The exciting thing about science is just that, it is exciting. Scientists and their camp followers live in perpetual danger of being ambushed by The Cold Hard Facts. Time and again, elegant hypotheses of respectable pedigree, well buttressed with evidence and gilded with general acceptance, are, if not brought crashing to the ground, at least so severely damaged as to be rendered uninhabitable by contrary data.
Sometimes the ruined edifice is one of the laws of physics: those sweeping, unrestricted generalisations such as the geocentric model of the universe or the pre-Galilean belief that projectiles travel in a straight line until the “propulsive force” runs out, at which point they drop to earth.
At other times the rubble marks the ruin of a particular set of beliefs which, although falling short of the status of Natural laws, nevertheless had a powerful influence. We now face just such a wreckage of our beliefs about the likely nature of extraterrestrial life.
The earliest speculations upon the possibility of life elsewhere are as old as theology. The models were by and large anthropomorphic, and for all that the many divinities could disguise themselves, their behaviours were distinctly human. However Zeus concealed his identity, he had the sexual mores of a recent United States president, while the various goddesses, on feeling slighted, could be more vicious than an offended diva.
During the last hundred years the focus of speculation on the possibility and nature of extraterrestrial life has shifted from being a theological pursuit to a scientific discipline. From humble beginnings, with Percivel Lowell struggling to gain acceptance for his Martian canals, we now have the high-tech search for signals from extraterrestrial intelligences: project SETI.
But life need not be intelligent or even advanced. Indeed, within our solar system we have discovered that none of the planets offers an environment remotely comparable with that of Earth, so developed life-forms similar to ourselves are most unlikely to occur on them. Cloud-shrouded Venus, our twin planet long beloved of science fiction writers, proves not to be cloaked in tropical forests and torrential downpours, but shrouded in greenhouse gases, free sulphuric acid and enjoying a surface temperature of 470°C—well above the melting point of lead. The atmospheric pressure is 90 times that on Earth, and even the most optimistic of exobiologists concede that this is now a sterile environment and probably always has been.
Mars, the next most favoured candidate, began to look like a poor bet with a surface never warming above -20°C, a mere wisp of an atmosphere, devastating UV radiation and no apparent surface water, even if it may once have been awash.
Beyond the shattered rubble of the asteroid belt, Jupiter and its four Galilean satellites were held to have the wrong chemistry and/or to be too cold.
In the 1950s and ’60s, exotic life chemistries were popular, at least amongst sci-fi writers, but as our understanding of chemical processes has improved, so the feasibility of life based on silicon and sulphur has been discounted. It appears that the triad of hydrogen, carbon and oxygen is essential for the construction and function of those stupendous molecules which are crucial for life processes, including proteins, DNA and that prince amongst solvents, liquid water.
Thus in 1976, when the Viking landers tested their scoopfuls of Martian soil, the results were interpreted as giving no evidence of life. (In fact, at least one experiment did yield a surprising result, but it was explained away.) What we knew then of the extremophiles—organisms adapted to extreme conditions of heat, cold, acidity, aridity, “toxic” food and so on—suggested that though life was tougher and more adaptable than we had thought, conditions on Mars still lay outside the envelope.
Even the discovery of the exotic fauna of the black smokers along the oceanic spreading ridges did not cause a fundamental alteration in our view of the biosphere, although it did demand some realignment of the spectacles. Here was an ecosystem of more or less familiar organisms surviving not on a drizzle of organic detritus from the surface layers where photosynthesis powers life, but on deep-sea bacteria gaining energy from inorganic chemical processes.
The last two or three years have seen a number of discoveries announced which dramatically extend our map of the biosphere and the range of living organisms and habitable environments. Deep explorations in both the USA and South Africa have revealed a subsurface biota extending down at least 3.5 km. Conventional wisdom put the limit for subterranean life, apart from cave dwellers, at about 1.5 km. This new extension of the frontiers of life is not just a matter of the odd bacterium discovered in fault lines and fissures, but a thoroughgoing colonisation of the sedimentary strata by several thousand species of bacteria, with a count of up to 100 million organisms per gram. Even such relatively complex forms as protozoa browsing on the bacterial fauna have been found down to depths of about 100 m.
Though the interior of igneous rocks is sterile, the most minute cracks are the habitat of a range of bacteria. Here life is hard, nutrients sparse beyond belief and cell division slow, perhaps as slow as once every 100 years. This is anabiosis with a vengeance; the metabolic processes are undetectable by any standard laboratory process (such as those tried by Viking), which is why it has taken us so long to find them.
Those infective bacteria which cause us so much grief may reproduce every 20 minutes and declare their presence in a culture medium within a day. However, their subterranean cousins ticking over about two and a half million times more slowly would take millennia to produce a similarly detectable colony.
With these discoveries the conventional picture of the biosphere must be redrawn, with the extension below the surface being comparable to that above it. The weight of living material below the surface may be as much as that on and above it—a stunning increment to the biomass.
The consequences for the terrestrial life sciences are considerable, but for the exobiologist investigating the other planets and their satellites, these findings are revolutionary. Finding so much life in such an alien environment here on Earth throws a quite different light on the search for life elsewhere in the solar system.
Europa is the current most favoured target in the search for subsurface life. Recent images from the Galileo spacecraft very strongly suggest that the icy surface overlies an ocean trapped below. Though why, like the Moon, Europa did not lose its water by evaporation to space during its early life, before it had cooled to below 0°C, remains to be explained.
That the surface ice does not extend to the rocky core is probably due to the heat liberated by the decay of radioactive isotopes locked in the rocks, together with heat generated by the continuous flexing of this moon under the ever–varying gravitational tugs of Jupiter and the other three Galilean moons.
Mars, with its evidence of abundant water in the past, is also a prime target, and at some depth in its mantle there must be a zone where water is liquid. If life either evolved or was transported to the planet when there was still liquid water, then it is an odds-on chance that its progeny are hidden in the rocks.
At the moment the consensus of opinion is that life of any sort is possible only in the presence of liquid water. Once the temperature drops low enough for the water to freeze, then its functions as solvent and transporter disappear, as does life. The fish and crustaceans of our polar icecaps manage to get below 0°C using a form of antifreeze, but only by a few degrees. However, it may be that we will discover that the ultra-slow life processes of these most extreme of extremophiles can occur even when they are frozen “solid.” This may be a fantastic speculation, but a decade ago a slowly thriving bacterial fauna two kilometres down in undisturbed sedimentary strata was just as impossible.
Whether or not such subfreezing life processes exist, we are having to digest the fact that life is fantastically tougher and more resilient that we had thought. Thus the seeding of one planet by splashes of material from another after a major meteorite impact lies well inside the bounds of possibility. Likewise, it may be that some of the rogue solar system bodies—asteroids on highly elliptical orbits and shrapnel from major impacts—are ferrying living contaminants. Did life develop on Earth or were we seeded? Are there other life-forms which, though similar, are indubitably non-terrestrial?
It is a pity that the effort being expended by NASA on projects of doubtful worth (such as the space station and a manned mission to Mars) could not be redeployed into several dozen limited mission probes around, on to and into bodies which may hold the keys to the evolution of the solar system and of life. Pace the cosmologists, for faithful readers of Stephen Jay Gould and Richard Dawkins the quest for the origin and varieties of solar system life is the great pursuit.