THE FIRST concrete demonstration that life exists, or existed, elsewhere in the universe will be a momentous day in the history of science. Although, that day is not yet here, it may be close.
Science has long claimed that life could arise elsewhere in the universe and was largely convinced until the 1960s that a form of plant life existed on, Mars. The pendulum then swung in the opposite direction. The Viking mission to Mars in 1976 seemed to show clearly that there is no life on the planet, and that present conditions preclude the possibility that new life could arise there. But, did the planet ever harbour life? Some weeks ago, NASA scientists described dramatic new evidence that a simple form of life existed on Mars about 3 1/2 billion years ago.
It is believed that life spontaneously arose on Earth about 3 1/2 billion years ago. This momentous event was powered by an external source of energy (the sun), which interacted with matter in accordance with the universal laws of physics. Obviously, if life could arise in this manner on Earth, it could also arise elsewhere.
Mars, the fourth planet out from our sun, was long considered the most likely location for life in our solar system outside of the Earth. Percival Lowell (1855-1916), carried out detailed astronomical observations and proposed an elaborate theory that, not only did life exist on Mars, but the planet harboured an advanced civilisation struggling for survival on a dying planet.
Lowell knew that Mars was very dry. However, he could see that the planet had polar ice caps, and he believed that these represented water ice with borders of liquid water at their perimeters. He interpreted the seasonal disappearance of one polar cap, coupled to the growth of the other, as a transfer of water between the poles. Lowell noticed that the semiannual transfer of polar "water" was accompanied by a darkening that spread in a wave from one pole to the other. He interpreted this as vegetation growth accompanying the movement of water across the planet.
Lowell could also see an elaborate system of straight lines on Mars, which he interpreted to be the product of intelligent activity. He concluded that these lines were canals, an elaborate irrigation system built by Martians to optimise agricultural production in the extremely dry conditions.
The civilisation hypothesis died with Lowell, but his other, ideas regarding water movement and vegetation growth survived, with much new scientific evidence to support them, up until the early 1960s. However, work carried oat after 1963 showed that conclusions based on previous scientific evidence were wrong. Incidentally, the canals described in detail by Lowell do not exist.
The atmosphere on Mars is very thin compared to that of Earth. Average total atmosphere pressure on Mars is 6 millibars - atmospheric pressure on Earth is 1,016 millibars. The atmosphere is 95 per cent carbon dioxide, 2.7 per cent nitrogen, 1.6 per cent argon, 0.13 per cent oxygen, and traces of carbon monoxide, neon, krypton, xenon, ozone and water vapour, while on Earth (nitrogen 77 per cent, oxygen 21 per cent, water vapour 1 per cent, argon 1 per cent, carbon dioxide 0.03 per cent).
Mars is an extremely dry planet, over 70 per cent of the Earth's surface is covered by ocean. The average water vapour pressure on Mars is 0.5 microbars on Earth it is 10,000 times greater. The polar ice caps on Mars are composed largely of solid carbon dioxide.
The average temperature on the Martian surface is -55C compared to 15C for Earth. Low Martian temperatures would not exclude the possibility of life. On the Martian equator, night time temperatures drop far below zero, but noon time readings up to 25C have been recorded. Growth of micro organisms on Earth is well documented for temperatures down to -10C, with some reports down even to -34C.
Conditions in the Martian atmosphere allow water to exist only as ice or vapour. Life as we know it on Earth is absolutely dependent on liquid water. The only readily conceivable form of life that might exist on Mars would also be dependent on liquid water and would have to extract it either from ice or, more likely, from water vapour.
On Earth, some forms of life that live under very dry conditions, e.g. desert lichen, can extract water vapour from the atmosphere and convert it to liquid. Conditions on Earth that come closest to Martian conditions are some extremely dry valley areas in Antarctica, although these valleys are very wet by Martian standards. The lichens that can utilise water vapour have been unable to colonise the driest parts of the Antarctic dry valleys. It therefore appears almost certain that life, similar in design to terrestrial life, could not exist on Mars.
Furthermore, Mars is not protected by an ozone layer and the surface is suffused with strong ultraviolet light. This would break up any organic molecules that might form, sterilising the soil.
Conditions on Mars were not always so inhospitable to life. In 1971, the Mariner 9 space probe cameras photographed dry meandering river beds and deltas on Mars, clear evidence that water had once flowed on the surface in a warmer, more hospitable climate. This liquid water has long since disappeared, but could life have arisen and evolved when it was present? Could some of this life still survive in Martian soil? The Viking space mission to Mars in 1976 answered the latter question. Landing modules scooped up Martian earth and performed experiments to test for biological activity. No such activity was detected and it was sadly concluded that life does not exist on Mars.
Of course, the Viking experiment would not rule out the existence of life below the surface of the planet.
The recent NASA announcement of the evidence that life once existed on Mars has been covered in detail in news reports and I will only briefly summarise the matter here.
About 16 million years ago, fragments of rock were expelled into space from Mars as a result of collision with a huge asteroid or comet. One of these fragments eventually crashed into an ice field in Antarctica about 30,000 years ago. It was discovered in 1984 and studied for signs of ancient life.
THE Martian rocks clearly shows what appears to be fossilised remains of primitive micro organisms. The most striking image is of a segmented, worm like object, about one hundredth the width of a human hair. Also, complex chemicals were found close to the "fossils", e.g. polycyclic aromatic hydrocarbons, organic molecules formed on Earth in association with biological activity. They are also formed when certain fossil fuels are burned. The age of the "fossils" has been estimated at 3.6 billion years, suggesting that they were formed when the rock was still part of the Martian surface.
Mars was then about one billion years old, and water covered at least part of the surface. The evidence in the Martian rock suggests that micro organisms lived in the Martian sea and became fossilised in cracks in the rocks.
The evidence in the rock is exciting, but not conclusive. First of all, how can we be sure that the rock originated on Mars? The evidence here is based on the composition of gases trapped in tiny crevasses in the meteorite. There is a close match between the composition of the gases and those in the current Martian atmosphere. However, the only way to be sure that a rock is from Mars is to go to Mars and pick one up.
Secondly, although the "fossils" look like biological organisms, they could have been formed by dried mud. Thirdly, the various organic molecules interpreted as arising from biological activity could have arisen by non biological mechanisms. Much further study is needed in order to resolve this matter, and this is certainly warranted by the evidence now available.