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
