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Jobs Classifieds -------------------------------------------------------------------------------- Services Advertise - print - online Delivery - paper - e-mail - handheld -------------------------------------------------------------------------------- Help Audio/video - SPECTRUM The great red hope Illustration: Greg Bakes Mars has never been home to little green men, but there's more to life than Man. Paul Davies examines the clues that may provide an answer to a nagging human question: is anybody out there? Across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.With those chilling words, H.G. Wells, that master of science fiction, began his most famous novel War of the Worlds. Published in 1898, it tells the story of a brutal invasion of Earth by desperate and hostile Martians. Wells wrote at a time when there was widespread belief that Mars was inhabited. A few years earlier, an Italian astronomer named Giovanni Schiaparelli reported seeing curious straight lines on the surface of the Red Planet, which he termed "canali". In America, Percival Lowell built an observatory to study these "canals", and soon became convinced they were the work of intelligent beings, constructed for the purpose of bringing spring melt water from the polar ice caps to the arid equatorial regions. Today we know that Lowell's canals simply do not exist. They were a figment of his fertile imagination. Mars has been mapped by orbiting spacecraft as closely as that of our own planet, and there isn't a canal in sight. Nor is there any evidence of intelligent Martians, past or present. The Red Planet is a freeze-dried desert that makes Antarctica seem positively balmy. The surface is bathed in deadly ultra-violet radiation and the atmosphere is pitifully thin. To paraphrase Elton John, it's not the kind of place to raise a kid even a little green one. In spite of this, the idea of life on Mars retains a certain compulsion for scientist and layperson alike. The lure of the Red Planet strikes a particular chord in us curious Earthlings. In fact, it would be hasty to write off Mars totally as an abode for life. To be sure, the surface conditions are highly inhospitable today, but in the distant past it was a very different story. Photographs reveal hundreds of sinuous channels etched into the hostile terrain that are almost certainly dried-up river valleys. Other broad channels tell a story of massive catastrophic floods. There is also evidence for ancient lakes and possibly the shorelines of a large ocean. Evidently Mars was once warm and wet, and maybe not unlike Earth. Sadly the Martian spring petered out a very long time ago 3.5 billion years to be precise. Scientists have struggled to understand what happened. Clearly Mars must have had a much thicker atmosphere in the past, creating pronounced greenhouse warming. The higher atmospheric pressure would also have enabled liquid water to flow freely on the surface without quickly evaporating, as would happen today. Whether there were regular rain showers and snow flurries on Mars isn't certain, but signs of multiple flooding episodes and repeated watercourse erosion suggest that liquid water was cycled continuously over millions of years. A key factor in understanding these features is the existence of Martian volcanos. The largest of these, Olympus Mons, dwarfs its terrestrial counterparts. Volcanic heat can combine with water to produce hot springs and circulate fluids through the planet's crust. The solar system formed about 4.5 billion years ago, so Mars may have enjoyed sustained warm and wet periods over several hundred million years. But it was always fighting a losing battle. At about half the size of Earth, Mars has too low a gravitational pull to retain much atmosphere. Over time, most of it leaked away into space, sending the temperature plunging. The water, never as abundant as on Earth, either evaporated into space with it, or disappeared into the porous surface to become permafrost. Some remains locked up in the ice caps. Could life have gained a toehold on Mars while conditions were relatively benign? Increasingly, scientists believe the answer is yes. In fact, it may be that Mars was a more favourable planet than Earth for life to get started. The reason for this concerns the way the planets formed. The solar system began as a white-hot blob surrounded by a swirling cloud of gas and dust. The blob became the sun, while the enveloping material aggregated into the planets. It took almost a billion years for the orbiting debris to get swept up. During that time, all the bodies in the solar system were pounded mercilessly by asteroids and comets of various shapes and sizes. The record of this early bombardment is dramatically written into the face of the Moon, which is heavily cratered. The destruction inflicted by the biggest cosmic impacts would have been awesome. A 500-kilometre wide object slamming into Earth today at 30km/second would excavate a hole the size of the British Isles, strip away much of the atmosphere, boil the oceans dry, and swathe the planet with a mixture of incandescent rock vapour and superheated steam. This global furnace would send a lethal heat pulse a kilometre or so into the exposed crust. There is negligible risk of such a major collision today, but comparison with the Moon suggests that dozens of cataclysms on that scale would have been inflicted on our world up until about 3.8 billion years ago. Eventually the ferocious bombardment abated, but it has never completely ceased. Smaller, yet still immensely destructive, impacts have continued throughout geological history. The most famous, 65 million years ago, exterminated the dinosaurs and many other species too. Although the cosmic barrage delivered water and vital organic substances to Earth, the mayhem it created was hardly conducive to the formation of life. Mars on the other hand got off lightly. Being a smaller planet, it took fewer big hits. And with its sparse inventory of water, there was less chance of Mars being enveloped in a superheated steam and rock vapour atmosphere. Another major factor favoured Mars over Earth as an abode for early life. When the planets formed, by the accumulation of rocks and other detritus, immense interior heat was generated. This alone would have prevented life from getting going on Earth until about four billion years ago. But smaller Mars cooled quicker, opening the way to life-encouraging conditions perhaps a quarter of a billion years sooner. Remarkably, H.G. Wells cited this salient fact on the second page of his novel. Many scientists suspect the earliest organisms on Earth didn't inhabit the surface, but dwelt instead deep in the rocky crust. Drill into the seabed today and it's teeming with microbes up to a kilometre down. On land, bacteria have been found alive and well much deeper still. Gene sequencing indicates that many of these sub-surface creatures are very primitive and belong to extremely ancient lineages. In effect, they are living fossils. If life started below ground, the comfort zone for the first microbes would have been squeezed into a narrow range. Too deep, and the planet's internal heat would prove lethal. Too shallow and the heat pulses from the cosmic bombardment would be just as deadly. Today, the comfort zone on Earth extends down several kilometres, but four billion years ago when our planet was still torrid from its formation, this habitable region would have been much narrower. Cooler Mars, on the other hand, would have had a more extensive comfort zone where sub-surface microbes could shelter from the big impacts. For all these reasons it's conceivable that life flourished on Mars long before Earth became habitable. Unfortunately for science fiction writers, it's highly unlikely that life on Mars ever got beyond the point of simple microbes. On Earth it took more than two billion years for more advanced organisms to evolve, and Mars degenerated long before that. However, if there were microbes on Mars before any life existed on Earth, a fascinating possibility arises. Could it be that terrestrial life did not originate on Earth at all, but came from Mars? A natural and obvious transportation mechanism exists to convey life from Mars to Earth. The same big impacts that caused such devastation on the young planets would have had the punch to blast vast quantities of rock into orbit. A 10-kilometre-wide asteroid hitting Mars might eject a billion tonnes of boulders into space. Some fraction of this debris would eventually land on Earth. We know this, because to date about 15 meteorites have been positively identified as having originated on Mars. Estimates suggest an average of one Mars rock per month lands here. One such Martian meteorite, ALH 84001, was propelled to fame a few years ago when NASA scientists thought they had detected fossilised microbes lurking within. Although that claim now looks shaky, the notion that Martian micro-organisms might hitch a ride in an ejected rock and travel to Earth is sound enough. Although outer space is a very hazardous environment even for simple bacteria, the ride might not be so bad. Tiny microbes could easily withstand the violence of being blasted off the Red Planet. Cocooned deep inside a rock, they would be well shielded from deadly radiation, and from the fierce heat of re-entry into Earth's atmosphere. During the journey, the cold of space would act as a preservative. Microbiologists believe many bacteria could remain viable for millions of years in a dormant state or spore-like form. That is easily long enough to reach Earth. Indeed, some microbes might get lucky and travel on a trajectory that brings them to Earth in just a few thousand years. It would need only a single viable microbe to successfully take up residence on the barren, primitive Earth for it to become seeded with life. The traffic of rocks between the two planets would have been much greater four billion years ago than now, because of the intensity of the bombardment. If Mars was infested with bacteria at that time, interplanetary inoculation looks to be inevitable. If this theory is right, then Earth was indeed invaded by Martians, as Wells imagined, but we are their descendants, not their victims! An attractive feature of the life-from-Mars hypothesis is that it explains a mystery about the speed with which life first became established here. The earliest known fossils are the remains of cyanobacteria found in Western Australia, dated at 3.5 billion years. But there is evidence that life was at work long before this. Rocks found at Isua in Greenland, which are at least 3.85 billion years old, contain carbon deposits that look like they have been chemically modified by living organisms. Since the deadly heavy bombardment continued through to about 3.8 billion years ago, it seems as if life took hold almost as soon as it could. This fact is neatly explained if a continuous rain of microbe-laden Mars rocks had been descending on our planet for eons. It only needed conditions to settle down here for life to rapidly take root. Of course, the scenario works in reverse too. Rocks knocked off Earth will have found their way to Mars, although our planet's higher gravity makes the journey in that direction a lot harder. Since Mars was warm and wet at the same time we know there was life on Earth, terrestrial microbes travelling inside rocks to Mars would have arrived to find conditions very much like home. So there could have been a two-way traffic of thriving organisms for a hundred million years. This considerably complicates the question of the ultimate origin of life. Did it start just once, on Mars say, and spread to Earth? Or did it start on Earth and go to Mars? Or could it have begun twice, on each planet independently, resulting in a titanic microbial war of the worlds, as rocky spaceships carried bacterial invaders back and forth, to slug it out with the indigenous microbes for supremacy? Scientists are sharply divided over the mystery of precisely how life began. Some think it was a stupendous chemical fluke, unique in the entire universe. Others believe that the laws of nature are inherently bio-friendly, implying that life will spring forth more or less automatically wherever suitable conditions are found. Nobody is quite sure what constitutes "suitable conditions", except that liquid water seems to be indispensable. Within our solar system, Earth, Mars, Europa (a moon of Jupiter) and possibly the interiors of comets all offer potential incubation sites. Unless somebody manages to cook up life from scratch in a test tube, the only way we shall know whether it really is written into the laws of the universe is to find a second genesis somewhere beyond Earth where it has emerged independently. Some researchers think Mars could reveal that life has happened more than once, in spite of the problem of Earth-Mars cross-contamination. If we find evidence for past life on Mars, chemical analysis could show it to be totally unlike Earth life, which would strongly suggest an independent origin. For example, all terrestrial organisms use amino acids to make proteins. These molecules are "left-handed" they differ in structure from their mirror images in the same way that a right-hand glove differs from a left-hand glove. If Mars life used right-handed amino acids (or some other molecular building blocks entirely), that would be a give-away. Undoubtedly the best hope for settling the matter is if we find that Mars still harbours some form of life even today. In 1977 NASA sent two Viking spacecraft to land on the planet's surface. They were equipped with robot arms to scoop up dirt and test it for life. None was found. But that was before deep sub-surface microbes had been discovered on Earth. Scientists now acknowledge that even if the surface of Mars is completely dead, deep down it may be a different story. Underground reservoirs may have formed where volcanic heat has melted the permafrost. Mars Global Surveyor has photographed runnels carved by flowing water thought to have spewed from hidden aquifers in the recent past. It is just possible that ancient Martian bacteria are clinging to life in such pockets, ensconced far beneath the forbidding terrain. Now that Mars is once again being taken seriously as an abode for life, worries have surfaced about safety issues. In Wells's book, the invasion of Earth fails because the Martians succumb to terrestrial germs. Some people worry about the reverse scenario humans being wiped out by a killer plague from the Red Planet. NASA is planning an unmanned mission to grab some Mars rocks and bring them back to Earth to study, fuelling fears that alien microbes might be released that will infect humans or our food chain. Although these anxieties seem misplaced after all, Mars rocks have been falling on Earth throughout history NASA will insist on elaborate quarantine measures. Most likely, surface rocks from Mars will provide only tantalising clues to life. The definitive answer will probably have to await a manned expedition involving a deep drilling project, which won't be easy, or cheap. But a great deal rides on the answer. If there was, or still is, life on Mars, and it turns out to be just another branch of the terrestrial tree of life, then planet-hopping contamination would be the simplest explanation. We still couldn't be sure if life has happened only once a freakish chemical accident in an insignificant corner of the cosmos. On the other hand, if it can be shown that Mars and Earth have different forms of life, then the bio-friendly theory will be confirmed. If life has happened twice in one star system, it will surely have happened again and again across the universe, on many other Earth-like planets. Our galaxy will be teeming with alien organisms. Perhaps some of them will have evolved by Darwinian processes to the point of intelligence, and may even be trying to communicate with us. Either way, the philosophical consequences are profound. The choice is between an almost totally sterile cosmos, albeit vast and majestic, in which life is just a random fluke, and a universe in which life, and perhaps mind, are deeply embedded in the fundamental scheme of things. It would surely be inspiring to discover we inhabit a universe with life-encouraging laws in which our existence is linked to the deepest processes of nature a universe in which we are not alone. Paul Davies is a physicist and writer. His latest book is The Fifth Miracle: The Search for the Origin of Life (Penguin). [go to top] In this section Sex and sensibility The great red hope Believe it or not ... 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