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Unmasking Europa: Of Ice and Controversy

You wouldn’t think the thickness of ice on a distant moon of Jupiter could emerge as something of a political hot-button, but that seems to be what has happened in the ongoing investigation of Europa. Thick ice or thin? The question is more complicated than it looks, because by ‘thin’ ice we don’t mean just a few inches, but perhaps ten kilometers, perhaps five. The key question is not a specific measurement, but whether the ice is thin enough to allow the surface and the global ocean beneath to be connected, in the form of occasional cracks, melt-throughs or other events.


Much hinges on the answer. As Richard Greenberg shows in Unmasking Europa: The Search for Life on Jupiter’s Ocean Moon (Springer, 2008), the small world quickly fell under the scrutiny of scientists with a geological bent after first Voyager and then Galileo imagery became available. The latter was a problem, for the failure of the spacecraft’s high-gain antenna meant the total number of images was sharply reduced, and many questions that might have been resolved by now continue to be controversial due to lack of data. What we do have from Europa is on the order of 1,000 images, a small enough selection that, as Greenberg recently told me, it fits on a single CD.

The upcoming Europa Jupiter System Mission should remedy the lack. But that mission won’t get us back to Jupiter before 2026, and in the meantime, there is much analysis that can still be done on older datasets. And if you read Unmasking Europa, with its painstaking analysis of numerous Europa images, you’ll come away with the sense that the geological model is on extremely thin ice here. Yes, solids can often mimic the flow of liquids, as can be shown right here on Earth, but take a look at an area of so-called ‘chaotic terrain’ on Europa and the visible evidence for melt-through of the ice, followed by reformation in a new configuration, seems palpable.


Image: Conamara Chaos region imaged during the Galileo spacecraft’s E6 orbit . Resolution is 180 m/pixel. Credit: NASA/JPL/University of Arizona Lunar and Planetary Laboratory.

We’re looking at chunks of crust that have undergone significant disruption, the re-frozen result of movement over the ocean beneath, following some kind of melting event. At least, that’s an interpretation that seems consistent with every image of such chaotic terrain that Greenberg shows, and he goes on to demonstrate that you can clip images of such terrain in order to ‘reassemble’ the features that had been there before the ‘rafts’ of individual material — often including distinctive ridges — broke away and floated into new positions.

I mentioned earlier the curiosity of the thickness of ice becoming a political issue, but in the highly-charged world of planetary science, that is exactly what happened. The debate has favored the thick ice model from the outset, but the fact that the new mission (Greenberg is on the design team for it, as are many proponents of the thick ice model) will have as a major goal to determine the thickness of the ice shows that Greenberg’s battle for a much thinner ice coating has had its effect.

The issue, of course, is critical for future study. An ice sheet a hundred kilometers thick makes investigation through some kind of ‘melt-through’ probe and submarine exploration that much harder, and diminishes the chances for life. But Greenberg, a celestial mechanician, demonstrated how powerful are the tidal forces acting on the Jovian satellites, particularly Io and Europa, and how much heat they may produce. Thin ice is a model that seems well supported by Greenberg’s evidence, and one that makes the possibilities of detecting life considerably more likely.

We discussed this by phone the other day in terms of the early study of these matters and how it explained what Voyager first found at Jupiter:

“When it was first proposed by Stan Peale and people at NASA Ames that you would have enough heat to do a lot at Io and even keep the H2O melted at Europa, that was such an outrageous idea that its proponents had to be extremely conservative. To make that case you had to say that even with a modest amount of heat, the water can remain liquid. If they’d said that a lot of heat could keep almost all the water liquid, then that would have been radical and hard to sell. But as time has gone on it has become apparent there is no problem having lots more heat. The amount of tidal heating of Io is way beyond what they had estimated, and the same is true of Europa.”

Yes, and what a surprise Voyager scientists had after Peale and team’s prescient call, just weeks before the flyby, that both Europa and Io should be very lively places on the basis of tidal effects alone! Greenberg recalls slapping his hand to his forehead when discussing the idea with Peale, realizing that the numbers pointed inexorably to the outcome that Voyager actually found — volcanic activity on a tortured Io, and growing evidence for substantial amounts of water inside Europa.

So much rides on this debate. For if there is demonstrated interaction between the global ocean and Europa’s ice sheet, we may not have to use submarines or drilling apparatus at all. A probe won’t last long in Europa’s radiation-bathed environs, but it could be shielded to last long enough to land near a Europan ridge where cracked ice may have exposed indigenous life forms to quick re-freezing. Moreover, not just tidal effects but underwater volcanic activity could be keeping this pattern of ice disruption alive and enhancing the likelihood of life.

This is a peppery, fascinating account that spares no detail in analyzing the varied topography of Jupiter’s ocean moon, but also offers a look inside the science of major space missions and how they are managed. It is filled with insights into the problems of observational bias in analyzing images and the all too human failing of seeing what we want to see. Can a geological model, of necessity assuming a thick ice mantle, explain the numerous features Greenberg analyzes here? It is hard to see how, for the model doesn’t adapt well to this non-Terran environment, even if it will take another mission to nail down the case once and for all.

Comments on this entry are closed.

  • James M. Essig February 27, 2009, 11:34

    Hi Folks;

    I would definately settle for an ET crustacean or some other cold water dwelling life. It might be the case that the European ice sheet has saline and briny water pockets within that; although perhaps much colder than any subsurface temperate or warm ocean. might still support life. An analogue here on Earth would be the existence of some type of optical light visionless fish, tube worm like creature, or other simple animal lifeforms as may exist in subsurface lakes beneath the 4 kilometer thick ice cap regions of Antartica.

    Either way, much of the readership of Tau Zero Centauri Dreams will still be alive in 2026 to see what the next Jupiter mission brings. All I can say is eat healthy and excercise if you do not do so already because things are really starting to get fun with planetary science and geology and the strong likelyhood that we will discover extra solar planetary systems with the biosignature of life.



  • andy February 27, 2009, 18:43

    The only certainty in exploration of the satellite systems of gas giant planets is surprise. The next obviously interesting target that hasn’t been surveyed since Voyager is Triton, but I seriously doubt I’ll see a Triton mission in my lifetime. Could do with a surprise there.

  • Adam February 27, 2009, 20:09

    Hi All

    I’m optimistic about Europa as we know it has some many varied energy inputs and oxidants. It would be fantastic if macro-fauna were confirmed, but even microbial life based on a different genetic substrate would be fantastic. If panspermia has delivered our kind of life to Europa successfully that would be interesting, though kind of disappointing. Life akin to ours, but subtly different though has a certain appeal. Could proteins with different amino acids be used? Or PNA instead of DNA? Or more sulfur and less nitrogen?

    Perhaps Freeman Dyson’s suggestion of freeze dried organic remnants in circum-Jovian orbit is one way could sample the biosphere beneath the ice?

  • Administrator February 27, 2009, 20:58

    My guess is that Triton will one day prove a seriously interesting target!

    And Adam, can you amplify your comment about Freeman Dyson? I’m not familiar with his idea about organic remnants in Jupiter space.

  • Goldstein Hovercraft February 27, 2009, 21:18

    Paul, regarding Freeman Dyson’s idea regarding freeze-dried fish, to read more, you can revisit the comments in this blog post: https://centauri-dreams.org/?p=1660 ! I’m continually impressed with the sheer volume and as well as the interestingness of your blog postings, and it is easy to imagine that you wouldn’t recall a posting and subsequent conversation in the comments from over a year ago!

  • Adam February 28, 2009, 3:45

    Hi Paul

    Haven’t you heard that line? He mentioned the idea in a piece about astrobiology’s prospects – best place to look is in orbit around Jupiter due to bits of Europa being blown into space by meteorite impacts.

    Europa’s ocean is interesting. It’s most likely a liquid ocean, warm and very deep. Europa’s the second satellite out from Jupiter. The inner satellite, Io, is blazing hot; it has volcanoes. The other satellites are frozen solid. In between there’s Europa, which has a thin layer of cracked ice. If you want to find creatures living in Europa’s ocean, you can do it the hard way – send a huge spacecraft carrying a submarine, dig through the ice, then launch the submarine to explore the ocean. Or you can do it the easy way. We know the other satellites have huge numbers of craters from being close to the asteroid belt. So what happens when Europa is hit with a huge asteroid? It will splash out immense quantities of water into space. If there are any fish present, they will be kicked out and freeze dried, and you’ll find them orbiting around Jupiter. There is already a ring of debris orbiting Jupiter, but nobody has gone to see if there are any freeze-dried fish. It’s a clever way to explore.

    From and old Wired interview here.

  • Administrator February 28, 2009, 10:43

    Goldstein Hovercraft, I thought something about the Dyson rang a bell — had no idea it was from a previous post here, but I went back at your direction and there it was. Strange the tricks memory plays! Adam’s link to the Wired interview is also helpful. Thanks to both of you.

  • george scaglione February 28, 2009, 11:00

    hello all – does anybody recall the old saying “your on thin ice?” and YES the moons of jupiter and saturn will in my mind be without doubt facinating places to explore. maybe even revealing the first life in space outside the earth! additionally i’d like to ad my voice to that of goldstein above in my sincere congrats to paul for doing such an outstanding job on this site!!! thanks one and all your friend george

  • Adam March 1, 2009, 6:20

    I must also mention that Ganymede in some ways is more interesting than Europa due to its intrinsic, rather than merely induced, magnetic field and the apparent ice/silicate volcanism that has extensively reworked its surface. There’s something going on between the three inner Galileans, some mutual eccentricity pumping or some such, which makes them all a lot more active than anyone imagined prior to Voyager – though Io’s volcanism was predicted just before the first Voyager 1 photos.

    John Lewis, and others, predicted the sub-ice oceans in the 1970s, but no one predicted the apparent thinness of Europa’s ice crust. I think it’s a bit unfair to describe the “thin or thick” dichotomy as political – the low temperature rheology of multi-kilometre thicknesses of ice is full of unknowns. A lot of the convection or conduction physics depends on the detailed crystal structure, salts and porosity, and the direct data is so limited. NASA erred on the side of caution on Mars and the presence or not of Life-like soil chemistry, so likewise on the issue of oceans on Europa. A vast organisation doesn’t want the collective shame of being proven to be incautious about something as explosive as the possibility of Life. Especially – as many of us now hope about Europa – complex Life.

  • Administrator March 2, 2009, 10:21

    I can’t deny the number of unknowns in the study of ice on a Jovian moon! But the political aspect of this study also seems evident, as it would have to be. Let’s face it, we’re dealing with matters where whose graduate student gets what preferment, which lab gets what funding, which scientists land on the appropriate planning team, are relevant not just to science but to careers and institutional well-being. So I think there’s an inescapable issue of money and gamesmanship whenever future big missions are discussed. Greenberg has many particulars in the book, and I’m sure they’re not uncommon.

    Re the inner three Galilean moons, Stan Peale’s team was able to come up with their sensationally accurate call on Io and Europa after Peale recognized that Europa’s orbital eccentricity was not zero. That set up the hugely significant tidal effects that make Europa so interesting. As to Ganymede, interesting place indeed! Orbital resonances have made Jupiter’s larger satellites a fascinating study, with poor Callisto odd man out.

  • andy March 2, 2009, 15:33

    Thanks to the fact it has an intrinsic magnetic field, Ganymede is probably the best analogue in our solar system to the icy super-Earths in extrasolar systems, as well as being a fascinating object in its own right.

  • ljk March 13, 2009, 17:18

    Is Europa and its three Galilean cohorts the survivors of five
    generations of earlier moons that were “consumed” by Jupiter?


  • ljk May 13, 2009, 20:32

    May 12, 2009

    What Life Might Look Like on Jupiter’s Europa: New Extreme Species Discovered

    Wonder what life of Jupiter’s moon, Europa, might look like? Checkout a new species of archaebacteria, Pyrococcus CH1,discovered thriving on a mid-Atlantic ridge within a temperature range of 80 to 105°C and able to divide itself up to a hydrostatic pressure of 120 Mpa (1000 times higher than the atmospheric pressure). Alieve won’t help down there.


  • Tom August 11, 2009, 15:21

    I don’t see that pictures like the one shown can be used to infer the depth of the ice. Yes, they show breakaway ice sections that have frozen in place, but that would be the case for any planet that had a very thin ice crust at one point and has slowly cooled. Imagine a planet cooling from an ocean world to a solid ice world, firstly there will be patches of ice on the surface, these will grow more numerous and float around, at some critical temperature, the thin parts of the frozen surface will freeze thick enough that further movement is only restricted to the large cracks seen in Europa pictures. From then on, the surface will be frozen in whatever state it was in, regardless of how thick the ice below becomes. So frozen chunks of ice do not give any indication of the depth of the crust today…. in my opinion.

  • ljk September 2, 2009, 0:44

    Is Jupiter’s Bizarre Moon Our Best Hope for Finding Extraterrestrial Life?

    NASA is gambling $4 billion that there’s life beneath the thin atmosphere, lethal radiation, and miles-thick ice on Europa.

    by Andrew Lawler

    From the September 2009 issue, published online August 28, 2009

    Full article here:


    To quote:

    To most researchers the nature of the ice determines whether Europa really is a plausible home for alien life. Bluntly put, the ice could be either a protective canopy or a killing lid, and the Europa Jupiter System Mission aims to figure out which.

    “Thick ice does mean it’s harder for life,” Pappalardo says. Then he hedges a bit: “It certainly doesn’t rule it out.” Even if the ice is thick, there may be ways life could have originated and persisted on Europa. For instance, an active, churning mantle below the sea might bubble up heat and chemicals that could serve as the basis for biology.

    “We don’t know yet how thick the ice is, but I think the surface tells us there’s an interesting geology there,” says Ron Greeley, a planetary geologist at Arizona State University who cochairs Europa’s science definition team.

  • Rob Flores March 7, 2013, 20:56

    I’d like to ask those who know their nuclear physics.
    The Jupiter Radiation belt is a fearsome thing, but how does it
    alter ICE. Is there chemistry going on that would create a reserve of
    Energy that is created and then falls through when there is a
    melting event. In other worlds could Solar Energy indirectly be
    part of the thermodynamics of Europan Life

    Also, thinking about those cracks on the ice produced by the interactions
    with the other moons. What if the cracking of ice has made permanent
    plates on Europa. At the bounadaries of these plates, deep in the ice, the fracture would hold liquid water, while just near surface there might be
    an insulating “cap” of a few hundred meters. If there is no place for these
    plates to move, they would always be in the same position to be uplifted by
    gravity again, so a crack in the ice maybe a permanent feature. Glad we sending probes to find out. (Even though I really wanted another Titan mission)