Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 18

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 sixteen million years ago and ejected the rock into space. After spending an uneventful few million years drifting through space, it was captured by the Earth’s gravitational pull and fell down on one of Antarctica’s blue-ice fields about eleven thousand years ago. In 1984, an ANSMET (Antarctic Search for Meteorites) team of scientists found and collected the rock, dubbing it ALH 84001.

The meteorite rock was packed in dry ice and shipped to the Antarctic Meteorite Laboratory at Johnson Space Center in Houston, Texas. There, it was catalogued and classified as a ‘common’ asteroidal meteorite. Its Martian origin was not discovered until 1993 when scientists took a closer look. Not only identifying it as coming from Mars (only the twelfth known Martian meteorite), researchers sectioned and examined the rock under the electron microscope and discovered globules of carbonate minerals and structures that looked remarkably like microbial fossils.

The Martian microfossils may look like bacterial fossils but, as any geologist will tell you, there are many natural rock formations that resemble fossils. The research team headed by Dr David McKay of NASA’s Johnson Space Center, supported their claim by also reporting chemical evidence of past life in the rocks, in the form of chemicals known as polycyclic aromatic hydrocarbons. However, if the structures do represent the remnants of bacteria, they are very significantly different from modern bacteria. Bacteria alive today are in the micrometer size range. The microbes that cause trachoma – an infectious disease that leads to blindness – are among the smallest. These chlamydia, have spherical cells measuring only a third of a micrometer (a millionth of a metre) in diameter. Yet the Martian ‘microbes’ are in the nanometre (a billionth of a metre) size range, and are usually only about 10 nanometres long. The cells could have had only a very tiny volume, about one millionth to one thousandth of the volume of a typical bacterium. Clearly, they couldn’t have held much material inside.

Yet, nanobacteria may also be found on Earth. Examination of the deep subsurface rocks recovered from Columbia River basin project, has revealed structures that look like nanobacteria; although their biological origin has not yet been confirmed. Robert Folk of Texas University claims to find nanobacteria in material from tapwater to tooth enamel.⁴ There have even been reports of nanobacteria recovered from human blood. Perhaps nanobacteria represent an earlier phase in the evolution of life. As we will be discussing in Chapter Four, it is highly unlikely that cells as big and complex as modern bacteria could have been the earliest life forms on Earth. The proposed nanobacterial structures formed on Mars at about the same time as life originated on Earth. If life was also in its infancy on Mars, then the nanobacteria fossils may be relics of the earliest life.⁵

Studying Martian life by examining rocks blown off its surface clearly has its limitations. The best way to look for life on Mars is to go there and examine the rocks directly. The late 1970s Viking mission to Mars did just that and hunted for evidence of life on the surface. Although it did discover a peculiar chemistry that mimicked biochemical activity, it is generally thought that the findings were negative. However Viking only sampled surface soils and it is likely that to find life on Mars you would have to dig deep. The current posse of Mars probes, including the Mars Pathfinder Mission’s indomitable rover vehicle, Sojourner, do not have any microbe-hunting experiments. But the interest generated by the recent Mars meteorite story prompted President Clinton to promise the ‘full intellectual power and technological prowess of the US behind the search for further evidence of life on Mars’. Let’s hope that future Mars missions have drills on board.

Beyond Mars, we come to the giant gas planets – Jupiter, Saturn, Uranus and Neptune. These have the necessary ingredients for life: hydrogen, methane (a carbon source), ammonia (a nitrogen source), and water. But they are very cold. The temperature on the cloud tops of Jupiter is a chilly – 153°C. Vast oceans of liquid hydrogen may lie beneath the clouds of the giant planets with solid cores probably ten to twenty times as massive as Earth. It is possible that liquid water may exist at some altitudes within their atmospheres. In a fanciful moment, the late Carl Sagan proposed that Jovian life might take the form of floating bag creatures that drift through the Jovian atmosphere. The Jovians would however have to endure a racy existence, driven by the two hundred and fifty miles an hour winds that blow through the upper atmosphere. All in all, the giant planets look unlikely habitats.

The outermost planet, Pluto, is smaller than the moon and has a surface temperature of about – 236°C. It looks the least likely place to find life in the solar system. More hopeful sites are on some of the moons of the giant planets. One of Saturn’s moons, Titan, has a thick atmosphere with water and traces of at least a dozen carbon-based compounds, including methane, ethane, hydrogen cyanide and carbon dioxide. The mixture is similar to the atmosphere many scientists believe existed early in Earth’s history, when life first emerged. But the temperature on Titan is a chilly – 180°C, far too low for liquid water. Its similarity to the early Earth has led to its description as Earth in the deep-freeze.

The young contender of exobiology candidates is the Jovian moon, Europa. About the size of our moon and with a surface temperature of – 145°C, Europa does not at first look a likely candidate. However, when the Galileo spacecraft sent back detailed images of the moon’s surface, it looked familiar. In fact the pictures could have been taken from above the Antarctic ice packs. Europa is entirely covered by a thick sheet of ice. The ice layer is probably about one hundred and fifty kilometres thick but evidence is accumulating that it is not all ice. Close-up shots reveal a cracked and broken surface and structures which look remarkably like icebergs. Something must be causing the ice to crack and break and the betting is that a liquid water ocean is churning up the surface ice, exactly like pack ice on Earth. Recent optical data from Galileo has detected mineral salts on the ice surface, probably the dried up remnants of briny seawater extruded onto the surface.

Scientists speculate that geothermal or tidal energy may be the heat source that has melted the putative ocean beneath Europa’s ice. Perhaps hydrothermal vents similar to those discovered by Alvin exist on Europa, spewing out hot mineral-rich water into the ice-locked ocean. Galileo’s instruments have detected complex carbon-based compounds on Europa’s sister moons, Callisto and Ganymede, making it highly likely that similar compounds are present in Europa’s seas.

The ingredients are all there. Europa almost certainly has a liquid water ocean with sources of carbon, nitrogen, minerals and a geothermal energy source. Similar conditions on Earth support complex ecosystems. Do Europaeans swim beneath the ice of Europa? On the principle that there is nothing special about Earth, my prediction would be (a hopeful) yes. Many scientists consider the imminent exploration of the terrestrial Lake Vostok as a rehearsal for a robotic dive beneath Europa’s ice, early in the next century. Perhaps the new millennium will be marked by our first contact with alien life.

And beyond the solar system, is there life among the billions of stars in our galaxy? Using the same approach applied to the solar system, we would predict life on planets possessing the necessary ingredients of carbon, hydrogen, nitrogen, oxygen, minerals and liquid water. These elements are certainly common throughout the galaxy so it is unlikely that life is limited by a lack of raw materials. The more difficult problem is to assess whether planets exist with liquid water. Until recently, nobody knew whether extra-solar planets existed. This has changed dramatically in the last few years with the discovery of many planetary systems around distant stars. The planets are usually detected by the periodic wobbling of a star, betraying the presence of a hidden companion object. So far the detectors can only pick up giant planets, about the size of Jupiter or even bigger. They are likely to be gas giants and therefore unlikely hosts (though they may have solid moons that could harbour life). About a dozen of these giant planets have now been detected, and many more are expected in the coming years. There is no reason to believe that the giant planets are alone. Earth-sized planets are also likely to be orbiting these distant stars. The optical signature of water has been detected in at least one putative planetary system.

Beyond our galaxy lie billions of other galaxies. I think it inconceivable that terrestrial conditions do not exist on many of the billions of planets probably orbiting those billions of stars. However, the gigantic distances that separate us from even our neighbouring galaxies (the Andromeda galaxy is a neighbour, but travelling at the speed of light it would still take two million years to reach it) ensure that such questions will, for a long, long time, remain entirely academic.

My guess, for what it’s worth, is that life is common throughout the universe. Just as life is found on Earth wherever we find the necessary ingredients alongside liquid water, then extraterrestrial life will be found wherever those conditions coincide. Astronomical evidence seems to be tilting towards an expectation that this combination is not so special.

We must now come down from the stars to return to this book’s central quest: to understand life on our own planet. Life’s success here on Earth has been contingent upon its most important action: the ability to replicate. Reproduction, the biological imperative, is clearly the most important action that (most) living creatures perform, so it is here we will begin our exploration of the source of life’s actions.