Monday, 26 February, 2001, 22:38 GMT
At a glance: Life on Mars?
Mars, as seen by the Hubble Space Telescope
A Nasa scientist claims to have found "conclusive evidence" that the planet Mars once harboured life. It is the latest twist in a long-running debate:
- The first speculation about life on Mars came after the Italian astronomer Giovanni Schiaperelli observed streaks on the surface of the Red Planet during a close approach to Earth in 1888. He called the lines "canali", meaning channels, but this was translated as "canals" implying an artificial origin for the lines.
- In the early years of the 20th Century, American astronomer Percival Lowell drew dozens of canals on Mars and believed Martians used them to irrigate the arid equator with water from the poles.
- In 1930, the US Army imposed a radio blackout across America for a few hours so it could listen for signals from Mars.
- In 1969, the Mariner series of spacecraft started observing Mars and showed that the canals were indeed fictitious. They did, however, see features on the surface that might have been produced by running water. This would have indicated that Mars had a warmer and wetter past.
- In 1976, two sophisticated US Viking spacecraft soft-landed on Mars. Each carried four experiments to look for traces of life in the Martian soil. Although there was initial excitement from one of them, it was eventually concluded that no evidence of life had been detected.
- In 1995, Nasa scientists reported finding complex carbon molecules in a rock from Mars that was found in Antarctica in 1984.
- In 1996, they expanded their work making the sensational claim that the rock contained evidence suggesting simple forms of life existed on the Red Planet three billion years ago.
- In 1997, the Mars Pathfinder probe soft-landed on Mars. Although not intended to search for life, it did confirm Mars' wet and warm past - promising conditions for the emergence of life.
- In 1998, the Mars Global Surveyor began its survey of the planet.
- Last year, Drs Michael Malin and Ken Edgett said the satellite had detected evidence that water may have flowed on Mars in the recent geological past.
- A 20,000-picture photo album of Mars was posted on the internet last year by American scientists. It was made up of images taken by Nasa's Mars Global Surveyor spacecraft.
- The archive covered a period spanning one Martian year, beginning with MGS's arrival in September 1997 and extending through to August 1999.
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