| The Search for Martian Life:
As you now know, the Mariner orbiters found evidence that liquid water had run on that planet, although perhaps only in the ancient geological past, and that there might therefore be hopes of finding life forms of some description. But there is a general question we should ask first: what sort of life should we look for? There are various aspects to this question, but the most obvious aspect is that we should be looking for what is usually called LAWKI.
This acronym stands for Life As We Know It. It may be true, if a little unlikely, that the universe teems with lifeforms that consist, say, entirely of interstellar magnetic fields in hot plasmas between the galaxies, and that separate individuals of this kind communicate by telepathy. (This kind of imaginative speculation is a common theme in `Star Trek', for instance.) But it is not clear how we would ever hope to detect a life form of this exotic and purely hypothetical nature. In the case of the Viking explorers sent to Mars, we must also be mindful of the fact that there is a limited volume and mass to any test apparatus we can land on the surface, so it is not possible to be ready for every imaginable life form. We have to start with more prosaic tests.
To begin with, consider what we know about the phenomenon of life. The only evidence we have so far (based on life on Earth) tells us that all known life forms use water as a solvent for chemical reactions, rely on on carbon-based organic chemistry, ingest atmospheric gases for metabolic purposes, are built around proteins which consist of chains made up of twenty common amino acids, and so on. Given that we can only pack a limited set of experiments into our space probes, it makes sense to focus on tests which search for LAWKI rather than some unsubstantiated speculation about alternatives.
Primitive or Advanced Life Forms?If we are to have any real hope of detecting life on planets around other remote stars, it will have to be because there are technologically advanced creatures there - those with the ability to communicate with us and make their presence known. An extra-solar planet teeming with forests full of trees or oceans full of blue whales would not appear special to us, or even be detectable from a large distance, but a planet which (say) emits clearly artificial radio signals (like our own broadcasts, which escape the Earth) will be instantly recognizable as the abode of life. Our search for extraterrestrials among the stars must therefore be aimed at very advanced species. In the case of Mars, however, there is no such restriction! Since that planet is within reach of our spacecraft, we don't have to wait until sentient Martian creatures signal their presence to us. We can simply go and look for more primitive life forms. More to the point, there is no merit in seeking advanced life forms on Mars because we do not really expect to find any there. There are a couple of reasons for this: The first is that if life sprang up independently on Mars, then it is likely to have evolved and progressed less rapidly than life on Earth. The Martian air is thinner, the climate is colder, the chemical reactions and metabolic processes will run less rapidly, and so on. So even if they started off together, life forms on Mars are likely to have developed more slowly, and attained a less-evolved form, for purely environmental reasons. Secondly, present-day scientific thinking is that the appearance of life on a planet is a chance event that happens when we fortuitously have the right mix of chemicals and energy sources. The fossil record shows that this happened fairly promptly on the Earth, where life dates back at least three billion years and perhaps farther. It may have happened equally early on Mars (if it did so at all), but it cannot have been much earlier. In other words, Mars was not granted a head start which compensated for the less-favourable evolutionary surroundings. Even worse, any life on Mars might have appeared much later than on the Earth, a delayed start which would surely produce even less evolved species by the present day. For all of these reasons, we might expect the dominant (or only) lifeforms on Mars to be microscopic, a thought which would seem to dictate the way in which we should design our tests. But is there a danger in this? Suppose, for example, that Mars also has big life forms, comparable in size to birds or mice (say). Do we have to worry about missing those if we spend too much time looking only for microbes? The answer is no: even if big life forms exist, there should be no real penalty in arranging our tests so that they are mostly (or only) sensitive to tiny organisms. Remember, for instance, that there are about a million ants on Earth for every human being, and almost literally countless numbers of bacteria. It is utterly implausible, in any speculation about life in its varied manifestations, to imagine a planet on which there might be many cat-sized creatures but no microscopic creatures. (Think, for example, about the nature of the food chain on Earth.) Any planet with life should have many microscopic animals or plants, whether or not it has evolved any larger ones. In short, you cannot go wrong if you test on small scales, so there is no point in wasting time, equipment, and resources in looking for bigger creatures. The corollory, of course, is that the absence of microbe-sized creatures would be very discouraging news. The absence of large ones would be no surprise, but would tell you nothing for certain.
Evidence of Absence.Before we begin the hunt, we should remember a very important point, one summarised in an aphorism which is applicable in a host of situations: Absence of evidence is not evidence of absence. In other words, if we fail to find evidence of life, we must not immediately conclude that there is none present! We may be looking in the wrong place, or in the wrong way; but until we explore Mars exhaustively, we should not be overly discouraged by an initial lack of success. Please note that this aphorism is sensible and appropriate in the context of the so-far-unsuccessful search for extraterrestrial life, but that it can be misused in other contexts. For instance, you would be quite upset if you heard me say that there is a reasonable chance that a lot of you cheated on your examinations because I have no evidence to suggest otherwise and thus cannot rule out the possibility! (I might conclude, for example, that I was looking in the wrong way. Perhaps I should have searched you all for miniature radio transmitters hidden in dental crowns rather than just make sure that you did not carry in 'crib sheets.') In civil and social contexts, we generally take the view that the absence of evidence of any guilty behaviour is indeed evidence of absence of such behaviour: we invoke a "presumption of innocence."
How to Search.Given the arguments I have presented, you will realize that there is no real merit in sending a lander to the Martian surface merely to take photographs to record whether there are any big trees or wandering stegosauruses. We could, however, think about doing something exactly like this on the microscopic level, but would that be enough? Should we rely simply on trying to get pictures of the life forms? We could, for instance, send a sensitive electron microscope to Mars, and scoop up and examine a bit of soil to see if it contains bacteria, viruses, or other tiny organisms. Whatever you think about the merits of this approach, the trouble is that we cannot readily adopt it! We are terribly restricted in what equipment we can send to Mars, and something simpler than an electron microscope needs to be our chosen tool. This is a theme I will come back to again and again: it is very difficult to launch a massive probe to another planet, requiring colossal amounts of energy. As I noted in the previous section, the two Viking landers had such tight size and weight restrictions that the entire life-search apparatus had to be carried in a box about one foot on a side. Think about it: what would you pack into such a box to permit you to test for life? The Viking scientists used what I think is a logical approach, perhaps made clear in a simple analogy. Suppose you found a small mouse, suffering from the winter cold, on your porch. How would you help revive it? The obvious answers are to warm it up (warmth is conducive to the efficient functioning of metabolic processes), to provide some nourishment, and perhaps even to provide a whiff of extra oxygen. If you feared that it was dead, you would look for signs like breathing in and out, with an intake of oxygen and exhalation of excess carbon dioxide, or the consumption of nutrients with a metabolic conversion to other forms (i.e. the mouse would digest the food it was fed). So that was the approach taken in the search for life on Mars, as follows. The Viking landers were equipped with small scoops on the end of robot arms, and with these arms small samples of soil could be picked up (just a teaspoon or so at a time) and deposited into test tubes. The samples were variously heated and given nutrients (things like amino acids, which are the building blocks of proteins, in a water solution) and air (extra carbon dioxide, since that is what the Martian atmosphere consists of and presumably what the creatures would like best). If there is microscopic life in the soil, you might expect it to show up in a variety of ways: the nutrients might disappear or be modified by metabolic processes into more complex chemical species, just as a plant turns water and carbon dioxide into sugars and cellulose; or the air `breathed out' might differ from that breathed in, just as we exhale less oxygen and more carbon dioxide than we inhale. This, then, was the guiding spirit of the tests: try to stimulate any hypothetical microbes to `do their thing' - speed up their metabolic processes - by providing the basic needs of life (warmth, food, air) in generous quantities. In fact, there were five tests for life on Mars, not all of which were exactly of this sort. Let us consider them in turn.
The First Test: Have a Look Around.The first test was the important and exciting one of sending back panoramic pictures of the Martian landscape, although, as noted above, this was not expected to be particularly informative in the context of the search for life, which is more likely to be microscopic in size. No one really expected to see a picture of a bush or tree, and indeed no such lifeforms showed up. By the way, the images would not have shown a bird flying past, or a brontosaurus walking through the field of view. The landers carried nothing as complicated as TV cameras; instead, the very simple optical elements created pictures simply by measuring the brightness of the scene, spot by spot, to slowly build up the image. (This was done through various filters to allow the determination of colours.) In this sense, the pictures were created rather as a painter paints a scene on a canvas - a blob of paint here, an added touch of colour there - over a long timespan. Something which moved briskly through the field would not register at all: it would be `blurred out.' The fact that we saw nothing other than rocks and sand is not a surprise; but neither does it invalidate the proposition that there may be life on Mars.
The Second Test: Find the Body.The second test consisted of the use of a device called a mass spectrometer to try to detect organic compounds in the soil of Mars. This is not a direct detection of life, exactly, but rather a detection of the `waste products' of life. For instance, if you landed in a forest in which a fire had recently killed all living things, you would still find lots of organic compounds: burnt leaf litter, bones from small animals, etc - all the leftovers from millennia of living creatures on Earth. By the way, it was once thought that organic compounds (mostly molecules which contain carbon) could only be produced by life forms - hence the name. About a century ago, science learned how to synthesise organics artificially, in techniques which are still used by the fertilizer industry for example, but the name has stuck. It is true, however, that most of the organic material in the Earth's soil and in the oceans was created by life. There is nowhere on Earth, not even on the Antarctic ice cap, that you would fail to find some organic compounds. (On Earth, the winds deposit spores, bits of soot, and so forth on the ice cap.) You may be surprised to know that organic molecules are even found on the moon, although there is no real expectation of finding life on the airless lunar surface under the unshielded ultraviolet rays of the sun. They are deposited there by the occasional infall of meteors of a certain kind - the so-called carbonaceous chondrites, which are relatively rich in carbon and its compounds. The Viking landers, by contrast, discovered absolutely no organics whatever in the samples of Martian soil, a finding which seemed to tell us immediately that Mars is utterly devoid of life. This was a very quick and discouraging result. (You should also find this surprising! If the lunar surface gets sprinkled with organic compounds from meteors, shouldn't the same be true for Mars? Where do those organic compounds go to? We will see below that the chemistry of the Martian soil is probably responsible.)
Tests Three Through Five: Look for Metabolic Activity.There were in fact three tests of the sort I described above: take a soil sample and see if our `feeding' it causes the hypothetical microorganisms to flourish in a way which shows up as some metabolic process producing a measurable result. One test, the Pyrolytic Response experiment, involved heating the soil (think of the words `pyrotechnics' and `pyromaniac' to recognize the reference to fire and heat). This experiment had a simple premise. The idea was to allow the hoped-for microbes time to ingest the fresh supply of food, water, and Martian air, and possibly to convert these raw materials to something more substantive, some more complex organic molecule. (The parallel again is to the way plants on Earth convert water and atmospheric carbon dioxide to starch and sugars.) The pyrolytic heating, after the incubation period, was designed to release any such compounds for capture and measurement. A second approach, the Gas Exchange experiment, involved examining the gases `exhaled' by the sample after the addition of food and fresh air. Our mouse on the doorstep, for example, would have breathed in and absorbed oxygen, but released extra quantities of carbon dioxide. Here we hope for analogous developments. The third experiment, Labelled Release, involved adding nutrients `labelled' with a tiny amount of radioactive carbon to trace where the material went, and in particular if it showed up in more complex organic compounds as the hypothetical microbes absorbed and processed the nutrients.
And the Results...The remarkable thing, after the completely null results of the mass spectrometry, was that each of the metabolic tests yielded an apparently positive result, suggestive of the existence of life forms in the soil! A full interpretation and discussion of the results would be rather complex; indeed, the experimental procedures were also necessarily complex, in ways I will not go into. Perhaps the second of these remarks deserves a little elaboration, which I will provide through the example of a typical precautionary step. The Viking landers picked up numerous soil samples, some of which were immediately heated to extremely high temperatures to kill any microorganisms which might be present. These now-sterile samples provided controls: they subsequently got exactly the same treatment as the unheated samples, so that we could see how diferently they might behave. Such controls are important in all branches of experimental science, including the health sciences. In the lecture, I showed you some of the graphs and figures which summarise the Viking tests. (Anyone who wants to know more should see me for specific references.) The details don't matter, but the exciting conclusion was that all the active metabolic tests gave strong positive results, although not without some ambiguity, in apparent confirmation of the hypothesis that there were micro-lifeforms in the Martian soil. But how can this be squared with the complete absence of organic compounds in the soil?
The Present Interpretation.Despite the apparently positive signals in the three `metabolic' tests for life, there were some ambiguities, as I noted. For instance, adding nutrients to the soil seemed to provoke an immediately positive response, as though the micro-organisms were enjoying the food and carrying out metabolic processes; but when a bit more nutrient was added, the response tended to die away rather than continue or increase. Simple inconsistencies of this or similar sorts were seen in all the tests. The perplexing behaviour remained a mystery for some time, but in recent years experiments in Earth laboratories have shown that exactly this kind of behaviour can be duplicated by an appropriate mix of chemicals in the surface soil - that is, the reactions are probably purely chemical, not metabolic at all. One interesting aspect is that this may imply that the soil of Mars is rich in hydrogen peroxide, a simple yet chemically active compound which seems readily able to explain the observed behaviour. (If you have ever accidentally put into your eye the hydrogen peroxide solution which is intended for sterilizing contact lenses, as I have done, you will understand it when I say that it can be chemically reactive enough to explain some of the features of the Viking experiments. Don't ever do that! It causes blinding pain, but fortunately no long-term damage.) Hydrogen peroxide is also commonly used to bleach hair, as you probably know. The chemistry of the soil, coupled with the fact that Mars's thin atmosphere does not block off the sun's ultraviolet rays, means that the soil will be pretty harshly bleached, and the absence of organic compounds is no surprise. As I say, such effects have been completely replicated in labs on the Earth, and the consensus view is that there were no LAWKI lifeforms in the samples tested by the Viking landers. (Some scientists are still unconvinced by this explanation and consider the question an open one.) This does not mean that Viking has proven that is no life at all on Mars, of course!
Other Prospects for Present Life on Mars.There are still prospects for life on Mars. The first point is that we have only looked in two locations - the landing spots of the Vikings. But this is not as strong a point as you might think, and the negative results really are a little discouraging. Consider, for instance, the fact that life on Earth is so far-flung that at least the organic `waste products' and more typically the lifeforms themselves would show up in every randomly chosen drop of lake or sea water and in every spoonful of surface soil. It is hard to imagine life existing on Mars and not having evolved to occupy every imaginable niche. However, there may be some merit in looking deep underground, perhaps by taking a core sample from one of the polar caps. The other point, of course, is that we may have been looking in an inappropriate way. Perhaps the life forms are not LAWKI, but are based on an entirely different set of metabolic processes. It will take a very considerable effort to consider all the possibilities and explore Mars sufficiently to make a decision one way or the other.
Where Did All the Water Go?Mars has some water now, in the polar caps and the atmosphere. But the geological evidence suggests that there was lots more in the remote past, running in river valleys and so forth. It seems likely, by the way, that the last episode of widespread liquid water on Mars was a few billion years ago. But where did all that water go? There are various possibilities. Perhaps, over time, the once-abundant water escaped to space. It is cooler on Mars than on the Earth, thanks to its greater distance from the sun; this means that the atoms and molecules in the atmosphere move around less rapidly. But the gravity is less, and one can show that water vapour would largely escape from Mars. (The carbon dioxide molecules are heavier and less prone to escape.) Indeed, the small amount of water which is still present in the form of ice in the polar caps suffers some loss (it `boils off') whenever it is turned to vapour by the onset of Martian summer. It may be, however, that there are large reserves of water in the form of ice deeply frozen under the polar caps or in the form of permafrost layers, perhaps kilometers thick, under the surface rocks and gravel. If the second possibility is correct, then we have to consider also the chance that Mars undergoes episodic flooding events if and when the deep permafrost is temporarily made liquid. In such a picture, a changing climate leads to melting of the deep ice layers, the water runs free for a time (some millions of years), during which it suffers big but not total evaporative losses to space, and then the climate turns cold again, restoring the permafrost layers. It is possible, in short, that the geological evidence for floods and rivers merely tells us about the most recent such event.
Will the Water Flow Again?Could it happen again? That depends on how the Martian climate changes over time. Will it warm up dramatically in the foreseeable future? There are two relevant factors: Mars is smaller than the Earth (and thus easily `pushed around' - or more correctly `pulled around' - by the gravity of the other planets); and Mars is relatively close to Jupiter, the most massive planet in the Solar System. Together, these considerations imply (as careful numerical analysis confirms) that the orbit and motions of Mars are much less stable than those of the Earth. Its orbit can change in shape: for long periods, it will be more-or-less circular, so that Mars is at a constant distance from the sun; then it becomes quite elongated, so that it ventures rather close to the sun at certain times of its year. The inclination of the axis of Mars also changes: sometimes it is tipped almost onto its side, so that the North Pole points towards the sun during one part of its year and away from the sun half a Martian year later. The effects of these changes on the climate are not yet completely understood, but it seems quite plausible that some combination of changed orbital and spin behaviour could indeed lead to a general increase in the global temperature on Mars, with large-scale flooding to follow (if indeed there are deep reserves of ice). Certainly, however, the existence of a large reserve of frozen water would make Mars a more attractive proposition for eventual human colonisation and settlement.
Fossilised Martian Life Forms?Many of you will have heard of the discovery, in August of 1996, of what may be fossilised life forms in a meteor which is known to have originated on Mars. Let us begin by considering how this meteor was discovered. The story is as follows: early in the history of Mars, about three billion years ago, solid rocks in a certain location coalesced from the molten state, perhaps following a volcanic outflow. Much later - about sixteen million years ago in fact - a large meteor hit Mars at this spot and completely knocked off various chunks of it, leaving them free to move around the inner solar system. About thirty-four thousand years ago, one such chunk fell to Earth, landing on the Antarctic ice cap (where it is quite conspicuous, of course: it merely needs someone to find it!). I will not tell you how these dates are determined, but want to emphasise that the place of origin (Mars) is quite securely known, again through techniques I will not belabour here. In fact, we have discovered on Earth about a dozen meteors each from the moon and Mars. The interesting thing is that this particular meteor, once cut open, revealed the presence of features of two distinct kinds. (See the figure on pages 728-729 of your text.) The features were: Carbonate `globules' about 0.25 millimeters across. While not thought to be fossilised life forms, these may have been laid down by life forms, and seem to have been formed at some moderately warm temperature, in a climate favourable for life. More interesting, however, were small `worm-shaped' forms found in the meteor, elongated forms which look very much like the fossil remains of some of the earliest identified lifeforms on Earth. The match is not perfect, since the Martian `fossils' are only about a thirtieth of the size of those found on Earth; but there are some striking compositional features, such as the presence of what are called PAHs (polycyclic aromatic hydrocarbons), which may point to lifeforms as the likely origin. NASA has been understandably cautious in claiming unambiguously to have detected fossil proof of life on Mars, and there are still problems of interpretation. (Understandably, there has been a lot of discussion of this in the scientific literature, including expressions of concern over the possible contamination of the meteor while it sat undiscovered in the Antarctic.) The possibility seems still to be open.
Back We Go....As you know, two American space probes have now revisited Mars (a third, a Russian probe, failed and fell into the Pacific). One of them landed a small `dune buggy' which trundled around the surface for a couple of weeks, sending back fascinating images. (Thanks to its small size, it could not carry sophisticated biological test equipment.) The other consists of an orbiter which has done a very detailed mapping of the Martian surface. Some of you may remember that the Martian Polar Lander, attempting a controlled landing about a year ago, was formally declared lost a few days after it failed to make contact with the Earth following its scheduled touchdown. (Perhaps it came down in a field strewn with boulders of moderate size and fell onto its side.) But what of the Viking landers? Well, one of them was shut off after a while to save money. (Running the lander costs nothing, but accumulating and analysing the data it sends back is a burden. As soon as NASA thought it had learned enough, it pulled the plug.) Once shut off, the lander dies as its batteries run down and no efforts are made to recharge or inspect them remotely. The second Viking lander subsequently suffered a catastrophic failure when an erroneous message from NASA caused it to aim its antenna straight down to the Martian soil rather than back to the Earth! There is an important lesson here. The erroneous message, once sent, could not be countermanded since it travelled to Mars at the speed of light as a radio signal that could not be overhauled. It is rather like sending a really nasty letter to a friend and then regretting it after it has gotten into the postal system, beyond hope of interception. Of course, you can always call your friend to ask her or him not to read the letter. NASA had no such recourse with respect to the Vikings, and although they tried sending corrective signals at very high power in the hope that some would reflect from the soil itself into the downward-looking antenna, this did not work. And so the productive Viking story came to an end. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Tuesday, 20 October, 2020.)