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The Case for Pluto’s Ocean

Sub-surface oceans in the Solar System may be far more common than we’ve realized. We’ve grown used to contemplating water under the ice of Europa, but similar oceans may well exist on Ganymede and Callisto, and there are signs of a possible ocean beneath Titan, not to mention the unusual activity we continue to observe on Enceladus. Where liquid water in cold objects seems least likely is in the Kuiper Belt, but Guillaume Robuchon and Francis Nimmo (University of California at Santa Cruz) have been making the case for an ocean inside distant Pluto, based on their models of thermal evolution and the behavior of the ice shell.

As this article in Astrobiology Magazine points out (and thanks to my friend Antonio Tavani for the pointer to this one), Pluto’s outer surface is a thin shell of nitrogen ice covering a shell of water ice. With New Horizons inbound to Pluto/Charon for an April, 2015 encounter, the researchers have been working out what surface features might help us make the call on whether such an ocean exists. One possibility is an equatorial bulge left over from the earlier days of Pluto’s formation, when it would have been spinning more rapidly. Such a bulge would be perhaps 10 kilometers high if it exists and New Horizons should be able to see it. A bulge would indicate no ocean beneath, as movement of the liquid interior over time would have reduced the protrusion.

But tensional stresses as the shell was stretched when temperatures changed over the course of Pluto’s lifetime would imply water beneath, a different kind of feature than we would expect from a solid layer below. The good news is that while New Horizons is a flyby mission, it will be able to map the entire sunlit surface of Pluto beginning in the three months before closest approach — we should get highest resolution (62 meters per pixel) when New Horizons closes to 12,500 kilometers, and ridges, valleys and possible geyser features should be discernible.

Image: Three Hubble images of Pluto. When New Horizons switches on its cameras three months before closest approach, even its most distant images of Pluto will be ten times more detailed than these. Credit: NASA, ESA, and M. Buie (Southwest Research Institute).

As to what would keep water liquid at an average 40 AU from the Sun:

The main source of energy likely stems from the rocky interior, where isotopes undergo radioactive decay. Among these elements, the researchers found potassium to be key – enough potassium in Pluto’s core would result in melted ice above it.

And signs look good – the amount of potassium needed would be about a tenth of that found in meteorites from the early solar system.

“I think there is a good chance that Pluto has enough potassium to maintain an ocean,” Nimmo said.

Robuchon and Nimmo calculate that a planet-wide ocean here would have an average depth of 165 kilometers beneath a crust of about the same thickness. Finding an ocean on Pluto would make the case for other Kuiper Belt oceans quite strong, especially on larger objects like Eris. A case for astrobiology in this extreme environment seems remote indeed, but the presence of an ocean here would remind us that the Kuiper Belt, which may contain a thousand dwarf planets or more, is likely to deal us surprises at every turn. And we can rejoice at the presence of New Horizons’ Long-Range Reconnaissance Imager (LORRI), which according to principal investigator Alan Stern, would be able to see individual buildings if flown over the Earth at New Horizons’ closest approach altitude. We are in for an incredible view in just a few years.


Comments on this entry are closed.

  • Anthony Mugan November 22, 2011, 11:12

    A very interesting article. I suspect the case for astrobiology in such environments may be stronger than it may seem at first sight, if these bodies do actually have liquid water beneath their surfaces. This suggestion is based largely on lithopanspermia being generally accepted now on interplanetary scales, assuming the surface is sufficiently active to allow some degree of influx into the water.
    In addition, if complexity theory is working along the right lines in terms of biological systems then abiogenesis may be more probable than a simple chance combination of the elements of a micro-organisms (an information theory approach) might suggest.

  • andy November 22, 2011, 13:54

    I recall seeing suggestions that Charon too might be expected to have an ocean thanks to similar mechanisms and the tidal effects from Pluto.

  • FrankH November 22, 2011, 16:05

    This isn’t the first article to posit an ocean under Pluto’s crust. From 2006:

    Hussmann,,Sohl, & Tilman “Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects”
    Icarus, Volume 185, Issue 1, p. 258-273


    It’s a pretty good article, since it explores the internal structures of the largest icy satellites (“large” means r >190km)

  • lepton November 23, 2011, 2:15

    I wish in my life time, we could have sent an orbiter for each planet. I would rather have that than man on Mars.

  • andy November 23, 2011, 4:30

    This suggestion is based largely on lithopanspermia being generally accepted now on interplanetary scales

    Is it really? Sure it is accepted that there is some transfer of rocks between certain planets, but as a viable transfer mechanism for life the evidence is sorely lacking. In particular, we have so far absolutely no evidence that life exists anywhere else in our solar system.

    So when you say it is “generally accepted”, the question arises “generally accepted by whom?”

  • Anthony Mugan November 25, 2011, 6:55


    Whilst this will have to be a short comment today my understanding is that:
    a) The transfer of material between planets is established fact (as you say)
    b) it has been established that micro-organisms can (if they are lucky in their positioning!) survive ejection into space and the stresses of re-entry.
    c) some micro-organisms can survive in space for a number of years (certainly at least two years in the case of bugs brought back from one of the early moon landers by Apollo astronauts – inside a camera if memory serves). Experiments have also been done more recently in orbit, again successfully, and in simulated laboratory conditions on earth with a percentage remaining viable for protracted periods.

    So if they can survive ejection from earth, the timescales in space needed for a trip to (say) Mars or another planet and the process of landing I can not really see how one can avoid the conclusion that transfer of life to suitable environments would occurr on interplanetary distance scales…Insterstellar distances are another matter, and whilst some material would certainly travel such distances the question of viability of any micro-organisms remains a subject of debate.

    A recent overview includes Wesson., P., 2010, Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space. (I shall come back shortly with where this paper was published, which I have a pdf copy of if you would like one). This contains a good recent overview with relevant references. Of course if I’ve missed a key point that would block the process occurring on interplanetary scales I’d be more than interested in learning about it, and be grateful for the information.



  • Anthony Mugan November 25, 2011, 6:58

    Andy – full reference:
    Paul S. Wesson, “Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space” [abstract | 33-page PDF], arXiv.org > astro-ph > arXiv:1011.0101v1, 30 Oct 2010; and [14-page PDF], doi:10.1007/s11214-010-9671-x, Space Science Reviews, online 14 Jul 2010

  • Holger November 25, 2011, 16:10

    The “moon bug” claim has been debunked: see e.g. [http://www.msnbc.msn.com/id/42932978/ns/technology_and_science-space/t/microbes-moon-mystery-solved-last/] – the camera was apparently contaminated with the microbes back on Earth.

    And a recent article [http://www.universetoday.com/90985/update-on-phobos-grunt-might-the-life-experiment-be-recovered/], while supporting your point “b)”, states that “very few controlled studies of microorganisms, plant seeds, and other life have been conducted beyond the Van Allen belts” – something they wanted to change by sending several microbes to Mars and back aboard the Fobos-Grunt spacecraft. So I don’t think that your point “c)” is supported by the current state of science.

  • Anthony Mugan November 28, 2011, 6:04

    Hello Holger

    Thanks for the reference to the criticisms of conditions around the Surveyor camera unit. I suspect that is one debate that will run and run.
    I was tempted to provide a series of references such as to the work of Wayne Nicholson, but as I think the debate here is focusing on the difference between drawing conclusions from current experimental data – largely, I agree, done in simulated space conditions and actually having microbes successfully survive a deep space mission I will limit things just a couple observation and a testable prediction
    The observations are:
    a) Current laboratory data indicates we should expect a proportion of microbes to survive the transit times required for interplanetary transfer (I really should have got all my references together before starting this, but Wayne Nicholson (and co-workers) and Ximena Abrevaya and co-workers) are amongst those who have both done work on that.
    b) It is perhaps interesting from an evolutionary standpoint as to what selection pressures have led to the characteristic observed which underpin observation (as) – such as survival of hard vacuum, deep cold, radiation etc.

    The prediction is that Gerda Horneck and her team will find a degree of survival from the experiments that have recently concluded from the EXPOSE-R patform on the ISS…although I suspect that won’t conclude the debate totally!

    Anyway I shall leave the discussion at that…thanks and an interesting discussion.

  • Eniac November 29, 2011, 0:14

    Let’s see, those super bacteria would have to survive:

    1) The heat of impact/vulcanism that hurls them into an escape trajectory
    2) The cold and vacuum and radiation of space itself, for many years.
    3) The reentry into the target body’s atmosphere
    4) The impact on the target body’s surface

    After all that, they would have to find the place in which they landed to be suitable for reproduction. Somehow, I do not think so, no matter what anybody says. I would need to see real evidence, like actual extraterrestrial life that shares our biochemistry. So far, there is zero such evidence.

  • Rob Henry November 29, 2011, 19:15

    Eniac, I feel that this whole discussion on lithopanspermia misses a key factor.

    There is growing evidence for the surprising high proportion of Earth’s biomass (perhaps even more than half of it) that subsists deep underground in the porespace of rocks and could only survive as such if it used the greater part of the energy available to it on superbly efficient mechanisms for DNA repair. How such life could ever evolve to such diversity on a timescale comparable to the age of our universe is beyond my comprehension, but it has.

    That changes everything!

  • Rob Henry November 29, 2011, 20:24

    This stage of New Horizons journey reminds me of a similar time in Voyager’s approach to Io. For long we had been ignoring our inability to explain major colour changes on the surface, but we all shared a belief that the explanation would be mundane.

    So, I really should put out the call, and ask it anyone has FULLY explained such changes on Pluto, or is this the time that we should all concoct exotic and detailed predictions and post them on the off-chance that posterity may label us sage?

  • Anthony Mugan November 30, 2011, 5:33

    Having said I would leave the discussion with my previous comment I find myself tempted into one more…this really is a most interesting and stimulating forum!

    First of all can I withdraw my earlier assertion that lithopanspermia is generally accepted. It evidently isn’t from this disucssion although I had taken it as such from the tone of a number of recent papers and my own assessment of the balance of evidence.

    Rob – good point about about subsurface organisms – I certainly didn’t mean my initial comment as being comprehensive of all possibilities, merely to suggest that environments such as Pluto may not be irrelevant from an astro-biological point of view, if it actually does have subsurface liquid water, with lithopanspermia as a suggested example mechanism (with whatever level of confidence!)

    Eniac – Whilst it is clearly an acceptable position to not accept a new theory until the level of evidence has become almost impossible to refute (and Kuhn pointed out that this usually requires a crisis in previous theories also) I would just like to suggest that your points 1, 3 and 4 are definitively established as possible for some micro-organisms in some circumstances. I must acknowledge my earlier error in taking current laboratory and other data on the survival of micro-organisms in space environements as definitive for relatively short timescales (years) – there is clearly room for debate as to if these results would continue to be relevant in a genuine deep space environment. At the moment however the evidence is that some micro-organisms do survive simulated space environments, so I would suggest that the balance of probability is in favour of your point 2 not being a show stopper. It certainly can not be said that lithopanspermia is falsified by current data.

    In terms of the mico-organism finding a suitable environment…it was really my initial surprise at the proposal that Pluto could have subsurface water that triggered the whole discussion. If a suitable environment is there and it is possible to access it (e.g. fractured surfaces etc.) then it really becomes a probability question for each specific event. I would not even begin to venture an opinion on if Pluto does or does not have such an (accessible) environment…but would suggest a distinct possibility (I would personally argue probability) that if it did then life would get to it.

    I do think you are right that ideas such as panspermia (or any idea that is outside the current paradigm) will not be totally accepted until it becomes essentially impossible not to accept it. On balance this is probably a good thing as normative science acts to provide an extreme hurdle in terms of the burden of evidence required for new ideas to be accepted, serving to significantly reduce the risk of prematurely accepting hypothesis…the cost being errors in the opposite direction, of course. I suspect that as someone who is not a professional scientist I am applying a slightly less asymetric requirement in forming my own judgements – whilst I entirely accept that this could be criticised and, I hasten to add, not having any absolute belief in anything (or at least I try not to!), just a judgement on the balance of probabilities.

    Anyway – I shall try to be more careful in my choice of wording in future…perhaps ‘increasingly well supported by the evidence’ might be the phrase I’ll use in future!

    Best wishes, and thanks once again to all commentators for a most interesting and thought provkoking discussion.

  • Rob Henry December 1, 2011, 18:27

    I’m ready with my version of the most shocking theory that I think could be physically possible. Here goes…

    Both Pluto and Charon will be found to posses powerful magnetic fields. The salty ocean currents that produce them are somehow energised by the interplay between them, such that they are rapidly (in the astronomical context) spiralling in toward each other. This causes such high levels of volcanism that it explains Charon’s anomalously high reflectivity, but Pluto’s proportionately lower levels of heating bring a lesser quantity of covering, such that periodic evaporation in its less cryovolcanic regions reveals surface deposits of organics that have been reddened by long periods of uv exposure.

    I note that it is not the spin as of itself that would help generate magnetic fields on any body, but the differential internal rates of spin that are produced as a secondary consequence of that spin, and in this scenario these would be expected to be much higher that produced in more typical scenarios for field generation.

  • kalish December 3, 2011, 15:42

    Hi, the panspermia theory is not really clear on what does it mean to be “possible”. I mean, it is really possible that SOME extreme organisms put on hidden parts of a space probe in route to pluto survive after all. But is it really possible that any organism stuck on the meteorite ejected from the earth to pluto survive during the journey?… I bet it is so tiny probable it becomes impossible. A meteorite is not a spaceship or a probe, a common organism is not an extreme one, and an extreme one is often so higly specific that it is unlikely it will adapt to its new environment.
    Just take a look at our common bacteria. Just put them in the desert, they will die quickly without feeding. How many disease have been stopped just by quarantine? A bacteria needs to sustain itself, if it doesn’t it dies excepted if it has some hibernation mechanism.

    Sorry for the English, I’m a foreigner.


  • Eniac December 4, 2011, 11:13


    That changes everything!

    It may change a lot, but I do not think it changes the validity of the Kalish’s (or my earlier) analysis very much … Critters that are used to the warmth and high pressure underground would not do too well when thrown into space. Being used to a stable environment without wind or water to spread them, they would not be likely to form spores or have other mechanisms for sustaining travel through hostile environments. Plus, they would likely find any surface on which they land to be inhospitable, being used to the deep underground.

  • Anthony Mugan December 7, 2011, 11:11

    hello Kalish and Eniac

    Can’t help but feel we’ve been reading different papers on the topic of panspermia…my ealier post with the reference to Paul Wesson’s paper may be relevant, together with the range of references therin.

    I can only speak for myself, but ‘possible’ to me means clear labororatory data and clearly established physics for every stage of the process. Not certain – I accept that, until someone actually tracks the complete process from end to end in reality, but possible…

  • Eniac December 10, 2011, 13:34

    Eniac – Whilst it is clearly an acceptable position to not accept a new theory until the level of evidence has become almost impossible to refute (and Kuhn pointed out that this usually requires a crisis in previous theories also) I would just like to suggest that your points 1, 3 and 4 are definitively established as possible for some micro-organisms in some circumstances.

    Anthony – I wasn’t asking for lots of irrefutable evidence. It would be progress if even a single shred of it existed. Not speculation that something might be physically possible under certain circumstances, but evidence that it has actually happened. Not lots, not irrefutable, just any evidence at all.

    There is a huge difference between possible and likely, and what I am questioning in this case is the latter.

  • ljk December 23, 2011, 12:29

    Surface of Pluto May Contain Organic Molecules

    SPACE.com Staff

    Date: 21 December 2011

    Time: 07:00 AM ET

    The Hubble Space Telescope has spotted new evidence of complex organic molecules — the carbon-containing building blocks of life as we know it — on the frigid surface of Pluto, a new study finds.

    Hubble observations revealed that some substances on Pluto’s surface are absorbing more ultraviolet light than expected. The compounds in question may well be organics, possibly complex hydrocarbons or nitrogen-containing molecules, researchers said.

    The dwarf planet Pluto is known to harbor ices of methane, carbon monoxide and nitrogen on its surface. The ultraviolet-absorbing chemical species may have been produced when sunlight or super-speedy subatomic particles known as cosmic rays interacted with these ices, researchers said.

    Full article here: