Mars has long been looked upon as a prime candidate for extra-terrestrial life, and is the subject of many a wild science fiction tale!

Towards the end of the 18th century, the German born astronomer William Herschel made many interesting comparisons of the Earth and Mars, sparking off the idea of there being life on Mars. He worked during the peak in early telescopic observations of the planets, and his observations of Mars heralded the beginning of the search for extra terrestrial intelligence. After all, where else should there be life other than on a planet which shows so many remarkable similarities to our own? Herschel's results were as follows:

• He determined the length of the Martian day as 24 hours, 39 minutes and 22 seconds, a mere 14 seconds below the currently accepted value.
• He noted that the rotation axis of Mars is inclined at an angle of 25° to the ecliptic, compared to Earth's 23.5°. Similarly to Earth therefore, Herschel noted the changing seasons, although the Martian equivalents are almost twice as long, because it takes almost two Earth years for Mars to orbit the Sun. He even went as far as correctly associating the seasons with the growth and shrinkage of the polar ice caps (which he inferred were thin layers of ice and snow). In a Martian hemisphere, spring and summer sees the polar cap shrink and the dark surface features become more distinct. Conversely, in autumn and winter the polar cap begins to grow and the dark features fade.
• He detected changes in the light and dark patterns of the planet surface, which he associated with cloud cover and cloud passage.

As a result of all of this data, by the end of the 18th century, Mars was seen as being very much like Earth in many respects. Both have a day of approximately 24 hours, both show successive seasons resulting in variations in polar ice cap sizes, and both have clouds obscuring parts of the surface. The growing resemblance of Mars to Earth soon led to the logical supposition that the dark features could be vegetation, growing in spring and summer before receding in the autumn and winter. This idea developed into the possibility of there also being intelligent life on Mars and this theory became so strong that in 1802 the German mathematician Karl Friedrich Gauss proposed that the Martians be signalled by drawing huge geometric patterns in the Siberian snow! His suggestion was never carried out.

The most famous observation that lead to the suggestion of the presence of life on Mars must be that of `Martian canals'. It is often put forward that the Italian astronomer Giovanni Schiaparelli first noted these features during his mapping through the favourable opposition in 1877. However, it was in fact Father Pietro Secchi who first observed 40 or so linear, straight line features criss-crossing the Martian surface in 1869 which he termed canali, meaning `natural water channels'. This term was soon mistranslated into English as canals and the idea arose that an alien race was desperately constructing canals to store water so as they could survive on their fast drying planet. This theory was popularised by the wealthy American amateur Percival Lowell (1855 - 1906) who financed the construction of a major new observatory near Flagstaff, Arizona which would be used primarily for continuing studies of Mars. By the end of the nineteenth century, Lowell had reported the observation of 160 canals.

Despite the public excitement brewing at theories of life on Mars at this time, many astronomers still could not accept the features as being true canals. In 1894, the American astronomer Edward Barnard said:

`To save my soul I can't believe in the canals as Schiaparelli draws them'

However, these more sceptical views were easily drowned out by the exciting announcements of Lowell and his colleagues. it soon became fashionable to view the Martians as a race who constructed massive canals as irrigation networks, carrying water from the polar caps to vegetation near the equators. The reddish appearance of Mars was suggestive of a dying, desert-like planet whose occupants had to struggle to irrigate their farmlands. The prospect of Martian invasions became the typical plot for an explosion of science fiction novels, with the aliens abandoning their arid homeland to invade the Earth for it's abundant resources.

The reason for such speculation stems from the fact that superficially, Mars appears to have many Earth-like characteristics. However, the barrage of spacecraft we have sent to Mars have shown us a very different world indeed. Mars, as it is now, would prove to be very hostile for the development and sustainment of life. However, in the past, the planet was probably far more like the Earth and could well have hosted all of the necessary conditions for life to begin. July 1996 saw excitement of this prospect grow when NASA released the results of their study of a Martian meteorite (called ALH84001) in which they claimed to have found fossilised bacteria. Although this result is still in question, it has raised the possibility of there once having been micro-organisms living on or beneath the Martian surface. These are not quite the canal building or spacecraft flying, bug-eyed aliens many were once expecting, but they are still more than enough of a find to warrant a huge public interest and a revival of spacecraft (and possibly even manned) missions to Mars.

Marvin the Martian image TM & © 1998 Warner Bros.