On 4 July, the US spacecraft Pathfinder, one of three launched last November, will enter the thin atmosphere of Mars. Though the Martian atmosphere is about 1 per cent of the Earth’s, the buffeting will slow the spacecraft down from 7.5 kilometres a second to about 400 metres a second, or 900 miles an hour – which is slow enough for a parachute to open (rockets will help). Pathfinder will literally bounce into touch, bobbing on a cocoon of inflated airbags, before coming to rest on Martian soil. The touchdown will be in a region called the Ares Vallis, chosen because it seems to be a huge wadi or dried-up watercourse. A hatch will open, and out will pop a little wheeled robot rover called Sojourner, which will beetle about the immediate terrain, examining rock chemistry and reporting back to the lander, which will relay data and pictures back to Earth.

In September, Pathfinder will be joined by a second American spacecraft, Global Surveyor, which will stay in orbit round the Red Planet. The third, a Russian satellite called Mars 96, burned up shortly after its launch: a reminder that in space the smallest errors impose huge costs. The loss, though awful for the British, European, American and Russian scientists who spent years designing the experiments on board, will not make much difference in the long term. Between now and 2005, pairs of robot spacecraft will leave the Earth for Mars every 26 months or so. The ostensible and often-stated reasons for the programme are simple: if we learn more about the climate and history of Mars we will understand more about the climate and history of Earth.

That is the official line: But many people inside space science and most people outside believe something more immediately interesting: that Pathfinder and Sojourner and the Global Surveyor are looking for life on Mars. This is because a team of US scientists announced last August that they thought they could see evidence of bacterial action in a meteorite, found in Antarctica, with a chemistry that identified it as having come from Mars: evidence, in other words, of life on Mars at one time, even if not now. When they made the announcement there were sharp intakes of breath in space science departments everywhere. (Altogether, 12 meteorites have been identified as bits of Mars, chipped loose in a bygone catastrophe – a collision with something big, like an asteroid or comet – which had spun around the solar system before bumping into Earth. A number of meteorites, Martian or otherwise, contain organic chemical compounds.) The claim was tentative, and some dismissed it, but since then other teams have claimed to have seen what might very well be confirmatory evidence. Unfortunately, the experiments and instruments aboard the spacecraft had already been designed and tested. It was too late to think of some really simple and dramatic extra: one of those sensors like the ones in Star Trek that enable you to say: ‘It’s life, Jim, but not as we know it.’ So the Mars probes will be looking for liquid water or evidence of liquid water in the past, and for resources that might support eventual human exploration. There isn’t any convincing evidence of life beyond Earth, but, once again, we are looking, and this time we expect to find it.

In 1969, the Apollo 11 astronauts, returning from the Moon, were freed from their capsule by men in masks and immediately clapped in quarantine, in case they introduced some dreadful alien infection to the planet. Even at the time, Nasa knew there was nothing much to fear. The Moon looked as though it had always been sterile. Robot spaceprobes went further afield. Venus, once imagined as a warm, tropical garden, an Eden for one of C.S. Lewis’s science fiction parables, turned out to be a hellish place: a surface hot enough to melt lead, covered by thunderous stormclouds which rain boiling acid. Mars, the planet on which astronomers once thought they could actually see canals, seemed an unlikely home for life: it was covered by huge dust storms and its polar caps were coated in frozen carbon dioxide. Just over twenty years ago, Viking landers touched down on Mars and sent back their instrument readings. Nothing. No remains of a vanished civilisation, no little green men, no little green plants, nothing. Humans contemplated the consequence. They were alone around their own star. The nearest star to theirs was four light-years away. That is, if you could accelerate a spaceship to 300,000 kilometres a second, which you couldn’t, it would still take more than four years to get to Alpha Centauri, and there was no evidence that any of the stars in that group had encircling planets. Mankind was effectively alone.

A new school of hardliners emerged. Maybe life was a unique event, a miracle of chance, or a miracle of God, a chain of improbabilities that began in a warm pool on a hot new planet billions of years ago. Debate focused on the odds stacked against life happening at all: people looked at the atmospheres of Mars, Jupiter and Venus, and deduced that Earth, too, must once have had no oxygen. If there had been oxygen, the gas would have oxidised the first fragmented attempts at life out of existence. Life’s chemistry had to get started by means of lightning or ultraviolet passing through a soup of methane and ammonia and carbon dioxide. The resulting amino acids had to form self-replicating chains of protein, and these to turn into cells deriving energy from the only supply around: the Sun. Which meant they had to develop the ability to photosynthesise, in order to suck carbon dioxide from the primeval air, capture the carbon to build more tissue, and excrete the waste product of oxygen. Only then could more advanced creatures exploit the oxygen. All this had to be done when the Sun was new and its light faint: too faint to keep water from freezing. No wonder it took 3.5 billion years to get as far as blundering bipeds with sharp stones in their hands.

This story – pieced together only in the later years of this century, to a large extent in the wake of planetary exploration – was part of the mythology of the ‘blue planet’, seen for the first time by the Apollo astronauts: blue, because it had seas, life and oxygen; blue, and therefore unique, and precious. Scientists took to calling Earth the ‘Goldilocks planet’: the only one where everything was just right, where the forces were so finely balanced that life could, after many false attempts, and against absurdly long odds, get started.

Some were not even sure about that. Some argued that the self-creation of the length of DNA needed to instruct a creature how to eat, drink and move around, to repair its own tissue and shelter itself from radiation, to defend itself against infection and then to replicate itself accurately – pretty basic stuff – would be like expecting a monkey hitting typewriter keys at random to produce the whole Bible without error. With 26 letters in the alphabet and six million characters in the Bible, the probability of that happening is 1/26th, multiplied by itself six million times. (The monkey/typewriter image keeps popping up in the extraterrestrial life debate. Sir Fred Hoyle, whose views on where life comes from are not orthodox, has used a different parallel: the creation of life from organic elements would be like a hurricane blowing through a junkyard and assembling a working jumbo jet.) That is where the debate rested a few years ago.

Perhaps as a consequence of this declaration of cosmic loneliness, the flying saucer industry went into a spin. Star Trek and Star Wars were wistful dreams of what might have been. Douglas Adams’s Seventies radio masterpiece, The Hitch-Hiker’s Guide to the Galaxy, was a nose-thumbing at the horror of it all: a whole universe, 15 billion light-years across, and hardly anybody in it at all, except on the surface of a small planet near the edge of an inconsequential galaxy – and some of them were hairdressers.

The faith in a populated universe is an old one. Lucretius introduced the notion of panspermia – the interplanetary diffusion of life by means of germs carried by meteorites – two thousand years ago. Svante Arrhenius, the man who predicted the greenhouse effect a hundred years ago, also believed in a universe permeated by the seeds of life. Fred Hoyle believes that the seeds are not just the seeds of life but the seeds of death: in his latest book he argues once again that viruses, too, can be explained as parcels delivered by comet express. In the medieval Christian universe, there wasn’t much room for alien civilisation – the cosmos of Dante and Aquinas had man at the centre, surrounded by the more perfect spheres of Heaven. The Copernican revolution changed all that. Even before it was clear that the Sun was a star like other stars, and that the galaxy was just part of a universe of galaxies, humans had begun to populate the cosmos with aliens. Some of those who speculated on these matters were giants of the Enlightenment. Benjamin Franklin wondered about the constitutions of those ‘who live on the planet Mercury’. Tom Paine used the plurality of worlds as an argument against Christianity–redemption implies uniqueness. The extraterrestrial assumption had the support of people who believed in science, as well as of those who believed in the literal truth of the Bible, and some determined to have the best of all worlds. Samuel Kinns, for instance, author of an anti-Darwinian work, Moses and Geology, or The Harmony of the Bible with Science (1882):

Spectrum analysis has revealed to us the composition of the distant stars. Sodium, magnesium, calcium, iron, bismuth, hydrogen and many other terrestrial elements are there; and as we find our earth is composed of the same minerals as our Sun, so the planets revolving around Alpha Centauri, Sirius, Aldebaran, Betelgeux and all the rest, may be composed of the same minerals as their suns. May not the presence of iron indicate probable industries of every variety? And would not sodium tell of seas, and hence of rivers, with all their accompanying vegetation? Would not calcium remind us that those seas might teem with life, and that the crustaceans, with their shells of carbonate of lime, would form strata of chalk, afterwards to be calcined into marble? And with that marble, might not palaces arise and noble architects to rear them? However imaginary this may appear, there are considerations that render it quite probable. Surely we need not now think that this Earth, which is but an infinitesimal atom in space, is the only spot where intelligent beings are living to enjoy and admire the beneficent and wondrous works of the great Father of the Universe? Nay, nay! As there are millions upon millions of suns, so there may be millions upon millions of beautiful worlds.

Many speculators have played the numbers game. There are 100 billion galaxies, each with 100 billion stars. If just 1 per cent of stars have planetary systems, and 1 per cent of those have Earth-like planets, and life got started on only 1 per cent of those Earth-like planets, and an intelligent civilisation developed on 1 per cent of those with any life-forms at all, and 1 per cent of those civilisations were smart enough (or dumb enough) to be looking out for other life, trying to get in touch, like we are, then all that winnowing still leaves the universe electric with civilisations: a thousand or so cultures advanced to the standard of Einstein or Jeffrey Archer in this galaxy alone. So where are they all? When John Logie Baird invented television, he also invented a microwave messaging system broadcast through interstellar space. For forty years, programmes like I Love Lucy and Rawhide and Dynasty have been spearheading planet Earth’s attempt to get in touch, expanding across space at the speed of light. So far, no one has asked us to turn the stuff off, or even down.

There are UFO-watchers and UFO-believers everywhere. Hilary Evans and Dennis Stacy have compiled some of the histories of the UFO years for a Fortean Times book: Evans calls the phenomenon ‘a rich and revealing myth’. That millions believe something doesn’t make it true; and the case against visitation is compelling. All sorts of people experience what they believe to be sightings, but the professional sky-watchers, the astronomers with instruments so powerful they could resolve a football twice as far away as the Moon, or collect photons from a distance of 13 billion light-years, or measure variations in the radiation left over from the Big Bang, have not seen any spaceships. That does not convince the converted, who believe that the aliens inhabit another dimension. And this is the huge joke about the alien life debate: we have been looking in the wrong place. Alien life turns out to be everywhere, living in conditions once thought impossible for life. There has been an error of scale. Because we occupy five or six feet of vertical space, we imagine it to be a norm for life. But 99 per cent of all living things are less than 3mm long. Dolphins and ducks, lions and lemurs, men and mice, are all very unusual. Arthropods – jointy-legged things which include insects and spiders – exist in millions of different forms and they, too, are unusual. Four out of five higher life-forms on the planet are in fact tiny nematode worms, but even they are not the dominant life-form. The aliens most likely to be detected on other planets are single-celled microbes. These are beginning to look like the masters of creation. Stephen Jay Gould points out that the first two billion years of life on Earth was the age of bacteria, and nothing much has changed since. Taxonomists used to think of evolution as a ladder, from microbes to us, and then as a tree, with humans at an uppermost tip. Creation is now drawn as three linked bushes, and all three bushes are of microbes. At the tip of one of those three bushes – a group of microbes called eukaryotes, which includes the yeasts – is a twig with three tines. On one are all the fungi. On another all the plants. On the third all the animals – worms, insects, elephants and people. Humans can be thought of as organised collections of specialised cells which must pay for their complex lives with death. Bacteria don’t die. They just divide and go on for ever, if they can manage it. Humans can be thought of as vehicles for microbes to exploit. There are about a million million cells in the human body, but each human body is host to about a hundred million million microbes. We are perambulating sacks of microbes, 10 per cent human, 90 per cent bacteria.

At about the time humans were failing to find alien life on Mars, oceanographers had begun to discover strange communities living two or three miles below the surface of the ocean. Here it is very cold and utterly dark: since life depends on the Sun’s heat, the ocean floor was imagined as an arid zone, populated at best by scavengers fossicking for the few carcasses dropping from the surface. But as submersibles explored the submarine volcanoes and thermal vents of the mid-ocean ridge, where new ocean floor is created to push the continents on their slow journey round the globe, they found something quite unexpected – huge quantities of bacteria, and large communities of worms and shrimps living on the bacteria. Instead of sunlight for energy, the bacteria used the heat of near-boiling water bubbling up from below the crust. And they were dining, not off rotting vegetation or decaying carcasses, but off minerals in the hot brines. Since then, wherever scientists have looked, they have found microbes making themselves at home in improbable places.

These creatures are sometimes called extremophiles. They live where oxygen does not exist, at huge pressures and in conditions of terrible heat. Some of them are identified as Archaea: the old creatures, the ones who might have inhabited the planet in its Hadean phase, when the atmosphere was thick with carbon dioxide, and comets and meteors pounded the surface. They might have been life’s founders. They were, for a while, only a curiosity. They are now becoming an obsession (naturally, they have their own website: www.reston.com/astro/extreme.html). There are ‘autotrophs’ that seem to eat basalt: they use hydrogen gas for energy and get carbon from inorganic carbon dioxide. Autotrophs have been found three kilometres below the surface, where the only source of heat is the heat of the rocks, and they have been found hundreds of metres below New Mexico, in water that sank below the surface 30,000 years before, living off the remains of things which died 100 million years ago. They have been found at temperatures of 113 Celsius and they have been found in the frigid Arctic ocean, grazing off the algae underneath the ice. They have been found in lakes of salt and of soda; in streams of acid; in toluene, benzene, cyclohexane and kerosene; and at 11,000 metres down the Marianas Trench. It has gradually become clear that life does not require a finely balanced set of conditions. Perhaps it just requires liquid water, a source of heat and some chemicals to exploit.

Thomas Gold of Cornell, an old colleague of Fred Hoyle’s, and also for years considered to hold surprising views, had argued for at least a decade that there was microbial life deep in the Earth’s crust, where the rocks are hot but the water stays liquid because of the pressure. In 1992 he claimed that there was so much microbial life that if it was smeared over the planet’s land surface it would form a layer of gunk 1.5 metres thick. Within a year or two, others began to dig up evidence of microbe communities miles below the surface. The vindicated Professor Gold then took the matter further. If there is life below the surface of the Earth, then there might have been life in the crust of Mars, he said at a conference in Seattle this year. And there might still be life there. The Earth had no special prerogative to develop life. ‘As long as you think life is possible only on planetary surfaces, the Earth is uniquely suitable. But when you talk about life deep below, the Earth is not unique at all. The deep, chemically-supplied life may be common, not only in the solid bodies of the solar system, but throughout the Universe.’ Several planetary scientists and oceanographers have begun to wonder whether the moons of Jupiter might not harbour life. The Galileo probe has visited one of them, Europa, and photographed a crust of ice under which there might be an ocean of liquid water. This could only happen if Europa had a hot core. A source of heat, and liquid water: what more do you need for life?

The discovery of microbes in funny places wasn’t entirely responsible for the sudden switch from doubt to faith. Once upon a time, evolutionary theory was expected to account for giraffes’ necks and pandas’ thumbs: now evolution’s principles have been harnessed to account for life itself, and even – in two books this year – the evolution of universes. Professor Cesare Emiliani of Miami University has put the monkey/typewriter argument a bit differently. A monkey’s typing is indeed random. But suppose the monkey’s computer worked by natural selection, so that each wrong letter typed would be erased. This is what the environment does to life’s errors: it rubs them out. ‘Typing away at one letter per second and assuming an average of 13 errors per letter (half of 26) the monkey will produce the Bible in 13 × 6,000,000 seconds: 2.5 years. Not only that, you are mathematically sure that the monkey will produce the Bible within that time and without a single error,’ he wrote last year in The Scientific Companion. ‘Given the chemical and environmental conditions of the primitive Earth, the appearance of life was a foregone conclusion. Only divine intervention could have kept the planet Earth sterile.’

This is why the touchdown on Mars will be watched with quite unusual interest. If Pathfinder and Sojourner find nothing, that will not be the end of hope. If they do find life, it will not be the end of argument. William Clark, in Sex and the Origins of Death, describes the extraordinarily resourceful life of bacteria. When the going gets tough, bacteria sporulate. They wrap themselves up and play dead. Bacterial spores have been heated to boiling point, and cooled to −270°C, which is the temperature of space between the stars. When things get better, they come to life again. Bacteria spores have been recovered from inside the guts of frozen mammoths, inside the bricks of Inca pyramids, from inside the intestinal tracts of bees trapped in amber millions of years ago. They could probably survive a journey through space. Could it have been Archaea from Mars which splashed into Earth’s primeval ocean and set off life on this planet? Alternatively, might some great cometary snooker cue have sent a ball of Earthly material soaring off into space, to land on Mars, delivering the seeds of terrestrial life? If planetary scientists do detect life on Mars, they still won’t know whether it is Martian life, or really Earth life sent on ahead.

And when, finally, a manned expedition sets off for Mars, it may well be that the scientists and astronauts on board will be ex-Martians returning from a three billion-year exile, about to begin the slow process of making a smaller, colder planet habitable, a process popularly known as ‘terra-forming’. There are a number of recipes for doing this. One sure way is to introduce machines programmed to change the original hostile atmosphere of the primeval planets into something that holds heat, reflects dangerous ultraviolet and can be breathed safely. We already have these things. They are called microbes: they made Earth a home for other life, and they can do the same for Mars. Maybe they did it once, long ago, before some abrupt cosmic catastrophe erased their handiwork. That is what we hope Pathfinder and all the other space probes will find out.

Books referred to in the writing of this piece:

Life on Mars? The Case for a Cosmi Heritage by Fred Hoyle and C. Wickramasinghe (Clinical Press, 222 pp., £17.50, 2 June, 1 85457 041 2).
The Case for Mars: The Plan to Settle the Red Planet and Why We Must by Robert Zubrin with Richard Wagner (Simon and Schuster, 320 pp., £16.99, 3 March, 0 684 81930 9).
Evolution of Hydrothermal Ecosystems on Earth (and Mars?): Ciba Symposium 202 (Wiley, 346 pp., £55, 15 November 1996, 0 471 9 6509 X).
The Rivers of Mars: Searching for the Cosmic Origins of Life by Piers Bizony (Aurum, 190 pp., £9.95, 24 April, 1 854 10495 0).
UFO 1947-97: Fifty Years of Flying Saucers, edited by Hilary Evans and Dennis Stacy (Brown, 272 pp., £16.99, 1 May, 1 870 87099 9).
Life’s Grandeur by Stephen Jay Gould (Cape, 272 pp., £16.99, 7 November 1996, 0 224 04132 0).
Sex and the Origins of Death by William Clark (Oxford, 208 pp., £16.99, 30 January, 0 19 510644 X).

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