Forty years ago, space engineers launched a probe that would play a pivotal role in changing our understanding of our place in the cosmos. On May 30, 1971, Mariner 9 was dispatched to Mars on an Atlas Centaur rocket and in November that year slipped into orbit around the Red Planet.

In doing so, the American robot spaceship became the first manmade object to be placed in orbit around another planet. Humans had added an artificial satellite to another world.

A few days later, two other spacecraft, Mars 2 and Mars 3, both built by the Soviet Union, followed suit and achieved Martian orbit. In three weeks, the Red Planet had become a scientific hotspot.

Thus began a revolution in our understanding of the solar system, a family of planets that space probes have shown to be far stranger and more exotic than expected, with Mars producing the most surprises.

Mariner 9 showed it possesses the solar system's largest mountain and its biggest canyon, while ancient riverbeds and streams were discovered at several sites, findings that have been confirmed and explored in far greater detail by subsequent probes and which continue to maintain hopes that we will one day find signs of life on another world.

Mars is still a place of fascination for humanity, though its investigation has been a rocky business.

"The story of Mars exploration has been a real rollercoaster," admits Oxford astronomer Professor Fred Taylor, who has worked closely with Nasa on several missions there.

"Once, it seemed destined to support life. Then we thought it was utterly dead and featureless. Then we discovered - thanks to Mariner 9 - that it had a landscape through which water had poured. After that, craft found its soil contained no signs of biological material. Since then, we have bounced back and are hopeful life may exist deep underground."

As a result, a swath of missions to Mars is being planned for the next few years with the US, Russia, China and Europe all preparing spacecraft.

These will include automated rovers, with one, ExoMars, being built in Britain; craft that drill deep below the planet's surface; one that will land on Mars' moon, Phobos, and survey the planet from there; and, ultimately, a robot spaceship that will fire Martian soil and rock samples back to Earth for analysis.

"Once we get our hands on that, we will have a real chance of finding out if there is life underneath the Martian surface," says David Parker, director of space science at the British National Space Centre.

If this flotilla of interplanetary craft succeeds in finding life, the study of Mars will have come full circle. In the years leading up to the space age, it was still hoped the most closely studied planet in the solar system possessed life. Changes in surface features, which could be seen through ground-based telescopes, were assumed to be due to seasonal changes in vegetation that were fed by melt-water pouring from the planet's icecaps. In the 19th and 20th centuries, astronomers such as Percival Lowell had even said these melt-waters were channelled through canals built by intelligent beings.

But by the mid-20th century few scientists clung to such notions. As telescopes got more and more powerful, Mars seemed less and less hospitable. Nevertheless, those colour changes on the surface kept alive some hopes of finding primitive life.

Then the first probes arrived at Mars, with the US succeeding with three missions - Mariner 4 in 1965 and Mariners 6 and 7 in 1969.

These "fly-by" missions took only a few dozen photographs and measurements as they flashed past the planet. From these, Mars looked like the Moon, a dead world. Its surface atmospheric pressure was a pathetic six millibars, compared with 1000 on Earth. There was nothing to breathe or provide protection against the sun's deadly ultraviolet radiation, from which Earth's thick atmosphere shields us. The landscape was also plastered with craters.

"It was a low point," says Taylor. "However, all that changed two years later with Mariner 9 which got us our first proper look at Mars. It was an orbiter and could study the planet for a long time.

"And yes, parts of it did look like the Moon. But there are also these wonderful mountains, polar caps and valleys as well as dried-up river valleys which showed water had once flown there. Suddenly, Mars looked as if it had a much more Earth-like climate in the past - though we didn't know if that was the recent past or the distant past. We still don't know."

It is for this reason that Mariner 9 remains of such importance today. It restored hopes of finding life on Mars and revealed a world of surprising variety - though this did not seem so when the spaceship first arrived. The planet was then engulfed in one of the incredible dust storms that occasionally sweep Mars, covering its entire surface. It looked utterly featureless, until the storm began to subside and the dust settled, revealing a strange sequence of markings at one site.

First, a dark, circular outline emerged, which scientists eventually realised was a volcanic caldera. As the dust dissipated, its outline filled out, revealing a 25,000m peak with a 30km wide caldera in its full craggy glory. This was the solar system's largest mountain: Olympus Mons.

"If you wanted to build one on Earth, you would have to excavate all of Texas to a depth of 8km for the raw material," says Oliver Morton in his book Mapping Mars. "An Everest on top of an Everest would not come to the summit of Olympus Mons."

Then a 4000km gash was discovered in the side of Mars, Valles Marineris (the Mariner Valley), an immense complex of canyons, more than 6km deep and 96km across.

Crucially, from the eastern end of this great rift valley, Mariner 9 sent back pictures of what appeared to be dried-up river beds connected to the bed of a former ocean.

Almost overnight, the forbidding impression relayed by earlier missions was replaced by a vision of a planet with a water-rich history like the Earth's and, therefore, a place much more suitable to the development of life. As British astronomer Sir Patrick Moore says: "The great breakthrough in our understanding of Mars was due to two spacecraft, Mariner 4 and, above all, Mariner 9."

Thanks to the latter, it had been discovered that Mars was a warm, wet place when life arose on Earth. So life could have evolved there, and could still be thriving below the surface, safe from harmful solar ultraviolet radiation and nourished by underground water supplies.

What was needed now were craft that could find those lifeforms, a task that has proved awkward and elusive but which may soon be resolved, say scientists. The first stab at following Mariner 9 was made when the US embarked on the Viking missions of 1976. These involved two craft being carried to Mars and landed with precision on its surface. Soil was scooped from the surface and analysed in tiny robot laboratories for evidence of biological activity.

"It was an incredibly expensive undertaking," says Taylor. "And it was a great success - except that there didn't appear to be any life. People were very disappointed."

After that, Nasa gave up on Mars for 20 years. But in 1996 Nasa scientists, led by David McKay, announced they had found possible signs of life on a Martian meteorite called ALH84001. This lump of rock, weighing almost 2kg, fell to Earth 13,000 years ago, after being blasted into space when an asteroid hit Mars.

Found in the Allan Hills of Antarctica, it is one of an estimated 100 - out of the 50,000 meteorites that have been found on Earth - that is reckoned to have come from Mars.

Their composition of elements is similar to those of Martian rocks, with traces of atmospheric gases identical to those found by spacecraft on Mars.

Analysis of ALH84001 revealed chemicals that could have been produced by living organisms, as well as rod-shaped structures that looked like terrestrial bacteria, said McKay and his colleagues.

The discovery was promoted as a triumph by Nasa. President Bill Clinton said it spoke "of the possibility of life". The search for life on Mars was back on track, though the announcement was treated with considerable suspicion by some scientists who argue the features seen on ALH84001 could just as easily be attributed to inorganic or geological processes. But McKay has refused to back down.

Nasa launched the Mars Pathfinder mission in 1996. A tiny robot rover, Sojourner, trundled round Mars' surface, beaming back pictures of rocks and craters. It was a major PR success and stimulated a renewed wave of Martian investigations which have included some notable successes, such as the larger and more advanced Spirit and Opportunity robot rovers landed in 2003. Opportunity is still in operation today.

Other missions were less successful and have included several losses.

Several even more advanced missions are also plannedwith the ultimate aim of returning a sample of soil to Earth, where it can be analysed for signs of life. Much will be learned about the geology, meteorology, soil chemistry and geography of Mars - and a vast sum of money spent.

The question is: do those billions represent value for money? It is a difficult one to answer, though we should be clear about the consequences of finding life on Mars, no matter how primitive it turns out to be. They will be considerable.

Consider the two competing scenarios. First, probes discover lifeforms that have a different genetic code from the DNA that is commonplace in life on Earth. That would indicate that Martian life had a very different evolutionary history from ours and would tell us that on two sister planets, in one humble corner of the galaxy, life appeared independently and separately, a fact that would suggest the appearance of life in general is a common affair on worlds elsewhere in the galaxy.

But even more intriguing is the alternative scenario: those samples could be found to have organisms that use DNA, as Earthly life does, as their genetic code. It is extremely unlikely that such a highly specialised, complex molecule like DNA could have evolved separately on the two planets, indicating that there must be a common origin for Martian and Earthly life.

And given that it is far easier for rocks carrying cells or bacteria to escape Mars' relatively low gravitational field compared with Earth's relatively powerful gravity, we would face an intriguing prospect.

Life based on DNA first appeared on Mars and then spread to Earth, where it then evolved into the myriad forms of plants and creatures that exist today.

If this was found to be the case, we would have to face the logical conclusion: we are all Martians.

Of course, it may be that we find no evidence at all that life evolved on Mars, no matter how hard we look. Even that would be illuminating, suggesting Earth is one of the few outposts harbouring life anywhere in the galaxy and is a place that needs special care and attention. Again, we would be wiser for the discovery.

In the next few decades, we should find answers.

Battling great ghoul

A mosaic of 102 photos of Mars taken by Viking Orbiter 1 in the late 70s. The exploration of Mars has a shaky record. Of the 38 attempts to send probes to the planet, 19 have ended in failure, consumed by the Great Galactic Ghoul, as astronomers describe the blight that has cursed so many missions.

Britain's Beagle 2, which was lost after it began its descent to the Martian surface on Christmas Day 2003, has plenty of company among the other bits of wreckage that pepper the Red Planet's landscape, as Oxford's Professor Fred Taylor knows only too well.

He led the team that built a device for analysing variations in water vapour, dust and temperature in the planet's atmosphere: the Mars Climate Sounder. It was launched on the US Mars Observer in 1992. "Everything was going fine until the engine was fired to put the craft into Mars orbit. It blew up. We never did find out why that happened."

So Taylor and his team built another sounder - for the Mars Climate Orbiter launched in 1998. This time there was a navigation error, caused by controllers who confused imperial and metric measurements, and the spaceship flew too close to Mars and crashed.

Taylor's Mars Climate Sounder was resurrected and put on the Mars Reconnaissance Observer which was successfully put into orbit round the planet in March 2006. "We are now getting lots of lovely data from it," says Taylor.

"We have found that the Martian climate has undergone massive changes over time, from conditions that used to be warm and wet, like those on Earth, to the ice age that now permanently grips the planet."