      • Bruce D. Mayfield November 23, 2018, 22:43

        This belief that life is likely wherever liquid water is found reflects the notion that evolution is easy. Diversification once life is established, perhaps, but life from scratch abiogenesis? There is no proof that spontaneous generation of life is common, just an assumption that it is because life is here.

      • andy November 26, 2018, 14:36

        Europa is at least thought to have liquid water in contact with the (presumably fairly active) rocky core, which makes it more promising than most of the subsurface oceans in the solar system which are thought to sit on top of thick layers of high-pressure ices. On the other hand, Earth has hydrothermal systems on land as well as in the oceans, which have no analogue on Europa and there is still debate over which environment is more promising for the origin of life.

        As for Mars, judging by this preprint that appeared on the arXiv last week there may not be much there any more (if there ever were a biosphere in the first place). But yes, would definitely be good to have more data!

  • steven Torry Rappolee November 24, 2018, 17:00

    I had fun chatting with Kipping on youtube during a Q & A in regards to the Super Jupiter Neptune size moon and Rufu & Canup
    capture?
    In situ?

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