"IF YOU compare Mars with Earth, Mars would have been an early developer which then caught a bad cold from which it basically never recovered," says the director of the Armagh Observatory Prof Mark Bailey.
Mars is the fourth planet away from the Sun, after Mercury, Venus and Earth. It was born around the same time as Earth, 4,500 million years ago. In its early childhood - for the first 500 million years or so of its existence - it was a warm place, with a thick atmosphere and torrential rains providing the running water essential to support life.
It was in this period that the primitive life, possibly indicated by the tiny traces of organic chemicals whose discovery caused so much excitement this week, must have existed.
Around 4,000 million years ago, again like Earth, Mars was struck by what astronomers call the "late heavy bombardment" of fragments from comets or asteroids.
This bombardment of the Earth and Mars caused a dramatic rise in both planets' temperatures, their atmosphere was sterilised and they became hostile to life.
At this point, however, the two planets' histories diverged. Mars had the misfortune to be half the size and considerably less dense than its nearest neighbour, with a gravitational pull only 37 per cent that of Earth. The result was that Mars was much less able to hold on to its atmosphere.
In contrast, primitive life forms reappeared on Earth a mere 200 million years after the bombardment from space, as it cooled within its protective atmosphere.
In contrast, primitive life forms reappeared on Earth a mere 200 million years after the bombardment from space, as it cooled within its protective atmosphere.
For the rest of its existence, poor Mars has suffered from the kinds of extremes of heat and cold, plus the unfiltered effects of the Sun's radiation, which make life impossible. Its extremely thin atmosphere, consisting mainly of carbon dioxide, is comparable to that 100,000 feet above the Earth.
In other ways, however, the two planets still share many characteristics, with surface conditions on Mars more like Earth's than any other planet. Its days are almost the same length, and it has four distinct seasons in its 687 day year.
However, temperatures on Mars are significantly lower than on Earth because it is nearly 50 million miles further from the Sun. Its average surface temperature ranges between minus 143 and plus 17 degrees Celsius. Mars has fierce eroding winds and towering volcanic mountains: the highest, Olympus Mons, is three times as high as Everest. Large parts of its surface are covered by craters caused by meteors.
Seen through a telescope, rust brown, desert like regions cover two thirds of the planet's surface. Most of the remaining third is greenish or bluish grey, and these areas are called maria (seas) by astronomers, even though they contain no measurable amounts of water.
Mars has canyons, gorges and deep, dry river beds which make the Grand Canyon look like a Wicklow glen. These appear to confirm scientists' belief that large quantities of water once flowed on the planet's surface. Its north and south poles have "polar caps" of frozen carbon dioxide, which may contain large reservoirs of frozen water.
Today Mars appears to be a lifeless "red planet" of sand, rock and rubble.
However, its crust contains the six elements essential to life: hydrogen, oxygen, carbon, nitrogen, phosphorus and sulphur.
Mars has fascinated humankind since earliest times. It was called Nergal by the people of Babylon, Ares by the Greeks and Mars - after their God of war - by the Romans. All its names have connotations of fire death, war, fear and pestilence.
One explanation put forward for this is that during the Bronze Age, perhaps 5-10,000 years ago fragments of comets and asteroids falling to Earth as terrifying fireballs were a far more frequent occurrence than today. When the comets and asteroids disappeared, ancient peoples transferred their fear of the vengeful gods' thunderbolts to brightly shining planets such as Mars and Jupiter.
In 1726 an Italian astronomer, Giovanni Schiaparelli, charted what he called canali or channels on the planet's surface. This was mistranslated into English as "canals", leading to speculation about Martian made waterways. That speculation was increased when an American, Percival Lowell, claimed in the 1890s that these channels were evidence of agriculture.
Successive generations of science fiction writers took their cue from Lowell's fancies. H.G. Wells in The War of the Worlds, published in book form in 1898, told of Martian refugees from a dying planet landing near London and wiping out the British army with their "Heat Rays" and "Fighting Machines".
In 1938 Orson Welles's radio adaptation of this story caused widespread panic in the US. As Welles described a Martian invasion of New Jersey, many fled from their homes, believing it was happening.
Half a century later Buzz Aldrin, the lunar module pilot during the first Moon landing, was co writing a thriller about an ancient city below Mars's surface. The 1996 version, by Kim Stanley Robinson, is set 30 years into the future.
Not surprisingly, it did not take long for Mars to become an essential ingredient of numerous B movie plots. Among these have been Flash Gordon's Trip to Mars (1938), Abbot and Costello Go to Mars (1953), Robinson Crusoe on Mars (1964), Mars Needs Women (1968), and Tony Curtis as the Lobster Man from Mars (1989).
Mars can now expect an armada of missions over the next 10 years. These start in November with Mars Global Surveyor, which begins a decade long exploration to discover the role of water in the planet's early evolution. They will culminate in Mars Surveyor 2005, which aims to be the first mission to return soil samples for analysis on Earth.