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 we know that there are no canal-building Martians on Mars, the planet remains one of the most promising candidates for extraterrestrial life. The atmosphere has plenty of carbon dioxide, together with nitrogen and small quantities of water vapour. The ingredients of life are there but the planet is cold. The average surface temperature is – 53°C: too cold for liquid water to exist on its surface. Yet liquid water did once flow on Mars. Networks of branching valleys with fine tributaries look remarkably similar to the Earth’s river valleys. Surface features record what appears to be catastrophic flooding by rivers more than one hundred times bigger than the Mississippi. The river valleys, lake beds and flood plains are all dry now but they record a warmer and wetter period in Martian history. It is thought that this warm wet period ended about three and a half billion years ago, but that might have left just enough time for life to evolve (like Earth, Mars formed about four billion years ago). Bacteria were already well established on Earth three and a half billion years ago.
If microbes once flourished in Martian seas, they must have gone through a catastrophic crisis when the planet’s surface dried up. The last stand of these microscopic Martians might have come when the dwindling seas, rivers and lakes were freeze-dried in the thinning atmosphere. But perhaps there are still outposts of life on Mars. Though the planet’s surface is now dry, its crust is estimated to hold a layer of water-ice five hundred metres thick. This permafrost layer would not be much different to that of the Dry Valleys of Antarctica, which does harbour life. Could Martian bugs – refugees from the ancient seas – survive still in the frozen subsurface? At present, we simply don’t know. The key feature allowing life to survive in Antarctica are the brief warm summer spells when the ice melts, releasing liquid water. Mars lacks a warm summer but it does have other sources of heat. Martian volcanoes like the massive Olympus Mons, five hundred and fifty kilometres across and twenty-five kilometres high, are potential sources of geothermal energy. The heat from volcanic eruptions must have melted huge quantities of the subsurface ice and probably caused the catastrophic flooding episodes recorded on the Martian terrain. Whether sufficient water remained liquid long enough to sustain life is, of course, very uncertain.
Geothermal energy may still be active under Mars’ surface. Mars almost certainly has a hot core like Earth’s. Although the surface is frozen, it is likely that temperature increases with increasing depth. There must exist a subsurface temperature window, hot enough to melt ice but not too hot to vaporize it. On Earth, microbes live in the deep subsurface where liquid water is present and may have survived there for millions of years. Similar conditions under the surface of Mars may yet harbour Martian microbes.
The possibility of life on Mars recently hit the headline with the publication of images of supposed fossilized microbes buried inside a Martian meteorite. The brick-shaped meteorite, known as ALH 84001 weighed nearly two kilos and was collected in the Allan Hills area of Antarctica. The rock was a basalt which had solidified from volcanic lava about four and a half billion years ago. But no earthly volcano spewed out ALH 84001. Analysis of gases trapped within the rock identified it as a small piece of Mars. Around about three and a half to four billion years ago, carbonate minerals were deposited in the rock, possibly precipitated from groundwater seeping through the Martian surface. The rock remained on Mars for the next three billion years and would still be there if a comet or asteroid had not crashed into Mars about