both on this planet and elsewhere. It doesn’t seem to matter too much how these are supplied; living organisms, particularly bacteria, are able to utilize sources as diverse as air, rock or vegetation. Active life also needs energy but organisms can capture either light energy or a multitude of chemical forms of energy.
Liquid water appears to be the chief limiting factor to life on Earth. Living organisms have a very limited ability to manipulate the freezing or boiling point of water: when the exterior temperature exceeds the limits of this ability, active life ceases. The most barren places on Earth are generally the driest. The relative sterility of the Antarctic Dry Valleys epitomizes the requirement for water but our own homes strikingly illustrate the same principle. Home maintenance is essentially a battle against moisture. We repair roofs and windows, paint surfaces with water-repelling chemicals and make endless trips to the DIY store in our battle to exclude moisture and promote desert conditions inside our houses. If we neglected this then microbes and moulds would quickly invade and undermine our homes.
Why water in its liquid state is so essential to life is a question we will return to in Chapter Five. On Earth, so long as liquid water is available, then life is also possible. Microbial life thrives in a diverse array of (watery) chemical environments from hot to cold, acid to alkaline and every other extreme of chemistry available on this planet. The source chemicals used to make up living cells are incorporated by a wide variety of chemical pathways and transformed inside living cells by a host of diverse metabolic pathways. There appears to be no common core of metabolic chemistry that drives all living cells. The diversity of the chemistry that underpins living cells in different creatures is surprising given the usual chemical explanation for the phenomenon of life: that it is a highly complex self-organizing chemical reaction (we shall be examining this in Chapter Six). If life is a merely a complex chemical reaction then we must explain how such a wide variety of chemical processes generates essentially the same phenomenon: life. This indicates to me that we need to look further than standard chemistry to discover the essential quality of life.
Another curious feature of life apparent from our brief exploration is its invasiveness. The inanimate world is characterized by a flowing down of energy: water flows down a hillside; electrons flow down to lower energy states and complex molecules break down to simpler ones. Yet living organisms will climb to fill any vacant niche. When in 1883, a volcano exploded on Krakatau, it obliterated two thirds of the island, leaving the remainder an inhospitable nutrient-depleted dustbowl. Yet some microbes (mostly cyanobacteria) can survive on a diet of volcanic ash and rapidly colonized the island. The growth of these microbes provided the nutrients to support further colonization, and a few decades later there was abundant plant-life and even a few small animals. Over the course of evolutionary time, life has relentlessly thrust itself in all possible directions to fill the planet’s every available niche. This aspect of life, its ability to direct itself forward, is hard to account for in terms of the perpetual running down that dominates inorganic chemistry. We are again pointed towards the realization that life represents a very different kind of chemistry from the reactions that drive the inanimate world.
EXTRATERRESTRIAL LIFE?
After finding the limits of life on Earth, the spacecraft would surely explore our solar system to discover whether life is limited to its third planet. We live on a planet orbiting one star in a galaxy of one hundred billion stars in a universe of a billion galaxies. Is it conceivable that we are alone?
Science fiction writers have dreamed of all sorts of non carbon-based life forms but, although entertaining, none is convincing. Life is a complex business that requires complex chemistry. As far as we know, carbon is unique in its ability to form the wide range of compounds necessary for the emergence and evolution of any life form. Let us, thus, concentrate our thoughts on carbon-based life. Carbon is relatively abundant in today’s universe. Our sun is about 0.3 per cent carbon. It is found with varying abundance on the planets and comets of our solar system in the form of carbon dioxide, methane and more complex hydrocarbons – all compounds used as carbon sources on Earth.
The next ingredient for life, hydrogen, is the universe’s most abundant element. Oxygen and nitrogen are more sporadically distributed but are nevertheless relatively abundant. Minerals are scattered throughout the galaxy. Energy sources are certainly widespread; we have only to look at the stars to see billions of them. Alternative chemical sources of energy such as volcanism and geothermal energy also exist within our own solar system.
The key requirement that would limit extraterrestrial life is likely to mirror that which limits life on Earth: the presence of liquid water. Wherever liquid water is present on Earth, life is also found. It seems reasonable to extend that principle beyond our planet and predict that wherever stable bodies of liquid water co-exist with sources of carbon, nitrogen, hydrogen and oxygen, then life will also be found.
How abundant is liquid water in the universe? Water itself is not a problem. It is found on other planets of our solar system. It is abundant in comets and has been detected around extrasolar stars. The important question is rather: is water present as a liquid? The range of temperatures even within our own solar system is enormous: from the billions of degrees found in the sun’s interior to only a few degrees above absolute zero in the outer solar system. Clearly, the upper end of the temperature scale is incompatible with even the existence of water as the molecule would disintegrate into its component atoms. Going down the temperature scale, there are thousands of (hot) degrees where water exists as a gas. A tiny window exists (just about 100°C at terrestrial atmospheric pressure) where water exists as a liquid. Below zero there are 273 degrees between freezing and absolute zero where water is present as solid ice. The feasibility of extraterrestrial life reduces to the bare question: does this liquid water window exist on other planets?
The closest planet to our sun is Mercury. It has the widest range of temperature for any planet in our solar system. At night, the temperature on the surface of the planet drops to – 183°C and during the day it rockets above 300°C. The planet has little atmosphere and no detectable water, so it is a highly unlikely supporter of life.
The second planet from the sun, Venus, seems, initially, a much better prospect. Venus has a thick atmosphere consisting largely of carbon dioxide but with both nitrogen and water vapour also present. The thick atmosphere obscures all detail of the planet, allowing nineteenth-century writers and illustrators to imagine a tropical paradise inhabited by carefree, amorous Venusians. However when probes were sent to explore Venus in the 1960s they brought back images of a reddish brown rock-strewn desert beneath an orange sky. With surface temperatures a baking 480°C – far too hot for the existence of liquid water – and thick clouds of hot sulfuric acid that rain onto the terrain below, Venus is far more like Hell than Paradise.
Conditions have not always been so harsh on Venus. There is evidence that the planet once had deep-water oceans similar to Earth’s. But high levels of carbon dioxide in the atmosphere set up a runaway greenhouse gas effect, trapping the solar heat, drastically raising the surface temperature and evaporating the oceans. Venus serves as a terrifying reminder of the dangers of ignoring the warnings of environmental catastrophe on our own planet.
The third planet from the sun and its inhabitants is the subject of the remainder of this book so let us pass quickly on to the fourth planet. Mars and Martians are of course synonymous with popular notions of extraterrestrial life. In 1877, the Italian astronomer, Schiaparelli, drew detailed maps of the planet and identified linear features on the surface of Mars which he called canali, channels. The word was incorrectly translated into English as canals and, although these features were later found to be optical illusions, tales of Martian civilizations building complex irrigation systems to distribute their dwindling water supplies captured the popular imagination. The first detailed images of the surface taken by the Mariner probes were thus a big disappointment to Martian-watchers. There were no civilizations, no canals – and not a drop of water.
Though
