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,