1 Young Adam: So tell us about the canals!
1 Martian 1: Canals? Well, they have a long and rich history.
2 Martian 1: Ancestral peoples built the first simple ones thousands of years ago for irrigation purposes. Thereafter followed larger ones for transport of goods.
3 Martian 1: At first they joined rivers, but as demand for goods and water grew, the engineering schemes became more ambitious, cutting across broad swathes of dry land.
4 Martian 1: The one in Panama is particularly impressive.
4 Young Adam: Panama? I meant the canals of Mars!
4 Martian 1: Mars? Canals?? Ah hah ha ha ha!

I remember growing up as a kid, and hearing wonderful fantasies about life on Mars, and tantalising stories of canals on Mars - structures that obviously pointed to the presence of intelligent life on the planet. At that age it was difficult to separate fact from fiction, and discredited scientific hypotheses of bygone eras from newly discovered scientific observations.

Although, as late as the mid-1970s there was still an open question on whether Mars harboured some sort of vegetation. Telescopic observations showed it had polar ice caps, which grew and shrank with the Martian seasons indicating, possibly, some sort of water circulation. And there were dark patches on the surface which seemed to grow and shrink throughout the Martian year - perhaps trees or bushes that changed colour like our own deciduous plants at different times of year.

NASA's Mariner 4 probe flew past Mars in 1965, before I was born. It returned the first relatively close-up photographs of the surface of Mars, from just under 10,000 km away - much closer than the previous best observations, made from Earth some 56 million or so kilometres away at closest approach. Up until this point - that's 1965, just 45 years ago - it was still scientifically respectable to posit that there could well be macroscopic life forms living on Mars.

Mariner 4, however, revealed that the surface of Mars was pocked with craters, much like our own moon. Other instruments on board made the first precise measurements of the pressure of Mars' atmosphere, returning the at-the-time surprising figure of only about 1% of Earth's atmospheric pressure. Putting these two observations together, many scientists concluded that Mars was so relatively unprotected from bombardment by both meteorites and solar radiation that it was unlikely that large life forms could survive there. Others, however, argued that the resolution of the Mariner 4 photos was so low that photos taken of Earth at the same resolution would not show any signs of human habitation at all - so the possibility of intelligent life on Mars still could not be ruled out.

And that was the situation for another 10 years. By 1976, most researchers had concluded that any life on Mars was probably microscopic, but there was still a less-well-supported but scientifically respectable argument that there might be macroscopic vegetation, or animals, or even intelligent life. That changed in 1976 because the previous year NASA had launched two new space probes: Viking 1 and Viking 2. These arrived at Mars in mid-1976 and went into orbit around the red planet. Each orbiting probe then dropped a landing capsule, which descended through Mars' thin atmosphere and touched down successfully on the surface. These probes gave us our very first images of Mars taken from directly on the surface itself.

They revealed a desert-like landscape, littered with rocks, and totally devoid of any visible indication of life whatsoever. But that's not all the Viking landers did. They carried four separate experiments designed to test for the presence of microbes on Mars. One experiment gave a strong positive result, producing a response that had been defined by the experiment makers as indicating life. However, the other three experiments returned negative results, including the gas chromatograph-mass spectrometer experiment, which failed to detect the presence of any organic compounds whatsoever. This result was confusing for some time, and remained controversial and subject to differing opinions by experts for some years afterwards. For the most part, the consensus was that the positive result had been produced by some unexpected inorganic chemical reaction, and not by life after all.

However, the question wasn't really settled to everyone's satisfaction. In fact, it's still not completely settled, with some researchers thinking that microscopic life could still be lurking somewhere in protected places on Mars such as within rocks, and others hypothesising that Mars once harboured life in earlier, more temperate climates, but that it has probably died out by now.

This was what was happening in the scientific world at the time. In my world, where I was attending primary school and just starting to wonder about things like other planets, this specialised and up to date information was hard to come by. I perused books from my school library, books which had probably been written in the 1960s, and which stated the then big unknowns about life on Mars, and I got the strong impression that there could well be plants and animals on that distant, mysterious world.

It really wasn't that long ago.