Long before we knew much about Mars, it was regarded as the most likely place where aliens might be living.
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We still talk of 'little green men from Mars' and Martians as if they might exist. This started back in the 19th century, when an astronomer noticed a bright spot on Mars. It was probably nothing more than a bright cloud, but it set author H G Wells thinking. What if it were the launch of an invasion of Earth by Martians?
At the same time, astronomers looking through telescopes saw what they thought might be canals on the planet - straight lines between what could be cities. We now know that these were just illusions caused by the eye joining up markings - not everyone saw them, including those with excellent telescopes - but the myth was born. Even up to the 1960s the possibility of some fairly advanced life on Mars, at least vegetation, was taken seriously. The dark markings on the planet changed with the seasons, so maybe they were caused by the spread of plants.
But then spacecraft started to visit the planet and our view changed. The atmosphere is thin, with little water vapour let alone running water, and the dark markings change from year to year as a result of shifting dust. But could there still be life on Mars?
Beneath the surface there are thought to be reservoirs of water ice. Orbiting spacecraft notice changes in gullies that suggest that water may flow from time to time even now. The vast canyons that cover parts of Mars's surface are ample evidence of a watery past, and photographs from the rovers also find evidence of dried-up lake beds. On Earth, life can exist in the same sort of conditions as on Mars today so there's a chance that there could still be life there.
But finding it won't be easy. It could be buried deep underground or in crevices, and using robot craft to search for life could take a long time. We don't expect to find anything creepy crawly, just bacteria or slime. But even this would be the scientific discovery not just of the century but of the millennium.
And there might even be signs that life was once more advanced. Not cities or even bones, but just possibly lowly fossilised multi-celled creatures. But don't hold out too much hope. It's only in the past few hundred million years that animals really got going on Earth, and bacteria held sway for billions of years. If we do find Martians, they might be little and green, but they certainly won't be men.
Credit: Space Telescope Science Institute - Mars, photographed by the Hubble Space Telescope in 2003.