This space ship will go and investigate life on mars.

Comments

  1. #1 John Callender
    November 26, 2011

    Small pedantic point: Isn’t it premature to say that this craft will study life on Mars? Aside from the uncertainty of its successfully landing, there’s the fact that to date there is no particularly compelling evidence that life has ever existed on Mars. If the planet turns out to be lifeless, then wouldn’t that reveal that a statement that this probe “will study life on Mars” was an example of overly credulous optimism?

  2. #2 Greg Laden
    November 26, 2011

    No. If if mars was always and is now lifeless, the study of life can still be done there. True, it will be negative evidence and rather boring …..

    But that is not important; Mars probably has life or did have life.

  3. #3 daedalus2u
    November 26, 2011

    I hope they spend as much on unmanned missions to search for life on Mars before they send a manned mission there because a manned mission will irreversibly contaminate it.

  4. #4 John Callender
    November 28, 2011

    Thanks for clarifying where you’re coming from on the probability-of-life-on-Mars thing. I think we disagree on at least one aspect of the question, but I also recognize that the position you’re articulating is more popular than the one I hold.

    The way I see it, there is no rational way, based on current evidence, to assign a probability greater than 50% to life existing or having existed on Mars (which is how I interpret your statement). That’s a subset of the larger question of whether life exists anywhere (or has ever existed anywhere) beyond Earth. Since you believe it likely that life exists (or existed) on Mars, you necessarily believe it likely that life exists (or existed) beyond Earth, but there again, I think it isn’t currently possible to craft a rational argument for that, either.

    Discussions in which I express this view often end up becoming discussions about the Drake Equation, so to save time, I’ll stipulate that the number of places in the universe that are capable of sustaining life (which relates to the Drake Equation’s term n-sub-e) is very, very large. I’ll further stipulate that Mars (or at a minimum, Mars at some point in the past) is (or was) one of those places.

    That still doesn’t get you past the Drake Equation’s term f-sub-l, which refers to the likelihood that a place suitable for life will actually experience the emergence of life; that is, to the likelihood of abiogenesis. I don’t see how you (or anyone) can rationally assign a probability greater than 50% to life existing beyond Earth without having better data than we currently possess on the probability of abiogenesis.

    We know that abiogenesis took place at least once, since we’re here to contemplate the question. So the probability of abiogenesis occurring must be greater than zero. But that’s pretty much all we know. Maybe the probability of abiogenesis is close to 1; that is, maybe life will nearly always emerge in a place capable of supporting it. Or, on the other hand, abiogenesis might be extraordinarily unlikely. It might, in fact, be so unlikely that the particular case of abiogenesis that led to us being here to contemplate the question was, in fact, unique.

    Both hypotheses (that life emerges whenever it has a chance, or that life emerges so rarely that we are unlikely to find any other instances of it) can account for the current evidence (that there is a place called Earth where it happened at least once in the distant past, and at least so far, no evidence of it having occurred anywhere else).

    So I’m curious: How is it that you are able to assign a probability greater than 50% to life existing or having existed on Mars? Is that a rational position? Or is it more the product of an emotional desire on your part for a universe that isn’t such a lonely and (in your word) “boring” place? If it’s the latter, I’m not sure that’s a position that is going to hold up very well to skeptical scrutiny.

    I’m curious what you think about this. (I’m also curious what will be found by Curiosity, and am really looking forward to hearing about its results.)

  5. #5 Greg Laden
    November 28, 2011

    Actually, I think you’ve misread what I’m thinking (or not so much misread but made some assumptions).

    First, I don’t see any basis for a 50-50 starting point for life on mars. Certainly not because the answer is “yes” vs. “no.” If I were to guess the chances of life on a randomly selected planet that I don’t get any info about, I’d say “no” because I think the chances are generally close to zero. If I were to guess what the chances are for a randomly selected solar system (star system) without any info about it, I’d say “no” if I had to pick “yes” vs “no” because I’m pretty sure there is a range of nastiness out there in typical galaxies and only a certain proportion of the solar systems are of the right age and position to possibly have life.

    My guess about the probability that there is or was life on Mars is not based on any such assumptions, however.

    There was a study of a Martian Tektite a few years back that had isotopic evidence of life on mars. That evidence was never refuted, although there are reasons to be cautious. Mars is not too far form or too close to the sun to totally rule out life. It has water, and evidence of active surface water being much more common in the past. that evidence suggests, to me, possibly a less variable temperature range and certain other conditions more conducive to life. I threfore conclude that Mars is conducive to the existence and possibly the origin of earth-like life.

    From there, the next step I take is based on a presumed universal principle. This is simply the annthrocentric logic (or whatever it is called). There is life on earth. There is nothing special about earth, but there is in fact life on it. Therefore life on any earth-like planet is not especially far fetched.

    So, conditions are right, it is not far fetched, then there is this isotopic evidence. A bit of optimism stirred into the pot and I figure there’s a good chance.

    Regarding the chances of abiogenesis: We know more than you are saying! For one, we know that (in geological terms) the very MOMENT that life could have emerged on earth it did. I would have a very different opinion if a life-capable planet existed for a few billion years before life sprang up, but that is not what happened. That is still an N of 1, but it is NOT evidence for low probability of life occurring. That would look differetn.

    Second, although experimental work on life origins has produced mixed results, there have been a handful of cases where stuff got organized rather suddenly or efficiently.

    Third, I regard the multiple hypotheses for conditions or places of origins (this or that primordial puddle, deep sea vents, etc) as NOT being competing mutually exclusive hypotheses, but rather, a list of opportunities. I would extend that logic to any planet with seas, rivers, puddles, sand, volcanoes, etc. etc.

    I can’t assign a probability to life being on mars. That is an unanswerable question. There is no way to do that other than to say “more than zero.”

    But my guess is that there was, very likely, and possibly (but much less likely) is now.

  6. #6 John Callender
    November 29, 2011

    Thanks for the reply.

    I agree with most of your latest comment. I wasn’t suggesting that a 50/50 chance of life on Mars was some kind of logical starting point; I was just restating your own statement (“Mars probably has life or did have life”). To me, that sounds like an assertion that the probability of life existing or having existed on Mars is greater than 50%. Did you actually mean something different when you said that? If so, maybe we don’t disagree after all.

    I wonder, because you sound like you basically agree with me when you write, “I can’t assign a probability to life being on mars. That is an unanswerable question. There is no way to do that other than to say ‘more than zero.’” That was pretty much my main point, so to the extent that’s an accurate description of your own position, I think we’re more or less on the same page.

    With that said, I think the following statement may be fallacious:

    “…the next step I take is based on a presumed universal principle. This is simply the annthrocentric logic (or whatever it is called). There is life on earth. There is nothing special about earth, but there is in fact life on it. Therefore life on any earth-like planet is not especially far fetched.”

    To me this sounds like tautological reasoning. You begin by assuming that there is nothing special about Earth. Life is a feature of Earth, therefore life cannot be special. But that is simply a restatement of your premise. What if Earth actually is special? What if the emergence of life on Earth actually was an extraordinarily unlikely event? How would things look different than they currently do?

    I’m not sure what you are referring to when you say, “the annthrocentric logic”. The term does not appear in Google’s index at the moment (though presumably it will once googlebot finds this page). The closest candidate I can find is the “anthropic principle”. But to my mind, the anthropic principle actually supports my argument rather than undercutting it.

    The anthropic principle emerges from the recognition that in order for us to even be having a discussion like this, life (and indeed, intelligent life) must have emerged in the place where the discussion is taking place. Given that constraint, it is logically invalid to try to infer the likelihood of that emergence based only on that one example. It’s not an independent data point that allows you to infer a probability. It’s a necessary precondition.

    The other pieces of evidence you cite are interesting, but they don’t rise to a level that alters that fundamental reality. It’s certainly interesting that life appears to have emerged relatively early in Earth’s history, but to say it happened at “the very MOMENT” it could have, even in geologic terms, sounds like hyperbole to me, given the limits of our knowledge of the precise timing of the events in question. Similarly, Martian meteorites, while intriguing, have so far failed to produce anything approaching smoking-gun evidence of Martian abiogenesis. Experiments in which “stuff got organized” in interesting ways and/or on interesting timescales are certainly intriguing. But are they definitive (or even particularly strong) evidence that abiogenesis is commonplace enough that the chances are better than 50% that it happened on Mars? I don’t see it.

    All of which is not to say that there is compelling evidence that life doesn’t (or didn’t) exist on Mars. I’m not saying I know what’s in the white areas beyond the borders of our current mental map. I’m just saying we don’t know, so asserting probabilities is logically suspect. We need to go and look.

    Which is what we’re doing with Curiosity. To go back to your original post, if you’d said, “This space ship will go and investigate the possibility of life on Mars” I wouldn’t have batted an eye. But when you said it would go and investigate “life on Mars”, that jumped out at me, since to my mind it is just as likely (to the extent we have enough evidence to assign any probability at all) that it is going to investigate what turns out to be the absence of life on Mars.

    I realize you would find that boring. But the universe is under no obligation to conform to your, or my, or anyone else’s preferences in that regard. It is what it is, regardless of what we want it to be. Scientific skepticism requires that we set aside irrational desires and focus on what the evidence actually tells us. And to my mind what the evidence tells us so far is that rationally, there is just as much evidence to support the prediction that Curiosity won’t find compelling evidence for life as that it will.

    I realize that this brings in another variable we haven’t talked about so far (the probability that if life ever existed on Mars, Curiosity will be able to find compelling evidence for it). But I’d be willing, if you’re game, to make an actual wager out of it. We could bet on the possibility that if Curiosity reaches the Martian surface and is able to carry out a substantial portion of its scientific program, that it will uncover evidence that a substantial fraction of scientists with relevant expertise view as compelling evidence of past or present Martian life.

    I don’t how you feel about gambling in general, but speaking hypothetically, if I were willing to bet $100 on the possibility that having successfully carried out its scientific program, Curiosity would not find such evidence, how much would you be willing to bet that it would? Would you bet $100? I think I’d take that bet. Would you be willing to bet $50 against my $100? $10 against my $100? I’d have to think about it some more, but I might be willing to take even those odds. If it turns out that Curiosity fails to find evidence of life, I’d at least have the consolation of having won the bet. And if I lose, well, I think that would make me even happier. So at least for me, it would be a win-win proposition. :-)

  7. #7 Greg Laden
    November 29, 2011

    To me, that sounds like an assertion that the probability of life existing or having existed on Mars is greater than 50%.

    Given a yes/no guess, I’m guessing yes (to did/does). I do not arrive at that guess after a series of probability statements. So, the 50-50 bit is derived from my guess, not the other way around.

    I could also say that I think the chances are good enough that it is worth spending billions of dollars looking. I would say that if the chances were somehow estimated at, say, 10%. So I don’t really need the 50-50, and prefer not to use it. It can be derived from what I say but only as an afterthought, not as the point or the meaning of what I’m saying.

    I think we mostly agree.

  8. #8 daedalus2u
    November 29, 2011

    I would put the likelihood of life on Mars at greater than 90%, maybe greater than 99%. I also think it is very likely there there is still life on Mars, but it may be hard to get at because most of it is likely km under ground. Even on Earth, it is thought that most living biomass is underground. There are probably living bacteria everywhere it isn’t too hot, with the upper limit being ~120 C.

    On Mars, the zone where the temperature of the crust is between 0 and 120 C is likely pretty large.

    I wouldn’t revise my estimate of life on Mars unless there were at least 10 holes drilled into porous aquifers where the temperature was greater than 50 C and none of these showed any signs of life.

  9. #9 John Callender
    November 29, 2011

    My own sense (as is probably clear from the previous discussion) is that estimates of a 90% or 99% probability of Martian life are examples of motivated reasoning, driven by the reasoner’s emotions, rather than any objective evidence we currently possess. There is a long tradition of otherwise-rational people going out of their way to construe the available evidence in a manner that allows them to populate Mars (or the universe in general) with high probabilities of extraterrestrial life. So far, though, the actual evidence we’ve managed to gather does not support that view.

    It’s an interesting example of how the human mind works that after each setback the proponents of extraterrestrial life simply regroup, pushing the hypothetical life farther away, deeper underground, or whatever, while continuing to assert that the evidence supports their optimistic willingness to believe, even going as far as to assign numeric probabilities to it (e.g., “Mars probably has or did have life”; “I would put the likelihood of life on Mars at greater than 90%, maybe greater than 99%. I also think it is very likely there there is still life on Mars…”).

    You’re free to believe that. But having looked at the question, I don’t think that belief is rational.

  10. #10 Greg Laden
    November 29, 2011

    OK, good! Now, daedalus2u, please explain your reasoning to John. He is claiming that you have come up with your conclusion based on emotions.

    Inductively, I doubt that, John; I’ve had lots of discussions with daedalus2u, arguments even, and I have yet to see him conclude or assert something on the basis of emotions.

  11. #11 daedalus2u
    November 29, 2011

    Yes Greg, I agree, that is kind of bizarre.

    Earth evolved life just about as fast as it could, a few hundred million years after the surface cooled enough for liquid water to be present in abundance.

    The late heavy bombardment occurred ~4 to 3.8 bya and obliterated essentially all crustal rock and very likely killed anything that was alive. By 3.5 bya there are fossils of microscopic organisms.

    The earliest organisms were not photosynthetic, so surface area exposed to light and light intensity doesn’t matter. There was high UV because the atmosphere didn’t have enough O2 for an ozone layer. If we consider the volume of Earth that was habitable by life at that time, it was only the near surface because Earth was still pretty hot underneath. The surface area of Mars is ~0.28 that of Earth. Because Mars is smaller, and wasn’t hit by the Mars-sized object that formed the Earth/moon system, Mars cooled much earlier than the Earth, so the initial habitable volume of Mars was greater than that of Earth and occurred sooner. Once the surface of Mars cools enough for liquid water, there is always a region that is in the habitable zone. Even now, there are regions of Mars that are warm enough for life. If you assume the formation of life is stochastic and not cumulative, and occurred on Earth in ~500 million years, life on Mars could have occurred at ~4 bya. If life on Mars took multiple times longer (for no reason that I can imagine), then there has been ~9x origin-of-life-times on Mars. If we consider that life had a 50/50 chance to form in each of those 500 million year periods (for no reason that I can imagine), then you end up with a 0.998 chance that life did occur on Mars. I think an estimate of 0.9 or 0.99 is conservative.

    We know that at one time Mars did have oceans deep enough to support and raft large ice flows. Outlines of those ice flows are still visible. Likely the ice is still there. When the liquid supporting that ice froze, it would freeze from the top, driving solutes down as the freeze layer propagated down. The frozen ice would block the loss of volatiles such as water, CO2, methane and H2. The surface of Mars is now known to have abundant nitrates, sulfates and perchlorates. These can be used by Earth organisms as electron acceptors and more easily than can O2 be used. Igneous rocks are reducing, water containing nitrates and sulfates percolating through such rocks does provide an energy source for organisms.

    The rover isn’t going to be able to drill into warm aquifers, so it not finding life on the very few surface places it will be able to look won’t change my estimate by very much.

  12. #12 John Callender
    November 30, 2011

    As I previously acknowledged, the relative speed with which life appeared on Earth is certainly suggestive. If you assume that there was nothing special about the emergence of life on Earth, such that Mars can be expected to have evolved life at roughly the same time in its habitable history as Earth did, then sure, there really should be life on Mars.

    But you have to make that assumption (that there was nothing special about Earth’s circumstances such that abiogenesis occurred). It is that assumption that I’m asserting is not rational, based on the available single example of abiogenesis, and the necessity that that single example must exist regardless of its likelihood or unlikelihood, per the anthropic principle. No matter how much number-crunching you do in other parts of the probability-of-life equation, as long as you leave that factor unaddressed, any assertion of an overall probability of life on Mars (or anywhere beyond Earth) is an act of wishful thinking rather than reason. You just don’t have sufficient data to determine the probability. The factors you know are overwhelmed by the factor you don’t know.

    This seems mathematically obvious to me, so it surprises me how many otherwise-rational, mathematically astute people disagree with me about this. I realize that a simple explanation is that I’m just wrong, so I appreciate your being willing to take the time to explain your thinking. If you’d be willing to point out the evidence that you believe justifies the belief that abiogenesis is a commonplace occurrence in the universe, rather than a rare one, I would really appreciate it.

    Thanks.