Q&BA: Which moon has the best chance for life?

By Phil Plait | April 20, 2012 10:04 am

Every weekend when I can I do an interactive live video chat on Google+ where people can ask me questions about space and astronomy. I call it Q&BA, and it’s always fun to hear what questions are on people’s minds.

Apropos of my recent post about Saturn’s moon Enceladus, I got this question: "Which moon has the best chance for life: Titan, Europa, or Enceladus?" This is a common question, and worth exploring! Here’s what I said:

Mars is still an interesting place to look for life, but those moons — all three — are very, very enticing. I’d love to see us launching future space probes with some icy targets in their sights.

[P.S. The aspect ratio of the video is stretchy for some reason; the video looked fine before I uploaded it to YouTube. I'll try to track this problem down.]

I have an archive of Q&BA links and videos. Take a look and see if there are other ones that tickle your imagination.

Related Posts:

- Q&BA: Can we build a space habitat?
- Q&BA: The Science of Science Fiction
- Q&BA: How does a gravity slingshot work?
- Q&BA: Why spend money on NASA?
- Q&BA: What happens if you are exposed to the vacuum of space?

CATEGORIZED UNDER: Astronomy, Piece of mind, Q & BA, Space
MORE ABOUT: Enceladus, Europa, life, moons, Titan

Comments (27)

  1. Life in the Europan seas- me, I don’t think I could stand all the pressure….

  2. Chris

    The answer is obvious to anyone who watched 2010.

    All these worlds are yours except Europa. Attempt no landing there. Use them together. Use them in peace.

    I was wondering, is that nebula you use for the Q a real nebula or some photoshopped blob?

  3. Brian

    We already know of one moon that has had life on it in the past.

    Who knows when it will see life again.

  4. Paul Laughlin

    Hello Phil,
    I think all 3 choices have life in some extremeophile form. Since Kepler is finding planets almost daily, it seems almost crazy to think that there is not life out there beyond our beautiful blue Earth. I used to agree with Chris above, Arthur C. Clarke
    was a huge influence for my interests and fascination with the universe. I still think Europa has life under the ice, I just don’t think we can get an ice melting, ice drilling probe there for about 30 more years or longer, bummer! Enceladus seems to be a better choice now that Cassini has already sniffed some curious stuff in the geyser blasts.
    I’m with Carolyn Porco on this one. Let’s go meet the little guys at Enceladus and then we can all be much less lonely in all of this vast beauty ! Thanks for all your contributions to science Phil ! Paul Laughlin @LoftyKC on twitter

  5. Jeff in Tucson

    There was a really sweet colloquium at the University of Arizona’s Steward Observatory yesterday about weather on Titan. Podcast is at the bottom of this page:


  6. Let’s say we can actually travel to a place that could potentially support life but doesn’t have life (at least not that we can find).

    Does anyone have any moral qualms against us dropping off some bacteria or whatever to see if it can take hold?

  7. Chris

    @6 CafeenMan

    You’re basically describing terraforming. You’d have to be 100% certain that there wasn’t some native life living there. Even if the surface is sterile, you can never know about the deep rocks or some cave which might have developed some ecosystem. If we’ve learned anything on Earth, it’s that life can usually find a way of surviving and thriving even under the most extreme of circumstances.

    And let’s not forget that there might be a race of superintelligent salt microbrains living in the soil who will call us ugly bags of mostly water. (ST:TNG)

  8. hmm, let’s see, consulting with the Hitchhikers Guide… Seems ever since the bypass went through, it’s not a question of which can support life, but which also has the best food, sleeping accommodations and of course live music.

  9. Keith K

    Yes! More probes! Especially to those three.

    Cassini is, like, the greatest thing ever. As if it’s not enough just to get such amazing close-ups of that gorgeous planet.. those moons!

  10. Tara Li

    @8 Keith – sure, more probes. Probes send back pictures, measurements, occasionally even samples.

    They don’t send back stories, songs, poems, dances, letters & postcards. I’m tired of the bleepin’ probes. I find myself actually getting tired of looking at spiral galaxies. I think we need to quit with the probes, and focus on building infrastructure in space – and on the Moon, where we’re not fighting vacuum, free-fall, and the need to transport every single bit of construction material to the site, makes the most sense to me. Manned missions to Mars? Great – but if there’s a problem, help is *MONTHS* away, not 2-3 days away as the Moon is.

    Of course, even the Moon is not that close if you’re more focused on doing it *exactly* right, rather than simply *doing* it. Cargo runs, especially – if you can launch for 1/2 the cost, though it raises the failure rate from 0% to 10% – for the same money, you can fly 20 missions and have 18 go through, as opposed to 10 missions the other way.

    How much of the cost of flying the Shuttle was the cost of inspections, and re-inspections, and over-engineering to shave 1/1000th of a percent chance of failure?

  11. Tim Gaede

    Enceladus and Titan may offer the chance to pull off some pretty spectacular engineering feats. With Enceladus’s surface gravity being 1/90th that of Earth’s, a submarine should be able to dive about 90 times as deep. The moon’s rocky core lying some 80 to 100 km below the surface should be reachable. Titan’s surface gravity is 14% that of Earth’s and the density of the atmosphere about four times as great. A back-of-the-envelope calculation shows that the power requirements for keeping an aircraft aloft there is about 1/40th that for Earth. An airplane powered by the decay of plutonium-238 could stay aloft for years.

  12. One of my biggest pet peeves of all of the space program politics over the past few decades (ask my friends and co-workers, who are tired of hearing me relate it to everything that’s wrong with space exploration) was the cancellation of JIMO. One mission, multiple moons explored–absolutely ground-breaking, frontier-pushing science at a very feasible technological level. Very sad it was canceled, despite the fact that it had congressionally-mandated funding. Think of what we could have learned!

  13. Go ahead, send down a probe- and don’t complain when they do the same to you personally…

  14. Brett

    Enceladus is much farther away than Europa, but it might be easier to actually enter a hypothetical Enceladus ocean than a Europan one. As far as we know, the Europan ocean is underneath kilometers of ice, which will be very, very difficult to drill through. Enceladus is covered by ice as well, but the geysers at the south pole regularly break that ice and shoot water high up above the moon, so we might be able to slip a lander into the water in that thin area of fresh ice coverage after the eruptions.

  15. Torbjörn Larsson, OM

    A very well reasoned Q&BA; I approve of its message.


    Certainly Plait makes a good point in that our early environment wasn’t that good either.

    Or it could have been ideal for chemical evolution attempts to pass to biological evolution, with a hefty rate making up for a high extinction rate. Or, most likely, it came out in between, since modern extremophiles are derived and have advanced stress mechanisms. (Say, D. radiodurans.)

    While there have been less reasons to suggest that extremophiles were early in recent years, this may make a comeback. I just got hold of an interesting paper which rely on combining a requirement for early autotrophs with phylogenetic methods. ["The emergence and Early Evolution of Biological Carbon-Fixation", Braakman & Smith, PLoS Comp Bio, april 2012.]

    The result is that they get a tree structure over deep time and have to infer a root. The upshot for this discussion is that the early branches are all extremophiles.

    Note that the tree LUCA is at the time of the rise of oxygen, so these extremophiles had oxygen poisoning and energy strictures as evolutionary drivers. They were probably pushed back into hydrothermal vent and volcanic pool refuges, placing the LUCA at ~ 2.5 billion years before present (Ga bp) during the Great Oxygenation Event (GOE).

    This is consistent with other results based on protein families, where an early Archaean Expansion (AE) had a rate bottleneck at the GOE. ["Rapid evolutionary innovation during an Archaean genetic expansion", David & Alm, Nature, 2010.]

    On the other hand the metabolic tree LUCA is robust, the authors suggest because of being regulatory primitive, and its universal structure can encompass all life and reach far back. It includes the CoA pathway, so this presumed UCA existed at pre-AE times ~ 3.5 Ga bp according to the 2nd paper.

    Now I’m sure there are many problems in this. You insert a root, but the author notes it is for parsimony. The root folds the Archaea clade into the Bacteria, but it is deep into the tree as it should. The original oxygen producers are unidentified, it seems.

    And, whether the authors state it or not, you assume individual autotrophy was reigning instead of having cases of ecological heterotrophy, say getting carbon by mutualism or by predation. I assume the counter argument is that those relationships are less parsimonious and robust.

    In summary, while modern extremophiles may be derived, the need for extremophily seems to be rather early. (But maybe not pre-GOE, as unlikely such a history seems.)

    And the UCA environment had strong drivers for robust solutions in the face of what looks like a stressful environment without much diversity so little trophic & ecological complexity driving evolution. So if life made it here, it may have made it “in” Europa.

    Another problem for Europa is the oxygen supplied by the radiation hitting its surface ice. As seen from the above 1st paper, oxygen may be very poisonous for early cells. And it is known to be poisonous for chemical evolution (lowers organic production many orders of magnitude in general, and blocks pathways specifically).

    But I hear that astrophysicists have put the oxygen diffusion after the ice formation and specifically many millions of years later than earlier believed. Sorry to say I can’t get hold of that work. But it is promising for a biosphere. You may benefit from oxygen, but not if it is produced too early!

  16. Torbjörn Larsson, OM


    I, on the other hand, find myself getting tired of the same false choice presented in this area and especially when it is presented out of context. If you are not supporting science, you are part of the problem.


    Even cheaper is to circle Saturn and dip into the geysers with aerogel collectors Stardust style. Or land and just let the ice fall into the scope, both ideas now supported by Cassini team leader Porco.

    Either of those could now use Titan for a cheap aerobrake to Saturn orbit, I believe. I’m assuming you have excess relative velocity to delta-v off!? It was suggested for pre-Huygens I think, but the knowledge of the atmosphere was marginal. Cassini/Huygens changed that!

  17. Torbjörn Larsson, OM

    The difference between geyser “dipping” and ice collection, even by scoping it up from the surface, is that the ice and perhaps not extant cells would survive the collision with the collector. So the collection process is partly destroying, I’ll bet, perhaps even changing the organic inventory by collisional heating. But you would still learn awesome stuff!

  18. Tom

    Perhaps a better question right now is which moon would be easiest (and cheapest) to investigate. As 15 and 17 say, Enceladus seems like the clear winner by that metric.

    Kiss the surface, scoop up some of the snow-like ice that has floated back down to the surface from an eruption (no worries about high-speed collection, 17), and let the onboard lab figure out what you found. No need to burrow down to the liquid water, right?

  19. Jason Weitzel

    @19 Tom

    It’s not that simple as any micro life that came out of the geysers would have been blasted to dust due to pressure. What Cassini “tastes” when it flies through the plumes of Enceladus is pretty much what we can expect to find on the surface of the moon – 80% water, -20% organic compunds.

    I’m with Caroline Porko and think it’s our best chance, but i think we’ll still have to do a little digging.

  20. Gwif

    Here’s a better question:
    Which moon has the best resources for building a human base? (resources for life support, as well as manufacturing, energy, growth, and eventual export)

  21. Good Q&BA Phil. Cheers. It seems quite possible that all these trio of outer moons have life inside if not on them.
    I can understand why you wimped out there. ;-)

    My own personal educated~ish guesswork answer would be Europa which is warmer than Titan and has a wider life zone for that to arise and evolve in than Enceladus does.

    But each moon has its pro’s and con’s.

    Europa has the pro’s of that larger sub-surface ocean with probably liquid water and energy – at reasonably “cosy” temperatures (& pressures?) and with lots of space available relatively speaking. (Everything is relatively speaking here really.)

    Europa’s cons – high radiation from the Jovian magnetosphere. Unknown chemistry and so many uncertainties on the exact details of that sub-surface ocean. The absence of lightning -at least we think its absent could also have negative implications for Europan life maybe?

    Titan has the pros of having the most space and most varied environments plus its atmosphere and we know there’s complex chemistry on its surface – but cons are the extreme cold of its temperature and thelife it might have may well not be carbon-based life as we know it.

    Enceladus has tiger stripes and sulci whichare unique and energetic and water rich environments but its so small and how much of that environment is really good forcretaing and sustaining life?

    There’s really insufficent evdience and the only way toknow for sure is to go investigate all three! 8)

  22. Nigel Depledge

    It seems to me that Titan is far too cold for anything that we would recognise as life.

    It is typical for chemical reactions to slow down by a factor of 2 for every 10 °C reduction in temperature, so if there is life on Titan, how would we go about recognising it as such? Its pace of existence would be orders of magnitude slower than anything that we see on Earth.

  23. Jeff

    I don’t know, I think all these would be too cold for life. I am confident carbon chauvanism, water chauvinism, and warm temp. chauvanism are the bare minimums to start life, and even then, only earth might have “life” in the entire history of the universe. I think Stan Friedman is correct that astrobiology is like throwing darts, although I know UFOs are bunk. I just don’t think we understand the details of how life started and so are missing big parts of the puzzle of life’s origin.

    And even on earth, why did animals evolve? I’m sure they have an answer, but I myself think plants are a much better design (and I also think ID and creation science are pseudoscience bunk). I mean, they don’t need a “home” , they are planted home and just stick their leaves upward and make their own food. Who needs animals?

  24. Messier Tidy Upper

    @ ^ Jeff : Plants do! Y’know for pollination, seed dispersal and stuff. ;-)

    (Ok, guess that’s only *some* plants but still.)

  25. Messier Tidy Upper

    @21. Gwif Says:

    Here’s a better question:
    Which moon has the best resources for building a human base? (resources for life support, as well as manufacturing, energy, growth, and eventual export)

    Europa is closest of the three and has abundant water – but is buried deep in the Jovian radiation zone making it perhaps too deadly to visit in person. Given Europa is about the size of our Moon and less dense, its gravity well is small so we should get stuff off it easily -but then Jupiter’s gravity well is pretty huge so could be an issue there.

    Titan and Enceladus are equally far away in Saturnean orbit – with Saturn’s graviy well and radiation far lesser than Jove’s but being more distant does make them harder to reach.

    Titan has pros & cons being larger and perhaps more varied in resources with an atmosphere which could both help (aerobraking, atmospheric flight, drawing in and processing atmosphere for various uses) and hinder. (air resistence, wind erosion, blocking solar power – not that’ there’s much of that that far out anyhow!)

    Ditto Enceladus which has plenty of water, geological activity which could again help or hinder colonisation. (Threat of vulcanism and maybe changing landscapes vs access to geothermal power source)

    There’s something to be said for and against each of them and so it all depends what technology we’ve got then and what we’re most interested in doing or gaining from settling there.

    Hopefully one day Humans will be able to live on or be able to visit all three of these worlds.

  26. Nigel Depledge

    Jeff (24) said:

    And even on earth, why did animals evolve? I’m sure they have an answer, but I myself think plants are a much better design (and I also think ID and creation science are pseudoscience bunk). I mean, they don’t need a “home” , they are planted home and just stick their leaves upward and make their own food. Who needs animals?

    Why should every organism go to the awkward and difficult lengths to manufacture food from CO2, water and sunlight, when there’s plenty of organisms all around you that just do it for you. All you have to do is ingest and digest them to get that benefit.

    The same argument applies to why there are predators.


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