- 1890s onwards: US astronomer Percival Lowell says Mars has a civilisation, notably shown by lines on its surface that are "canals" linking cities. He spawns a global obsession with aliens, starting with H.G. Wells' "The War of the Worlds."

- 1950s: Science-fiction novels and movies about Mars are in full swing, painting the planet as either source of terrestrial invasion or a fertile future home for humans.

- 1964: That vision is dented by the first close-up pictures of Mars sent back by the NASA spacecraft Mariner 4, which show a barren, cratered world.

- 1976: Disappointment is complete. Pictures sent back by the US landers Viking 1 and 2 show a dry, rocky landscape. Mars is pronounced sterile.

- 1996: A group of NASA scientists reports they may have found proof of past life on Mars. The evidence, contested by many, is the existence of outlines of what are said to be fossilised bacteria, buried in a Martian meteorite that fell to Earth.

- 1997: The water debate is revived. NASA's Mars Pathfinder rover sends back reams of data suggesting that Mars was once warm and wet, rather like the Earth. But the precious liquid either evaporated into space or receded underground, say theorists.

- 2002: Data sent back by the orbiting US Mars Odyssey spacecraft suggests, according to its NASA interpreters, that vast quantities of frozen ice exist under the Martian surface, from the edge of the polar icecaps to mid-latitudes of 60 degrees. Doubters say the intriguing gullies, valleys and channels that cover Mars were sculpted by wind and dust or liquid carbon dioxide, not running water.

- 2004 (January): European orbiter Mars Express confirms presence of frozen water on the Martian South Pole, adds visual clues that, in the past, seas and glaciers helped mould the planet's surface.

SPACE.WIRE