PREVIOUSLY I described the scientific consensus on how life spontaneously arose and evolved on Earth and how the basic building blocks of carbon based life are found throughout the universe.
I concluded it was almost certain that such life has arisen elsewhere in the universe. Today I examine the possibility that life has arisen in our solar system, outside of planet Earth.
Long before the 20th century, scientists and philosophers believed life proliferated on other planets. Philosophical considerations fuelled this conviction. Copernicus and Galileo had shown the Earth was not at the centre of the solar system but merely one of several planets revolving around the sun. It was reasoned therefore that Earth was a typical planet, containing abundant life, and many other planets would harbour life.
Now at the end of the 20th century, what can we say about life elsewhere? If a planet is to bear life as we know it, it will have to meet certain criteria. Carbonbased molecules are destroyed at temperatures significantly above 100 C, which puts an upper limit on planet temperature.
Planet conditions must allow liquid water to exist, and for that reason, and to allow chemical reactions to occur quickly enough to sustain life, mean temperature should not be much less than 0 C.
A life bearing planet will have an atmosphere in a state of dynamic equilibrium reflecting exchange of matter between it and life on the planet. Finally, it must receive sufficient radiant energy from its sun to power biological life.
We know more about our own solar system than about any other part of the universe. Is there life in our solar system beyond Earth? Almost certainly not, according to the criteria just outlined. Let us look briefly at the planets, moving outwards from the sun.
Mercury has no atmosphere and surface temperatures of about 430 C on the sunlit side and about -180 C on the dark side.
Venus has an atmosphere almost entirely of carbon dioxide and its mean surface temperature is about 450 C. Liquid water cannot exist at Venus's surface temperature. Conditions for life may have been more favourable in the past but life has been impossible for a long time. The first planet beyond the Earth is Mars, to which I will return later.
Then we come to the giant gassy planets: Jupiter, Saturn, Uranus and Neptune. In these it is thought chemical evolution could never get properly under way. While organic molecules might form in the upper cooler regions of their atmospheres, the constantly moving atmospheric currents bring these molecules down to lower hotter regions where they are destroyed.
Finally, there is Pluto, the outermost planet in the solar system. This has a surface temperature of -220 C, and a less hospitable abode for life could scarcely be imagined.
And now let us consider Mars. Until the 1960s, astronomers agreed that plant life existed on Mars. Indeed in the latter half of the 19th century there was a respected scientific view that the planet harboured an advanced civilisation. This inspired the Mariner and Viking space probes to Mars in the 1960s and 1970s. Disappointingly, they showed conclusively that Mars was a lifeless planet.
Belief in life on Mars was based on astronomical observations made from Earth. The planet shows dynamic behaviour through the telescope. Polar icecaps wax and wane with the Martian seasons. Darker and lighter patches show changing patterns that correlate with the waxing and waning of the polar icecaps. It was decided that these were made of waterice and their waxing and waning reflected the transfer of water between the poles.
The changing pattern of the dark patches was interpreted as growth of vegetation triggered by the passage of water across the planet, as it moved from pole to pole.
The most detailed observations of Mars were made by Percival Lowell (1855-1916). Lowell drew attention to a complicated pattern of straight lines on Mars which he called canals. He concluded these were irrigation ditches constructed by an intelligent civilisation to make maximum use of scarce liquid water resources.
The Viking missions to Mars in the 1970s took detailed photographs of the planet surface, measured atmospheric composition and carried out experiments on samples of Martian soil designed to detect biological life. Disappointingly the results showed that Mars was a lifeless planet.
Liquid water cannot exist on Mars. The polar icecaps are largely composed of solid carbon dioxide, although under laid by waterice. The tiny amount of water vapour present in the atmosphere means the planet is extremely dry, far drier than the most desiccated regions on Earth.
The experiments carried out on Martian soil found no evidence of organic compounds or any activity characteristic of biological life.
But the detailed pictures taken by the Viking orbiter showed interesting details. Long meandering channels were seen on Mars's surface. Geologists believe these channels, which are not visible from Earth, were formed by running water billions of years ago.
The Martian canals described by Percival Lowell do not exist. They were optical illusions present in Earthbased observations. The patterns of darker and lighter patches seen on Mars by Earthbased observations are not evident when the planet is inspected at close quarters.
In July 1996, NASA scientists made an amazing announcement. They reported evidence that life had arisen on Mars about 3.5 billion years ago. This evidence was present in an asteroid recovered from the Antarctic ice sheet. Analysis of the composition of gas bubbles indicated that it came from Mars. Microscopic examination showed the presence of fossilised bacteria like organisms.
The rock is about 3.5 billion years old and was formed when liquid water was present on Mars. The speculation is that life arose at that time in a manner similar to its origin on Earth. Some of the organisms became trapped and fossilised in rocks. Later a large meteorite smashed into Mars exploding fragments of Martian rock into space. One of these eventually reached Earth and was buried in Antarctica.
If investigations continue to support the contention that the meteorite contains fossils of living organisms, this will justify future scientific expeditions to Mars in search of further evidence in deeper layers of soil.
Finally, a few weeks ago the Galileo space probe beamed back sharp photographs of Jupiter's ice covered satellite Europa. This moon has a planet wide ice cap floating on an ocean of water. This is an exciting finding as there is a possibility that some life form may have gained a foothold in that ocean. I rather doubt it, though. Liquid water is an essential prerequisite for life, but only one of several.