ResearchBlogging.orgEuropa is a moon of Jupiter, the smallest of the four Jovian moons discovered by Galileo in 1610. Juipter has 63 objects circling it that are called moons, though only eight of them are “regular” in their orbit and other characteristics. The rest are bits and pieces of clumped up matter that were probably captured by Jupiter’s big-ass gravitational field, and have irregular orbits, i.e., they go the wrong-way around the planet, or are not in the solar plane, etc.

Europa is almost as big as the Earth’s moon, probably has an iron core, and is otherwise made of silica. There is an atmosphere made mostly of oxygen, and the surface is covered with ice made of water. Beneath the ice might be liquid water. Although Europa is not sufficiently warmed by the sun to have enough warmth for nice M-class conditions, there is heat provided by the giant planet’s gravitational interactions with its moon. Any largish object in a strong gravitational field will experience differential pull across its mass. Depending on what the object is made of and other factors, it can simply heat up due to the forces of gravity being converted to kinetic energy at a molecular level.

Europa’s surface is nearly devoid of impact scars and is covered with cracks and other features that together indicate a liquid water layer beneath the ice surface, as well as melting and refreezing of the surface. Unless catastrophic life-unfriendly events are common on Europa, if it does indeed have constant liquid water owing to tidally induced warmth, then there IS life on this moon!

Maybe. If live is the kind of thing that evolves wherever certain conditions are present, then there is a pretty good chance that there is life there. Or at least, the chances of life on Europa in the past or present are good enough that if one is serious about one’s astrobiology, one would want to go there, a feat best accomplished vicariously through the use of probes and robots.

There is a controversy among planetary scientists as to how think the ice on Europa is. Some say that it is very thick, others say that it is either thin or has the capacity to become very thinned out over large period and for considerable periods of time. The thickness vs. thinness of ice on Europa, and the frequency with which water is exposed directly to the atmosphere will impact both the likelihood of life being there and what that life may be up to, as well as any strategy to examine it directly. For instance, a thin layer of ice would allow some interaction between the atmosphere and the liquid below, and certainly, any regular breaks would enhance this. This brings up several issues not only related to methods and approaches to the exploration of Europa, but also, ethics.

What if there is life there? What if, for instance, life is present in the sub-ice sea like so many walleye in a Minnesota lake in January, but exposure of the sea via holes punched in the surface (by meteors, for example) is deadly and causes local extinctions? An Earthling probe would arrive on the surface of the ice, drill a hole through it like a fisherman on a Minnesota lake in January, and as soon as the hole is punched through, mayhem ensues. Or, if the estimate of thickness of the ice is way off, maybe the machine we send will go crashing through the ice. Like a fisherman on a Minnesota lake in late February. If the life is complex and intelligent, perhaps the Earthling probe’s blunder will kill Europa’s leader. Or a school full of beloved Europan children. Or, if we are lucky, the Europan equivalent of Adolph Hitler.

Of course, most likely, life on Europa would not have intelligence. For that matter, it might not even have multicellularity. Or any cellularity. Which is why we want to look … if there is life there, we want to see what the heck it looks like, and learn how it works, and try to figure out how it came to be and how it may have changed over time, and so on and so forth, because a) that would be so freakin’ cool and b) we would learn so much about what life is, simply by adding to our sample of instances of life in our solar system.

If Europan life includes children and philosophers and workers and criminals, it would be highly unethical to do something that did damage to it … them … whatever. But even if Europan life is essentially bacterial, it would be wrong to mess up the ecology of another planet by simply arriving there and mucking around. And there are plans to go there. Therefore perhaps there should be plans to go there in a way to minimize harm, and that take into account the full range of models of how the ice is distributed, how thick it is, and how it changes over time. This is the subject of a recent paper by Richard Greenberg, who is the author of Unmasking Europa: The Search for Life on Jupiter’s Ocean Moon. Greenberg is one of the scientists who thinks the ice surface of Europa is thin and dynamic and that there is significant ocean-atmosphere interaction. That and the nasty big-science politics linked to this debate are covered in his book. I’m sure the paper, which he sent me last week, will spark some controversy.

Here’s the abstract from Greenberg’s paper, published in Astrobiology:

Europa has become a high-priority objective for exploration because it may harbor life. Strategic planning for its exploration has been predicated on an extreme model in which the expected oceanic biosphere lies under a thick ice crust, buried too deep to be reached in the foreseeable future, which would beg the question of whether other active satellites might be more realistic objectives. However, Europa’s ice may in fact be permeable, with very different implications for the possibilities for life and for mission planning. A biosphere may extend up to near the surface, making life far more readily accessible to exploration while at the same time making it vulnerable to contamination. The chances of finding life on Europa are substantially improved while the need for planetary protection becomes essential. The new National Research Council planetary protection study will need to go beyond its current mandate if meaningful standards are to be put in place. Key Words: Europa–Planetary protection–Mission planning–Spacecraft–Geology. Astrobiology 11, 183-191.

The long term plan for exploration of Europa involves three phases: First, an orbiter around the moon, then a lander to explore the surface of the ice, and later a rig to drill through what is assumed (by the dominant thick-ice school of thought) to be a few thousand meters of ice and into the ocean below. This model is predicated on the idea that the Europa’s ocean is fully separate from its atmosphere, and that life on that moon would have evolved along some Europan equivalent of Earth’s hydrothermal vents.

One of Greenberg’s complaints about this stems from the mission’s obvious near-impossibility, especially with respect to the deep drilling part. The assumption of thick ice leads to an assumption of great technical difficulty in the mission which in turn leads to the schedule for the mission being extended significantly. At this point, the orbiter, originally scheduled to have been sent there already, will not be in place until nearly 2030. Greenberg claims that since that plan was originally developed in the 1990s our conception of Europa has changed enough that the mission itself needs to change as well. From the paper:

If Europa’s ice is in fact permeable, it would have major implications for mission planning, because organisms or their markers would be readily accessible near the surface. The prospects for discovering extraterrestrial life in the foreseeable future would improve immensely. At the same time, a europan ecosystem would be highly vulnerable to contamination, so planetary protection becomes a critical issue.

Greenberg goes on to describe his updated model of Europa’s surface conditions and the dynamics of the ice surface, and to suggest alternative views of Europa’s possible biosphere. And, of course, he makes the point that if the ice is permeable, the prospects for life there are enhanced significantly.

There is a lot more to the discussion of ethics (and the evolution over time of ethical guidelines in space exploration) that I will not go into here, but it’s all in the paper. Unfortunately, you may have to subscribe to Astrobiology to get it. If you have questions, put them in the comments and perhaps I can persuade Dr. Greenberg to have a look.

Greenberg, R. (2011). Exploration and Protection of Europa’s Biosphere: Implications of Permeable Ice Astrobiology, 11 (2), 183-191 DOI: 10.1089/ast.2011.0608

Comments

  1. #1 daedalus2u
    April 4, 2011

    The radiation at the surface is not insignificant. It is about 5.4 Sv per day.

  2. #2 Juice
    April 4, 2011

    Define “thin.” Are we talking a few miles or a few yards?

  3. #3 ppnl
    April 4, 2011

    It seems likely that there would be evidence of life on the surface even if the ice is thick. Even on earth the deep layer of molten rock is separated by a thick layer of solid rock and yet some of manages to find its way to the surface.

  4. #4 Greg Laden
    April 4, 2011

    You’all have to read the book.

  5. #5 daedalus2u
    April 4, 2011

    A few tens of km. I would not call that “thin”.

    It has a surface gravity of 0.134 g, and a surface temperature of ~100 K.

    Because ice has a lower density than water, there is always going to be a layer of ice at the surface unless there is a gigantic heat flux. The maximum density of liquid water is ~4 C, well above the freezing point, so there can be a stable film of liquid water adjacent to the ice. Ice is going to sublime unless it is very cold, -100 C or so. The heat of vaporization is ~7x the heat of vaporization, so the sublimation of one inch of ice removes enough heat to freeze 7 inches of ice. Any liquid surface will be very short lived. Freezing from the top down is going to cause impurities to migrate down.

    There is probably a very large production of H2O2 in the ice from the radiation. That is heavier than ice, and H2O-H2O2 form a eutectic at ~ -55 C. In a temperature gradient, H2O2 will tend to migrate to the hotter region (that would be down).

    The high radiation field will generate lots of oxidizing species, O2, O3, O2-, hydroxyl radical, H2O2. There might not be much organic on the surface, it might be all oxidized away.

  6. #6 Patrick
    April 5, 2011

    This is cool stuff. Question from a non-scientist:

    Doesn’t the ice on the surface of Europa originally come from deep down? If so, then even if life on Europa is deep down, couldn’t the ice on the surface contain frozen bacteria-equivalents in it? When liquid water erupted onto the surface the bacteria-equivalents would have been carried upward with it, and frozen with it too.

    It seems that examining the ice on the surface skirts the potential ethical issues. The bacteria-equivalents on the surface are fair game. Messing with them cannot hurt the ecosystems from which they originate.

    If we found dead (or at least dormant) bacteria-equivalents in the surface ice, then we’d have grounds for return trips to drill deeper and to look for more complex critters.

  7. #7 Greg Laden
    April 5, 2011

    The grizzlies TT encountered were well fed – that makes a difference because their natural food sources were satisfying them and they did not need to seek new food sources like human garbage.

    At Europa’s air pressure in primarily oxygen?

    Regarding ice thickiness: The life is not on the surface. (If it was, I would think there’d be a signature of that.) The standard model says the is is 20 km thick. The whole point of this discussion, though, is that Greenberg is arguing that the is is not that thick (though he does not calculate an actual value) and that the surface features of Europa indicate frequent communication between the “isolated” ocean and the surface. (The standard model interprets the surface features differently.)

  8. #8 Greg Laden
    April 5, 2011

    Patrick: Good question.

  9. #9 Alan
    April 5, 2011

    Although I’d love to see what’s below the ice I think the best I can hope for in my lifetime will be to find out what that reddish scum on the surface is. We expect impurities to sink, so it’s mere existance is intriging.
    I also think the contamination thing is a bit over-rated, the locals will be much better adapted to the Europan environment and are likely to simply eat any Earthly hitch-hikers that somehow manage to survive the dehydration, starvation, and radiation of the journey.

  10. #10 Richard Greenberg
    April 5, 2011

    Interesting discussion! Let me try to clarify a few points. First, Europa does not have much of an atmosphere. Radiation produces oxidants (oxygen and hydrogen peroxide), which get mixed into the water ice. I have calculated that a lot of the oxidants may get into the ocean and supply a complex ecosystem. Second, the issue of “thick vs. thin” is better described as permeable vs. impermeable. I have long been a vocal advocate for the permeable ice model (with ice thinner than about 10 km), which enhances the prospects for life and also makes it more vulnerable to contamination. Finally, the likely direct linkages from the ocean to the surface (through cracks, melt-through, and impacts) mean that we might be able to sample life (or its remains) within cm of the surface, rather than waiting to drill down to the ocean. This concept has important implications for exploration strategy. For more, read my book, “Unmasking Europa, the Search for Life on Jupiter’s Ocean Moon”.

  11. #11 Mu
    April 7, 2011

    While a nice philosophic argument, the idea that a single probe will somehow change Europe’s ecosystem on a planetary scale is ridiculous. What Earth-bound organism would survive long enough in the environment described (100K, oxidizing, high radiation)to outcompete any local species adapted to the environment? And the likelihood that the lander will land on the head of the leader is negligible on simple geometry ground alone.
    You might argue that impacters or detonation charges to measure ice thickness are unethical before we’ve determined the sterility of the surface, but that doesn’t make the lander itself “forbidden”.
    So you might back off if you find any large black monolith orbiting.

  12. #12 JesseS
    April 7, 2011

    The comments here seem to focus on hitchhiking earth based organisms as ‘contamination’ but when I read the article my first thoughts were ‘pollution’. Aside from the new hole in the ice letting significantly more radiation through there’s all the chemicals the lander and drill will bring with them.

    Not a Europa-wide problem but still an ethical question.

  13. #13 Sigmund
    April 8, 2011

    Europa is not the only non-Earth body that is suspected of having liquid water present. Enceladus, one of Saturn’s moons is also believed to have large bodies of water close to its icy surface and, in contrast to Europa, that water may be accessible without drilling – or even landing on the moon. Enceladus regularly releases jets of water vapor from these liquid water bodies under its surface. These jets travel through space and end up in Saturn’s rings. Would it not be more ethical (and scientifically achievable) to try to collect and analyze some of this water from the jets of Enceladus before we try for the difficult to reach waters of Europa?

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