The prospects of finding life elsewhere in the Solar System
Thirteen thousand years ago, a dazzling meteor flashed into existence in the skies above Antarctica. As it ploughed earthwards, the heat of its fall ripped apart the atoms of the air, leaving behind a brilliant trail that lit up the icy landscape. It would have made a beautiful sight, if anybody had been there to see it. The meteor's surface melted, then vapourised, and as the rocky ball tore towards the ground it slowly shrank. Ordinarily such an object would have been completely destroyed long before making contact with our planet's surface, but this one was not. A small chunk - about two kilograms of it - survived the fall, and lay there, hot and steaming, on the cold Antarctic ice.
Most meteorites that are found on Earth are simply interplanetary debris - small pieces of junk left over from the violent formation of our Solar System, four and half billion years ago. But this particular meteorite was special. It had come from Mars, blasted from its home planet fifteen million years ago by a cosmic impact even more spectacular than the one in which it fell to Earth. This potato-shaped lump of rock, codenamed ALH84001 by the scientists who first discovered it, was a messenger from another world. And buried deep within its baked interior lay what may have been humanity's first tantalising glimpse of an alien lifeform.
Every few years, the red disc of Mars passes particularly close to the Earth, and human observers are afforded an especially good view of the planet that has fascinated mankind since ancient times. This fortunate configuration of the planets is known to astronomers as a "favourable opposition", and one such event occurred in the year 1877. Among the many observers who turned their telescopes towards the Red Planet in that year was the Italian Giovanni Schiaparelli, who was surprised to see a network of dark, greenish lines criss-crossing the planet's rusty surface. He named these features canali, meaning 'channels'. The word was wrongly translated into English as 'canals', and fueled wild speculation about who might have built these canals, and why. The American astronomer Percival Lowell, amongst others, entertained the idea that they were the work of an advanced Martian civilisation, forced to construct these enormous channels in a desperate attempt to irrigate their arid and dying world.
Fanciful though his ideas may have been, Lowell was not the first to speculate about life on Mars. Ever since the seventeenth century, astronomers had studied Mars through their telescopes, and reported that in many ways it is a planet very similar to our own. A day on Mars is only 37 minutes longer than a day on Earth; the planet has white polar ice caps, just like those at the Arctic and Antarctic; and the 25-degree tilt of Mars' axis gives it seasons very similar to those on Earth. If Mars is so Earth-like in so many respects, perhaps it, like the Earth, would be capable of sustaining life.
It was not until the 1960s that the first interplanetary spacecraft journeyed past the Red Planet, and the images that they sent back were disappointing ones. Lowell was right about one thing: Mars is arid, a dusty desert world. The canals, however, had been an optical illusion. Photographs of the Martian surface revealed no network of lines, and no green vegetation. The first pictures of Mars merely showed a faintly cratered landscape, as blank and barren as our own lifeless Moon.
Closer examination of the photographs, however, revealed features that looked startlingly like the remains of ancient, dessicated riverbeds, meandering gracefully across the planet's scarlet surface. In other places, there are the dry remains of 'outflow channels', created when flash floods gushed suddenly across the Martian terrain. The vast green channels observed from Earth by Schiaparelli may have been imaginary, but on a smaller scale it seems that there really are canali (or at least their dried-out remnants) on the surface of Mars.
Today, of course, no liquid water flows on Mars. The planet is around 80 million kilometres further from the Sun than the Earth is, and has a climate comparable to that of Antarctica. Worst of all, the existence of liquid water requires not just suitable temperatures, but also suitable pressures; when the pressure is too low there is nothing to stop molecules from simply flying away from a liquid's surface, causing it to boil furiously. The carbon dioxide atmosphere that envelops Mars is vanishingly thin, and the two Viking spacecraft that landed there in 1976 recorded an atmospheric pressure more than a hundred times lower than on Earth. Even at the planet's equator, where temperatures can rise to quite a comfortable 20°C, any liquid water would evaporate almost instantly into the rarified Martian air.
Nonetheless, the presence of ancient riverbeds and channels suggest that Mars once had a warmer climate, and a much thicker atmosphere. The two facts are related: a thick carbon dioxide atmosphere would have trapped solar heat in a 'greenhouse effect' very similar to the one that operates on Earth today, warming Mars to a temperature at which liquid water could flow.
Planetary scientists now believe that the primordial atmospheres of Earth and Mars were initially very similar: a thick mixture consisting primarily of carbon dioxide, water vapour, and nitrogen, all of which would have outgassed from volcanoes in enormous quantities as the young planets cooled. The idea that the two planets were once very similar makes obvious sense - after all, they both accreted from roughly the same materials under roughly the same conditions. There was little if any free oxygen in these primitive atmospheres, since oxygen is a highly reactive substance and would quickly have combined chemically with other materials. (The oxygen present in Earth's atmosphere today has built up over the past three billion years as a result of the tireless efforts of photosynthesising organisms.)
If Mars once had a thick atmosphere, where did it go? An unfortunate property of carbon dioxide is that it combines with water to produce carbonic acid, which corrodes into rocks, locking the gas away in the form of carbonate minerals. Over millions of years, this could have all-but-destroyed Mars' atmosphere. The same process does occur here on Earth, but luckily our own atmosphere is maintained by volcanic activity, which boils up carbonates and pumps the resulting carbon dioxide back into the air. Mars, being smaller than Earth, cooled down much more quickly after its formation, and as a result its volcanoes quickly stopped. With surface gravity only a third as strong as on our own planet, many of the other gases, such as nitrogen, slowly escaped into space. Eventually, Mars became so cold that its oceans froze, preventing further loss of gas. By now, sadly, there was very little left.
Although it is dry today, few scientists doubt that the early Mars did once have liquid water - and a rather pleasant climate. This idea has again fuelled speculation that the planet may once have harboured living things, albeit primitive and microscopic ones. But of course, living things are not composed simply of water. Their cells are built up from a fantastic array of complex carbon compounds: so-called 'organic chemicals'. Could these have existed on the early Mars?
In the 1950s, scientists Stanley Miller and Harold Urey filled a flask with a mixture of simple chemicals similar to those found in the primordial atmospheres of Earth and Mars, heated it (to simulate volcanic activity), and subjected it to electrical discharges (to simulate lightning). The result was an astonishing brew of complex organic compounds, including most of those that are important in living organisms. There were sugars, amino acids (the building blocks of proteins), and most of the chemical components that make up DNA, all in surprisingly large quantities. Miller and Urey were, of course, trying to demonstrate that organic material could have been formed on the early Earth, but similar processes must surely have occurred on Mars.
The building blocks of life need not even have originated on the planets. We now know that the Universe is full of organic matter, and that it is formed just about anywhere that basic carbon-containing materials are subjected to ultraviolet light or electrical discharges. There are thousands of 'carbonaceous' metorites flying around the Solar System, some of which are known to contain a rich variety of organic compounds. Such materials are not even confined to our Solar System. In 1994, analysis of the light shining out of the distant interstellar dust cloud Sagittarius B2 revealed the tell-tale spectral signature of the amino acid glycine, an important biological molecule. Similar clouds have also been found to contain other familiar carbon compounds, such as ethyl alcohol and formaldehyde.
It therefore seems very likely that the early Mars had not only water, but also many of the biochemical building materials upon which life depends. What else would an extraterrestrial organism require in order to survive on a planet? What about oxygen gas - a material that was never present in large quantities on Mars? We are all taught from a young age that oxygen is essential for living things, yet in fact this simplistic statement conceals a horrible irony. Oxygen, being highly reactive, is a ruthless destroyer of organic materials. The animals and plants that inhabit the Earth may have learned to cope with oxygen, and even depend on it. But within each of our cells there is a continuous biochemical battle against the ravages of this element - a battle that we all eventually lose. The most primitive microbes on our planet - those that are thought to be most similar to the common ancestor from which all life on Earth is descended - are poisoned if they come into contact with oxygen. And experiments like those undertaken by Miller and Urey fail miserably if carried out in an oxygen-rich environment.
At the moment, the most hotly debated scientific question about early Mars is not whether life could have arisen there, but whether it actually did. This is a much harder issue to investigate, since it comes down to one of the most fundamental - and unanswerable - questions in modern biology: is the origin of life, given suitable conditions, a highly improbable event? Scientific opinion is divided on the matter. "I suspect that, given the composition of early atmospheres and oceans, life's origin was a chemical necessity," wrote the American palaeontologist Stephen Jay Gould in his book Wonderful Life. Other scientists are more skeptical. The prominent evolutionist Richard Dawkins, amongst others, has pointed out that there are probably tens of billions of planets in our Galaxy alone, yet we only know for certain that life has appeared on one (our own). It is therefore not unreasonable to speculate that the origin of life is a very, very improbable event, and that the chances of it happening twice within one Solar System are virtually nil.
The entire issue is made more problematic by the fact that biologists can not even agree on what a 'living organism' is. Molecules capable of self-replicating, and even evolving, have been synthesised in laboratories, but few people would consider these to be 'alive'. The most basic thing that would be universally accepted as 'life' would be a single-celled organism resembling a bacterium, yet even this apparently mundane lifeform is in fact a fantastically complicated chemical machine.
A bacterium, like any other living cell, is essentially based around proteins. There are several hundred different types of protein in even the simplest organism, and each of these typically consists of a string of several hundred amino acids, which must be arranged in a precise sequence in order for the protein to function. Proteins hold an organism together, catalyse important biochemical reactions, and provide molecular markers that allow each cell to 'identify' its neighbours. The instructions for making all of the different proteins are stored, in an intricately coded form, in the form of long chain molecules of deoxyribonucleic acid (DNA). The DNA of even a very simple bacterium contains about as much information as the entire New Testament. This whole DNA sequence must be copied, almost perfectly, every time a cell divides. The 'translation' of DNA code into a protein chain is carried out with the help of an additional type of molecule, ribonucleic acid (RNA). Like DNA, RNA consists of a long string of chemical 'letters', but RNA is more versatile than its sister molecule, and can catalyse chemical reactions as well as storing information. Of course, the continuous construction of proteins and DNA strands requires raw materials such as amino acids (of which there are twenty that are used by living organisms), and the chemical components from which DNA and RNA strands are assembled. These must be synthesised from inorganic materials such as carbon dioxide, with the help of yet more proteins. All of these processes require energy, which microbes obtain by carrying out controlled chemical reactions, once again catalysed by proteins. In more advanced organisms, these reactions involve the controlled 'burning' of sugars and other organic molecules, via an almost incomprehensibly long chain of intermediate substances, but the first bacteria to appear on Earth probably thrived on more bizarre fuels, such as iron and sulphur. A microorganism's entire chemical apparatus is protected by being enclosed within a fatty droplet: a cell.
Most of the individual chemical components of a bacterium can be put together in the laboratory, given the right conditions. A solution of the molecules that make up RNA will, if left alone in a warm test tube, spontaneously assemble into molecules capable of self-replication (but only if the mixture is supplied with a suitable catalyst). Fatty droplets resembling cells can form quite easily under natural conditions. However, putting all these parts together into a working organism has yet to be achieved even by intelligent scientists, and envisaging how the process could come about in nature clearly requires a great deal of imagination.
It appears that the only way in which scientists could find out if life ever arose on Mars would be to physically look for its fossil remnants, in a suitable lump of Martian rock. Most people associate 'fossils' with dinosaur bones and similarly gigantic structures, but micro-organisms can also leave fossil imprints in certain minerals, and these 'microfossils' can be seen and examined under powerful microscopes.
With a manned mission to the Red Planet probably several decades away, in the early 1990s it began to look as if scientists would have to wait a while for a chance to go looking for Martian microfossils. Enter ALH84001. In 1993, it was announced that this meteorite - found in the Allan Hills area of Antarctica nine years earlier - was of Martian origin. Three years later, in August 1996, a NASA-funded team of scientists proclaimed to an astonished world that they had discovered what appeared to be the fossil remains of tiny micro-organisms, deep within the meteorite. After centuries of waiting, it seemed that mankind had finally found the first solid evidence of extraterrestrial life.
The idea that ALH84001 contained fossil lifeforms was based upon three main observations. Firstly, and most obviously, the meteorite contains numerous microscopic globules made of carbonate minerals, and these look strikingly similar to some of the bacterial fossils that have been found in terrestrial rocks. Secondly, the metorite contains a combination of materials, such as iron sulphides and magnetite, which are often formed here on Earth by bacterial processes. And thirdly, the meteorite contains complex organic compounds called polycyclic aromatic hydrocarbons (PAHs) that are associated with decaying organisms. All of these things are found remarkably close together within the meteorite - just as would be expected if they had formed from decomposing microbes.
Since this remarkable finding, many papers on ALH84001 have been published. A few of them endorse the original assertion that the meteorite contains evidence of Martian life, but many others have criticised it. There is little doubt that ALH84001 is from Mars: detailed analyses of the meteorite's chemistry produce results that match those obtained from genuine Martian rocks by the Viking landers. Whether the microscopic structures inside come from Mars, and whether they represent evidence of life, is another matter. Similar features have been found inside meteorites of lunar origin, yet nobody is mad enough to suggest that our barren Moon has ever harboured life. The structures in the lunar rocks must either be recent fossils of terrestrial bacteria that have contaminated the meteorites, or they must be of non-living origin. If this is the case for other meteorites, why should it not be true for ALH84001? The idea that the 'bacteria-shaped objects' (as they are tactfully described) are not fossils at all is supported by work published last year, in which a team of scientists managed to create identical structures within just a few weeks using entirely inorganic processes.
The original interpretations of the meteorite's chemistry have been similarly challenged. Some scientists have concluded that the minerals in ALH84001 formed at temperatures that would have roasted any living organism - however alien. As for the PAHs, it is becomingly increasingly clear that organic chemicals need not necessarily be formed by organic processes. But by far the most compelling evidence that the objects in the meteorite are not microfossils is that they are simply too small. All the complex chemical machinery required by a free-living organism requires a certain minimum amount of space, and this places a limit upon how small a bacterial cell can become. It might be reasonable to speculate that Martian bacteria were slightly smaller in size than their terrestrial counterparts (after all, they did come from another planet!) but the structures in ALH84001 do not even come close. Some of them, only a few billionths of a metre in diameter, are barely large enough to contain even the thin outer membrane of a living cell.
Of course, the fact that one lump of Martian rock does not contain the remains of ancient bacteria is not proof that such organisms have never existed. Martian microfossils may yet be found - and there are plenty of people determined to keep looking. However, what really excites modern exobiologists is not fossil remains, but the possibility of finding Martian microbes still living somewhere on the Red Planet. Modern Mars may be a dry and hostile place, but bacteria living in Antarctica are known to survive under similarly hostile conditions.
Back in 1976, the Viking landers did test the Martian soil for signs of life. The results were inconclusive. In one experiment, in which nutrient medium was added to a sample of soil, gases were given off, apparently indicating that the nutrients had been metabolised by living things. However, instead of proving that micro-organisms do live in the Martian soil, this experiment may in fact be the most definite evidence so far that they do not. The current interpretation of the experiment is that the Martian soil is full of powerful 'oxidizing agents', oxygen-containing chemicals that were formed over billions of years from water that was broken down by ultraviolet sunlight. (It was this process that slowly rusted the planet's iron-rich surface, giving Mars its deep red colour.) Like bleach - the familiar household oxidizing agent that has been 'killing all known germs' for over a century - these chemicals would destroy any living organism that came into contact with them.
If life does thrive on Mars today, it will be found deep underground. Perhaps there are warm sub-surface aquifers somewhere on Mars, where warm water still flows and living things might survive. Mars still holds many mysteries - and although future robotic space missions will answer some of the questions, it will take a manned Mars mission to truly explore the Red Planet.
So far I have concentrated entirely on the planet that has been the subject of the most speculation regarding extraterrestrial life - Mars - but is there any other world within our Solar System on which life might be found? Until forty years ago, the planet Venus seemed a promising candidate. It has volcanoes and a thick atmosphere of carbon dioxide, and in its size and density the planet has been described as 'Earth's twin'. Unlike in the case of Mars, however, the era of unmanned space exploration quickly settled the question of living things on Venus once and for all: the planet is sterile. Venus's pretty white cloud tops are made of concentrated sulphuric acid, and at its gloomy surface the atmospheric pressure is ninety times higher than on Earth. Underneath such a thick, heat-trapping blanket, Venus roasts at temperatures of over 470°C. Not even the hardiest living thing could survive in such a place. The other planets are similarly hostile to life. Mercury and Pluto are barren lumps of rock, and the giant planets Jupiter, Saturn, Uranus, and Neptune have no solid surface on which life could develop.
Another place to look for extraterrestrial life was suggested by the astronomer Fred Hoyle. In his book The Intelligent Universe, Hoyle puts forward the controversial idea that bacteria are capable of travelling from one planet to another, hitching rides inside meteorites. Amongst other sources, he cites evidence from Hans Dieter Pflug, who had examined a meteorite that fell in Murchison, Australia, in 1969. Inside the Murchison Meteorite, Pflug claims to have found not only a complex variety of organic compounds, but also microfossils that look remarkably like the primitive terrestrial bacterium Pedomicrobium.
However, Hoyle's ideas were almost universally condemned by the scientific community. Although some microbes have been known to survive space journeys (bacteria on the outside of the Surveyor 3 spacecraft continued to survive on the Moon for two years before being brought back to Earth), it seems highly implausible that organisms could remain viable for thousands of years in the hostile conditions of interplanetary space. If anything, Hoyle's book - which predates the ALH84001 controversy by several years - only serves to remind us of how easily inorganic structures can be mistaken for bacterial fossils.
One final hope for the existence of life elsewhere in the Solar System lies with the various moons that orbit the planets. Many moons are little more than giant boulders, but there are seven 'planet-sized moons' that deserve mention. One of these is the blatantly lifeless world of our own Moon: dry, dusty, and scorched by solar radiation. Another is Neptune's moon Triton, the smallest and most remote of the seven. On Triton, the distant sun is little more than a bright star in the sky, and temperatures are typically only forty degrees above absolute zero. At this temperature, the little moon's nitrogen atmosphere freezes into an unearthly snow. Triton is obviously not somewhere that we could expect to find life.
A far more promising candidate is Saturn's moon Titan. Although much smaller than Earth, Titan - unlike the other planet-sized moons - has a substantial atmosphere, similar in thickness to our own. Titan's atmosphere is made up largely of nitrogen, just like the Earth's, and the remainder consists primarily of an intriguing mix of complex carbon-containing compounds, built up from simpler chemicals over billions of years, as the energy of the sun's rays battered the distant moon. If any one place deserves to be called the home of organic chemistry, it is Titan.
The most important carbon compounds on Titan are hydrocarbons - molecules consisting of chains of carbon atoms flanked by smaller hydrogen atoms. Hydrocarbons come in numerous varieties, each with differing numbers of carbon atoms and differing physical properties. We are familiar with many of these compounds in our everyday lives: they form the basis of natural gas, lighter fluid, petrol, paraffin, diesel oil, tar, and many important solvents. They are also the raw materials from which most plastics are manufactured. On Earth, the main source of these compounds is crude oil, formed underground over millions of years by the decay of fossil micro-organisms, but the hydrocarbons on Titan are of inorganic origin.
The lightest hydrocarbon is methane (containing just a single carbon atom), and it is this gas that predominates in Titan's atmosphere. The two-carbon molecule ethane, which is also abundant on Titan, is denser and would be a liquid under Titanian conditions. And there are also solids: large, plastic-like molecules that were formed when smaller hydrocarbons reacted with each other under the influence of sunlight. As well as reacting with each other, some of the hydrocarbons can react with the nitrogen compounds in Titan's atmosphere, forming a number of biologically interesting molecules, including the ubiquitous amino acids. We do not know exactly what Titan's surface looks like, since the two Voyager spacecraft that passed it in the early 1980s could not see through the thick smog that envelops the giant moon, but the Cassini-Huygens mission scheduled to land on Titan in 2005 will hopefully solve the mystery. Some speculate that Titan harbours vast, cold oceans of liquid ethane, smothered in a black, sticky tar.
Does life exist on Titan? In short, no. Most scientists agree that any form of life requires liquid water in order to develop, and with average temperatures of -180°C, water cannot possibly flow on Titan. Perhaps billions of years into the future, as the ageing sun becomes brighter (and life on Earth is roasted to death), something will stir in one of Titan's sticky ponds. For the time being, we must look for life elsewhere.
Four of the seven planet-sized moons - the so-called 'Galilean satellites' - orbit the giant planet Jupiter. The Galilean satellites are a microcosm of the Solar System, with barren rocky worlds at the centre and larger, lighter ones orbiting further out. At the centre of the whole system lies the fierce orange disc of Jupiter, vast and gaseous. In the real Solar System, the one world known to harbour life is neither too close to the sun nor too far out, neither too small and barren nor too gigantic; a planet with a solid surface enveloped in a thin, watery hydrosphere. Is there any such place among the moons of Jupiter?
Intriguingly, there is - the little Galilean satellite of Europa. Viewed from Earth through a pair or binoculars or a small telescope, Europa is just a tiny white dot in the glare of its huge parent planet. Close-up photographs from Voyager spacecraft revealed it to be entirely flat and smooth, except for a network of bizarre cracks that criss-cross the icy surface. The large Galilean satellites of Ganymede and Callisto are also covered in ice, but their surfaces are pock-marked by a vast number of craters - the legacy of the cosmic bombardment that the entire Solar System endured after its violent formation four and a half billion years ago. If Europa lacks such markings, its surface must be geologically younger than Ganymede or Callisto's. This fact, together with the strange cracks, has led scientists to speculate that Europa's surface is continually replenished by a process similar to the plate tectonics that occur on our own planet. Instead of rock, however, Europa's 'continental plates' are made of plain, white ice. The implications of this are startling: Europa's apparently bare surface may conceal an ocean of warm water!
The energy to heat this water comes not from the Sun, but from within Europa. As the little moon orbits Jupiter, it is caught in a continuous gravitational tug-of-war between its parent planet and the larger moons, Ganymede and Callisto, which orbit further out. This squeezes Europa violently, causing its rocky interior to heat up. A similar process is known to occur on the neighbouring moon Io, producing huge volcanoes that spew plumes of molten sulphur many miles above Io's orange surface. Since it orbits further from Jupiter than Io does, Europa is not heated quite so strongly, but the process could still provide sufficient energy to melt the lower layers of Europa's ice.
It is just possible that living things might thrive in the tepid waters beneath Europa's pristine surface. However, the idea has its problems. Any carbon compounds present on Europa would be hopelessly diluted in an entire ocean, and under such circumstances it is hard to see how the materials of life could first have come together to form a living organism. Maybe life developed on Europa when the moon was young, at a time when volatile organic compounds were more abundant and a thick ice layer had not yet smothered the cooling surface. The only way to be certain whether or not anything is alive on Europa today would be to go there and find out, and such a mission is probably a long way off. Nonetheless, Europa - a cold, distant world smaller than our Moon - remains one of the most likely homes for life elsewhere in the Solar System.
Perhaps our origin was a one-off, a fluke, or perhaps it was chemically inevitable. But one thing that we can be sure of is that, once life has appeared, it has a great ability to adapt and meet new challenges. Microbes here on Earth have not only survived journeys to the Moon, but have also been found living inside nuclear reactors, volcanic vents, and even the rocks several miles beneath our feet. So if life ever did appear elsewhere in the Solar System, it is hard to escape the suspicion that it is still out there - somewhere - waiting to be discovered.