OTHER ARTICLES
THE OUTER REACHES OF LIFE
John PostgateCambridge University Press 1994 pages 252-268
What are the absolute pre-requisites for our kind of life? Obviously the elements that constitute living things must be available in accessible forms: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, iron and another twenty or so. These elements are abundant in the universe, and are unlikely to be seriously lacking where other requirements of life are satisfied. So what exactly are these requirements? The clearest indications come from the study of microbes, because microbes have colonised the terrestrial habitat to its limits. And they tell us one truth above all others: the features we humans take for granted on Earth's surface today are far from obligatory for life.Let me recapitulate briefly. Life manages very well without oxygen, evolving into flourishing communities of anaerobes. Acidity, at least to the strength of weak sulphuric acid, presents no problem, as sulphur bacteria and their co-habitants illustrate, nor does a considerable degree of alkalinity bother alkalophiles - indeed, certain alkaline lakes in Africa hold the terrestrial record for biological activity. Water purity is a trivial matter: saturated salt brines support abundant bacterial life. And pressure is quite as irrelevant, with bacteria growing happily in a near vacuum or at the huge hydrostatic pressures of deep ocean trenches. Temperature, too, presents little problem: boiling hot springs support bacterial life, and bacteria have been found growing at 112° Celsius in superheated geothermal water under hydrostatic pressure; conversely, other types of bacteria thrive at well below zero, provided the water is salty enough not to freeze. And even if they do get frozen, many bacteria revive when their habitat thaws. Even organic food is not a pre-requisite: though organic matter was necessary when terrestrial life first flickered into existence (I told that story in Chapter 14), today plants, and several kinds of bacteria, use sunlight to form their own organic matter from carbon dioxide.
Does that mean that light is essential, then? Sunlight does indeed seem to be a more serious matter. Plants cannot manage without it, and neither animals nor the great majority of microbes can manage without plants. Photosynthesis, by plants and bacteria, provides the primary supply of organic matter on which the rest of the living world subsists. With photosynthesis at the base of our food network, we are all here by courtesy of solar energy. It is true that a specialised group of bacteria exists called the chemotrophs, species of which, instead of using sunlight, can obtain energy from mineral substances such as iron, hydrogen gas, sulphur or ammonia, and use it to make organic matter from carbon dioxide. At first sight chemotrophs might seem to be able to form the basis of a food chain which is independent of sunlight, but in fact they need oxygen gas to attack their minerals with, and this would not be available were it not for plant and cyanobacterial photosynthesis. Chemotrophs depend on sunlight at one remove, so to speak.
Are there any species of microbe which could manage on a sunlight-free Earth? Yes, there are a few. They include certain anaerobic methane-producing bacteria, which need only gaseous hydrogen and carbon dioxide to generate organic matter, and some species of sulphate-respiring anaerobes which can make organic matter from hydrogen and carbon dioxide, given a supply of sulphate. On Earth, today, they are not actually independent of sunlight: the hydrogen these bacteria use comes from organic matter which has been made by other, sunlight-dependent, creatures - But free hydrogen is abundant in the universe, and so are sulphate and carbon dioxide. They could manage without sunlight, and that fact means that light energy, though jolly useful to life, is not absolutely essential.
So what are we left with that is essential? The answer is water. Though evolution has provided living creatures with ways of living on dry land, they have to keep their interiors wet. Certainly some organisms have developed ways of surviving extreme drought and desiccation, often for long periods, but they do so by becoming dormant, as seeds or spores. They revive only when wetted. The fact of the matter is that terrestrial life processes can take place only in water. It is not just that a liquid is essential for moving molecules around and causing them to interact; the chemistry of water, with its ability to split into its hydrogen and oxygen components, which themselves interact with biological molecules, makes it an integral part of life. Scientists have tried to imagine how life-like processes might carry on in other liquids, such as petroleum, or acids such as sulphuric or formic, or fluids which might occur on colder planets, such as liquid ammonia, liquid nitrogen or liquid methane. But even the most elementary features of terrestrial life's chemistry, let alone the build-up of complex molecules such as proteins and DNA, simply could not happen in any of these fluids.
Water, then, is the fundamental background against which all the processes of life take place. It is an absolute pre-requisite of our kind of life. (I refer to 'our kind of', or 'terrestrial-type', life because I wish to disregard, but not wholly dismiss, the possibility of life-like entities based on parameters wholly independent of our planetary chemistry. Such as sensate stellar plasmas communicating by radio? Yes, I like science fiction, but not just here.) On Earth, the physics of water determines life's theoretical temperature limits: from about -30° Celsius, the coldest unfrozen Antarctic brine, to above 350°, the temperature of certain deep sea hydrothermal vents. I said theoretical deliberately; in practice it seems that the upper limit is, as I mentioned earlier, much lower at 112°. I discussed why in Chapter 2; it probably has to do with the rate at which biological chemicals concerned with energy transfer are decomposed by heat. Had there been call for it, a more heat-stable terrestrial biochemistry would probably have evolved here.
Elsewhere in the universe, things could be different, for there is no reason to think that the terrestrial limits need be universal: ice melts if it is compressed, and water boils at elevated temperatures; on a high gravity planet, the temperature range of liquid water could be three or four times as wide as here, and the scope for terrestrial-type life would be accordingly greater. (Lifetimes would be short, and life fast-moving, at high temperatures; long and sluggish lives would be the feature of cold worlds. Would those lives feel like ours, with time gently accelerating as one ages? I rather think so.)
Other WorldsWhat else, apart from availability of liquid water, distinguishes an inhabited planet, such as ours, from one that has no life?
The answer is metastability. This is a word that chemists and physicists use: an object or substance is metastable when it appears stable, but when its stability is maintained only by consuming or conserving energy. A clear example of a physically metastable object is a top which is spinning, because sooner or later the pulse of energy which initiated the spin will run out and the top will fall over. Less obvious is the case in which energy is retained: a top-heavy object which, suitably poised, stays upright until a minute disturbance initiates its collapse - the top, if exceptionally finely balanced, might behave like that when spinning ceased. Another example of physical metastability occurs when pure water in clean glass is cooled steadily without disturbance: it remains liquid for several degrees below its freezing point. It becomes 'supercooled'. Disturb it by shaking, or drop in a solid particle, and it forms ice with a snap. Again a miniscule input of energy is needed to de-stabilise it, but as it freezes it warms up a little: it gives out thermal energy. Actually, glass itself is metastable because it, too, is a supercooled liquid, but it takes tremendous physical stresses to make it crystallise.
Metastability can also be chemical. Explosives are spectacular examples: give them a pulse of heat or a physical impact, as the case may be, and they undergo catastrophic chemical change. And a mixture of cooking gas and air is equally metastable. In a far more gentle way, living things are metastable. They build themselves up, expending energy derived from food or, in the case of plants, from sunlight. But the molecules that compose their functional parts - the cells of their soft tissues rather than structural organs such as shells or wood - are metastable. They could not be truly stable, because living depends on their interacting with each other. So the functional parts of living things have to be kept in continuous repair. Their components are constantly being used up and disposed of, while fresh molecules of the same kind are generated to replace them, all of which requires more energy. Ultimately some critical parts of the replacement processes fail, and then the organism will die.
The living and non-living worlds are so intimately linked that the metastability of living things is reflected in our habitat. The clearest example of such a reflection is provided by the composition of our planet's atmosphere. Today it is rich in oxygen, but 3 billion or so years ago there was none; as I have mentioned in earlier chapters, we have oxygen because plants generate it continuously from water during photosynthesis. Free oxygen is metastable on Earth, a tell-tale indicator of life. Even a billion years ago, when there were neither animals nor plants, and all life was microbial, the atmosphere had a modest oxygen content which would have revealed life's presence. Move back another billion years, when there was no oxygen, and the tell-tale signs of life in the atmosphere would have been hydrogen sulphide and hydrogen: gases which would disappear without microbes to regenerate them. If, through some global catastrophe, life on Earth were now to cease abruptly, free oxygen would vanish fairly rapidly, entering into chemical combination with various terrestrial minerals.
Today's atmosphere has other components which are metastable, such as methane: it reacts with oxygen but is constantly generated by microbes. Apart from determining the atmosphere, living things have altered the planet's geology in ways which are blatantly evident: chalk, peat and coal come at once to mind; readers who have dipped into Chapter 8 will know that several ores and mineral deposits are also of biological origin. The biological elements, carbon, nitrogen, sulphur and so on, are cycled through air, water and soil - itself a partly biological product - by living things; forests influence weather and cloud formations. The physics and chemistry of the Earth's present surface together offer a supreme example of the way in which living things, collectively, have not only changed their environment to suit themselves, but sustain that environment in a metastable but steady state, so that it and they change only slowly.
Metastability on a planetary scale is a presumptive indicator of life. Analyses of moon rocks reveal no sign of comparable metastability, nor did anyone seriously expect them to. A couple of decades ago, most of the scientific community was less certain about Mars - except for James Lovelock, a distinguished British environmental scientist, who pointed out that remote sensors, which had sent back reliable information about its atmosphere, had found nothing metastable about it. Therefore the planet was not likely to be inhabited. When the Viking lander of 1976 reached the Martian surface and sent its first signals back, much excitement was generated because, though the atmosphere was just as expected, one part of the probe seemed to have detected life-like processes in a wetted sample of Martian surface dust. But it was a misleading result (of a kind that researchers are all too familiar with): an unexpected chemical reaction between a watery solution provided by the lander and samples of Martian soil had given a false impression. I shall write a little more about life on Mars later in this chapter.
Space explorers in future centuries, human or robotic, will use remote sensing to seek evidence for both water and chemical metastability on or around potentially inhabited celestial objects, long before they or their landers arrive. They will be aware that an inanimate world can mislead. Apart from unexpected chemical
reactions that can occur when an alien material is introduced, as happened on Mars, geological activity can simulate metastability. The aftermath of volcanic activity can leave metastable materials in the form of fluid and gaseous emissions which change physically as they cool, or react further chemically; wind erosion can produce bizarre geographical features suggestive of deliberate activity; and fluid movements can sieve, sort and re-shape objects in positively Earth-like ways.
Our space explorers will also be aware of another point. Our dependence on the sun as our primary source of energy has predisposed terrestrial life to occupy our planet's surface, and conditioned most of us to think of planetary surfaces as the only extra-terrestrial habitats. But had our kind of life depended, for example, on the Earth's geothermal heat as its primary energy source, the surface is hardly likely to have been colonised at all. In the last two decades, space probes have revealed a bizarre selection of worlds within the solar system, especially among the satellites of the outer planets. There is Jupiter's Io, a constantly erupting, hot world of sulphur; Saturn's Europa, a smooth snowball encasing liquid water, and Titan, with cold seas of petrol-like liquids and non-biological organic matter on a bedrock of ice; and Neptune's Triton, with lakes of liquid nitrogen in a landscape of solid methane. Imaginative space scientists have noticed that Europa, being warm enough to have liquid water, could have developed aquatic, anaerobic inhabitants beneath its crust; they have also pointed out that there is a zone in the vast atmosphere of Jupiter where water is liquid and the sorts of organic compounds from which life emerged on Earth are constantly being formed: buoyant, floating organisms analogous to microbes could exist there. In both instances, such biological systems would be powered by planetary heat more than by the attenuated light of their far distant sun.
After the disappointment of the once promising Mars, floating microbes around Jupiter and anaerobic fish within Europa are but imaginative extravagances - at present. More seriously, is there any likelihood of life in the universe outside the solar system?
Certainly there is. The probability is that we are far from alone. Water (mostly as ice) and the biological elements are widespread and abundant, and locations in which the elements can come together as organic matter are numerous - radio-spectroscopy has identified many simple organic compounds in interstellar space, and they are definitely present on Jupiter and Triton. There are likely to be millions upon millions of planets in the universe as a whole, and 5 to 10% of them will have liquid water. Even within our own galaxy, the Milky Way, there is probably a vast number of planets, since it is an unimaginably huge conglomeration of between 100 billion and 1000 billion (101' to lO'Z) stars, and many of these are likely to have planets circulating around them. In 1992 David Hughes of the University of Sheffield estimated that the Milky Way has about 40,000 million (4 x 101° planetary systems, each of which might include one planet that is temperate and wet enough for terrestrial-type life. Hughes's figure is a huge increase over earlier estimates of some 100,000 (105) inhabitable planets, but either number is enormous.
Life may not have taken hold on every wet and temperate planet, and scientists have no real idea of what the proportion might be. But on statistical grounds alone it is likely to be high rather than low, especially because a wet-and-temperate planet need not be very Earth-like to support one or another kind of microbial life (see my opening paragraphs). Airlessness, high or low temperature, saltiness, acidity, alkalinity and high pressure are all tolerable provided liquid water is present. Of course, in such environments evolution is likely to have generated some beings which, by terrestrial standards, are truly exotic. But one thing is certain: where life has moved in, it will have altered the planetary chemistry to suit itself, sustaining local or planet-wide metastability, just as on Earth.
Even if Hughes's number is correct, and even if most terrestrial-type planets are inhabited, there is still no likelihood of such a planet being within 20 light-years of Earth, which means that light or radio signals from here would take two decades to reach the nearest. Over such distances, unless cosmologists' ideas about space and time are spectacularly awry, there is little chance of humanity's interacting with the life forms that have probably emerged on those planets - even if, as is likely, evolution on some has produced beings who are asking themselves similar questions.
Yet if beings at our level of technological development have evolved elsewhere, there is a chance that we could become aware of them. Early in the twentieth century, radio, and later television, were invented, and in consequence the Earth started emanating radio signals, spreading like ripples into interstellar space. They get weaker as they spread, but if a sensitive alien space probe, far out in the galaxy, were to pick up what was left of them 20, 40 or even 1000 years hence, its proprietors would recognise that they corresponded to no ordinary cosmic process, and know that something living was, or had been, active in the neighbourhood of our sun. And the reverse is also true: any alien community which uses radio for communication is unintentionally signalling its existence. That is why NASA has a programme for scanning the skies for unexpected radio signals. NASA has also deliberately beamed radio signals towards the denser part of the galaxy in case they, too, are looking out. After all, most of us would like to know whether there are other beings out there, and no doubt they would like to know of us, too.
Personally, I rather enjoy the thought that the universe probably has other inhabitants, yet I find, to my surprise, that even among scientists there are some who resist the idea of extra-terrestrial intelligences. Their outlook stems from the nineteenth-century naturalist and thinker, Alfred Wallace, who was co-discoverer of natural selection with Charles Darwin. Wallace argued that terrestrial life, and mankind, must be unique in the universe: life was balanced so finely and intimately with the Earth's climate, physics and geology, and even with its position within the solar system and galaxy, that a repetition elsewhere of so astonishingly close and delicate a relationship was beyond the bounds of probability. It is a thread of reasoning that persists even today, though rarely among biologists, who have generally perceived its flaw. It disregards the now overwhelming evidence that living things and the planet have evolved together, generating that profound intimacy gradually but spontaneously. We, and now I speak for microbes and plants as well as for my fellow people and animals, have altered the planet to suit ourselves, and if we had not done so, we should not be here. We might be somewhere else (and equally impressed by how well it suited us); more likely, we should not be at all; there would be others instead.
However, among non-scientists, the wish to believe that terrestrial life is unique can become almost passionate. With some, this desire stems from a sense that religious dogma is threatened; and I suppose that, since all the world's religions were invented by people, it is understandable that they place mankind at the centre of the universe, and that firm believers wish us to remain there. But in addition, I have been astonished to read of those who feel that humanity would be in some way diminished if other living things were discovered to be sharing the universe with us. And of yet others who truly fear invasion by aggressive aliens in space ships (quite as idiotic as those who yearn for the coming of highdomed savants, who will enforce universal love and peace upon wayward mankind). Well, they are all safe; with interstellar travel out of the question and conversational exchanges restricted to intervals of many decades, perhaps centuries, they need fear no piratical space monsters coming to attack us; no little green men stealing in and out (leaving only corn circles); and no benevolent savants, to belittle us and our gods.
Our WorldMankind has more immediate problems, however. Like a colony of microbes which has suddenly been provided with a glut of nutrient, we are multiplying exponentially: doubling in numbers roughly every 50 years. Our numbers recently passed 5.5 billion, almost spot-on the course predicted by demographers over 20 years ago. This means that by the year 2015 there will be about 8 billion people on Earth. We know this for sure, because the children are already among us who will mate and produce the extra billions, and they will do so before their parents and grandparents die. Thereafter the numbers become more speculative, but they will certainly go on going up.
Over historical times, our expanding population changed the appearance of this planet's surface spectacularly and, despite some unfortunate effects such as the deforestation of North Africa, the changes have been greatly to humanity's benefit. But since our population growth became explosive in the twentieth century - 'population explosion' is a very apt description in the time-scale of our history - we have become well embarked on changing things for the worse. Global pollution, global warming, over-fishing, declining ozone layer and such catastrophes have rightly become contemporary buzz-words, though far too many of us choose to forget that the population explosion is the root of so many of these environmental troubles.
Like human beings, microbes often alter their environments in ways that initially favour themselves, but that later become distinctly unfavourable. Unlikely as it may seem (for behavioural analogies between microbes and Man must be few), it is instructive to reflect on what causes a microbial population to cease multiplying.
First, there is the matter of resource depletion. Microbes cease multiplying when a nutrient runs out - usually the principal energy-providing food, but sometimes a minor nutrient such as a nitrogen source or a vitamin. Humans have vastly more complex resource requirements and, despite local shortages, our global habitat is nowhere near exhaustion. As far as food is concerned, our population growth has been sustained by intensive agriculture. Food shortages, even famines and starvation, occur in some parts of the world, but they do not arise from any inadequacy in current agricultural technology - which, according to the late Sir Kenneth Blaxter, could support some 9.5 billion people. They happen because we lack, as a global community, both the political and the economic will to cope. And as far as industrial resources are concerned, we now have sophisticated ways of winning some of them - such as deep mining for coal and offshore drilling for oil - and we use substitutes for resources that have become truly scarce (e.g. using plastics to replace scarce metals).
Our responses to shortages, whether of food or of raw materials, amount to increasing the quantities of energy we have to devote to winning those resources. Technologically it will be perfectly possible to go on doing just that for some time to come, especially if, probably in the late 2000s, hydrogen fusion energy becomes a reality. In principle, we could increase our energy consumption per caput sufficiently to feed and supply almost double the world's present population, perhaps more, at present nutritional standards. We have resource problems today, of that there is no doubt; but none is really insurmountable. Not yet, anyway.
However, there is also the matter of the accumulation of damaging wastes. Microbes are especially cogent examples here, because most higher organisms avoid fouling their habitats. But the growth of microbial populations is often limited by the accumulation of toxic end products, such as when the multiplication of yeasts in a fermentation is arrested by the alcohol they have produced, or when the growth of sulphur bacteria is slowed to zero by the sulphuric acid they make. These things happen because microbial habitats are generally closed: the toxic residues cannot escape easily. By this criterion, mankind has joined the microbes - and is already in difficulties. We have become so numerous that our habitat is effectively the whole planet, and is thus closed. Current anxieties over refrigerants (the 'CFCs'), greenhouse gases and global warming, atmospheric oxides of sulphur and nitrogen, detergent and pesticide residues in food and water, radioactive waste disposal and so on, arise because the waste products of human activities are beginning to cause us damage on a global scale - though so far the damage has not been sufficient to limit our population growth significantly.
Thirdly, there is the matter of disease. Epidemics are symptoms of over-population and actually limit the size of populations of higher organisms: a prime example is the effect of myxomatosis on rabbit populations. But any analogy to the biology of microbes per se is bland. Bacteria are susceptible to pathogens - to special viruses and to predatory bacteria (called Bdellovibrio) - and attacks by pathogens are commonest among dense microbial populations. So it is with people. But microbes are relevant in a way independent of analogies: it is microbes that cause human epidemics, and they are greatly favoured by overcrowding. However, in the last half of the present millennium, medical advances have contained our epidemics and ailments with great ingenuity - the last time mankind experienced such problems on a global scale was as the Black Deaths of the Middle Ages. Today's troublesome scourges, such as malaria, drug-resistant tuberculosis, schistosomiasis and AIDS, are terrible in human terms, but they do not limit our population growth. At their worst they will have barely perceptible effects on global population figures.
Finally, there is the matter of aberrant behaviour, a problem which has no microbiological analogue but which is perhaps the least tractable consequence of humanity's microbe-like multiplication.
Overcrowding predisposes gregarious mammals such as ourselves to aggressive, competitive and anti-social behaviour. If you doubt that assertion, reflect on the behaviour of normally polite and considerate people in rush hours, traffic jams, mass meetings and so on. Such behaviour is precipitated by individuals in the crowd who, giving way to anger or frustration, initiate what amounts to social breakdown among their fellows. Those individuals are examples of the deviants from the social norm who, in all sorts of ways, become the foci of local, regional and occasionally global behaviour patterns, for good or for evil as the case may be. It is self-evident that, because there are more people around than ever before, there are more such deviants than ever before. And those that catalyse social breakdown have, because of more frequent overcrowding, more opportunities to do so than ever before.
We have seen these trends operate impressively during the latter half of this century. Within small communities, there is the familiar progression from vandalism and hooliganism to street crime and violence. Between communities, the situation can become catastrophic: racism, nationalism, or aggressive religious fundamentalism lead to terrorist atrocities and wanton massacre, sometimes erupting into war. The overt causes of such breakdowns differ from case to case. Social analysts might cite poverty, boredom or escapism as causes of localised incidents; dogma, greed or varieties of tribalism beneath more widespread episodes. But they all have deeper biological roots in the response of gregarious mammals to overcrowding. In an ironic way, they are diseases of the twentieth century, because they spread like infections; diseases which have replaced the microbial epidemics which medical advances have enabled mankind to avoid. Even in Western societies, where population growth approaches zero, there are centres of high population density, the big cities, which are sliding into social breakdown; and outside the West, countries with high population growth rates retain some kind of stability only by means of oppressive regimes, autocratic, oligarchic or theocratic as the case may be - or, if political government is weak, by means of mafias.
It is a dismal spectacle, and biology offers little comfort: it tells us that, if humanity goes on multiplying, things can only get worse. Our descendants will live embattled lives, impoverished by dogma, corruption, crime and oppression.
One is compelled to the conclusion that, even if mankind could find, and use, remedies or palliatives to the environmental and social problems which we are generating, we must still stop multiplying. This is a truth that many otherwise environmentally conscious people still prefer to forget, but there is no getting away from population control. We have only one Earth to live on - at present.
New WorldsScientists are romantics at heart, even if most keep their fantasies to themselves. The thought that it might be possible to explore the skies beyond the confines of our planet has attracted scientists, as well as thinkers and writers, at least since the era in which the tale of Icarus was first conceived. Exploration has always subsumed colonisation - sometimes regrettably- and as the scope for exploring the Earth's surface dwindled in the course of the nineteenth century, space exploration became an increasingly attractive theme. Throughout the whole of the twentieth century, starting in 1903 with a Russian teacher and writer, Konstantin Tsiolkovsky, there have been a few scientists prepared to write down thoughts about ways in which mankind might colonise extra-terrestrial space. Some favoured artificial constructs: Tsiolkovsky* conceived space stations, and the British crystallographer J. D. Bernal, writing in 1929, imagined huge closed hollow spheres, able to house twenty to thirty thousand people, with animals and plants. Others preferred to consider how to establish settlements on the Moon, Mars and Venus, our nearest planetary neighbours. Such ideas are no longer the stuff of science fiction; the Russians' Mir space laboratory is a reality, in regular use; and feasibility studies carried out in both the USA and the old USSR have provided detailed plans for enclosed settlements on both the Moon and Mars (Venus is agreed to be too hot - though ways of cooling it down have been considered), as well as for constructed space colonies on Bernal's lines. ( * Tsiolkovsly's contributions are not widely known; perhaps it is appropriate that his name is perpetuated as a crater on the dark side of the Moon. )
At the present pace of technological advance, one or more of these projects will probably come to fruition during the earlier centuries of the next millennium. I find it difficult to imagine that any constructed colony in space, or even a few enclosed Lunar or Martian settlements, could do much to alleviate population pressure, simply because of the cost, in energy terms, of transporting from Earth all the installations needed.
But modifying Mars, to make it a self-sustaining environment for unenclosed colonisation, ought not to be beyond the bounds of human ingenuity. It has water, though not much by terrestrial standards, and it has sufficient gravity to retain a breathable atmosphere. But its present atmosphere, mainly carbon dioxide with a little nitrogen and argon, is so tenuous as to be incompatible with human beings, even with oxygen masks (it has less than a hundredth of the density of Earth's atmosphere). Moreover, its temperature ranges from zero Celsius to -100°: well below that of Earth's Antarctic. Nevertheless, terrestrial bacteria of various kinds survive well in simulated Martian environments in the laboratory provided they are warmed to above zero. Mars was not always so uninviting. The Mariner fly-past space probes and two Viking landers have provided strong evidence that, around 31/2 billion years ago (when terrestrial life was in its infancy), Mars was quite Earth-like. It was sufficiently wet for rain, rivers, flooding and sedimentation to erode and modify its surface, leaving distinctive features that the Mariner probes photographed; it had quite a dense atmosphere, though still based mainly on carbon dioxide and with barely any free oxygen; it was warmer by several tens of degrees; and it had active volcanoes, which would have liberated geothermal heat, lava and gaseous emissions. The more obvious pre-requisites for terrestrial-type life, albeit anaerobic microbial life, existed. But the Viking landers, as I told, provided no evidence that life had taken hold: perhaps the warm, wet period was too short. On the other hand, negative evidence is always tricky. Perhaps life did become established; perhaps Mars is now in the terminal stage of habitation, with the environmental traces of its biology no longer obvious at its surface, yet with anaerobic life forms persisting in residual wet environments beneath its surface.
The water that once splashed around Mars was largely lost into space, together with much of its atmosphere - I must refer you to books on planetary science for the reasons - and today it is a cold, harsh planet, with so exiguous an atmosphere that the little remaining water is in the form of ice or water vapour. Liquid water no longer exists on its surface: with the changing seasons, ice at the poles evaporates direct to water vapour and returns as frost. (This fact gives rise to a paradox: it can be argued that Mars is as wet as it can be, because its atmosphere is actually saturated with water vapour - given its low pressure and sub-zero temperature ranges.) Mars also has quite a lot of oxygen, combined loosely in its surface sands (as compounds called peroxides), as well as more sulphur than was expected.
Can Mars be restored to relative warmth and habitability? Possibly, but I cannot prescribe a step-by-step plan. The question has been discussed in depth among space scientists, and in August of 1991 the science periodical Nature published a fascinating survey of the possibilities. One thing we need to know for sure is whether any kind of life persists beneath the surface of Mars and, if so, what to do to conserve it. And sites with liquid water (doubtless as a strong brine) need to be discovered or established. But assuming such a project proves feasible, an essential early step in the 'rehabilitation' of Mars will be the introduction of bacteria of the kind which tolerate Antarctic conditions on Earth, including types capable of generating oxygen through photosynthesis, with the long-term objective of setting up the sort of planet-wide metastability that the Earth's global biology relies upon (including, of course, a spot of our much maligned global warming). Lower plants and simple animals will come later; humans (without space suits) much, much later. Patience will be needed: over a few centuries the planet will have to be managed so as to follow, with due attention to its own geochemistry, transformations which took the Earth over 3 billion years.
But, provided that humanity can contain the stresses of population pressure, and that our world does not degenerate into several hundreds of warring, neo-mediaeval principalities, advancing technology will enable us to know, or learn, what those transformations need to be.
It will be a supreme example of life's ability to extend its outer reaches - as so often, by altering its world to suit itself.
Mini BiographyJOHN POSTGATE, FRS, is Emeritus Professor of Microbiology at the University of Sussex, where he was also Director of the Unit of Nitrogen Fixation. He was educated at Kingsbury County School, among others, and Balliol College, Oxford, where he took a first degree in chemistry before turning to chemical microbiology. He then spent fifteen years in government research establishments - studying.mainly the sulphur bacteria and bacterial death - before moving to the Unit at Sussex, where he spent the next twenty-two years. He has held visiting professorships at the University of Illinois and Oregon State University and has been President of the Institute of Biology and of the Society for General Microbiology. He is the third Professor John Postgate: the first his great-grandfather taught medicine at Birmingham University, the second his grandfather taught classics at Liverpool University. His other grandfather was George Lansbury, the Socialist leader, and his father was Raymond Postgate, the historian and gourmet. Long ago John Postgate led the Oxford University Dixieland Bandits (on cornet), and he is known as a jazz writer. He and his wife, who read English at St Hilda's College, Oxford, have three grown-up daughters.
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