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Life hitching a ride to Earth: Bugs could travel to Earth in comfort aboard Martian meteorites

January 09, 2002

FOR the first time, millions of bacterial spores have been purposely exposed to outer space, to see how they are affected by solar radiation. The results support the idea that life could have arrived on Earth in the form of bacteria carried from Mars on meteorites.

        The idea that life started elsewhere and spread through space is called panspermia. It was first proposed in 1903 by the Swedish chemist Svante Arrhenius, who suggested that solar radiation might propel single spores across solar systems. Then, in the 1970s, astronomers Fred Hoyle and Chandra Wickramasinghe studied the infrared spectra of interstellar grains of dust and concluded that they were dried, frozen bacteria. They put forward the controversial suggestion that life on Earth originated when such bacteria arrived from space. But critics of their work said that cosmic rays and ultraviolet radiation from the Sun would kill unprotected spores.
        Recent discoveries of Martian meteorites that have reached Earth have raised the possibility that bacterial spores could have hitched a ride on these rocks (New Scientist, 15 January 2000, p 19). Most meteorites spend millions of years in space, but meteorites taking a direct route would make it from Mars to Earth in just a few years-too short a time to experience much damage from deadly cosmic rays.

        The Sun`s UV radiation might still pose a danger, however. To assess its effects, Gerda Horneck of the German Aerospace Centre in Cologne and her colleagues carried out a series of remote-controlled two-week experiments aboard the Russian Foton satellite. They started by exposing nearly 50 million unprotected spores of the bacterium Bacillus subtilis outside the satellite. This is the first time any living organism has been purposely released into space. "You are not allowed to do that if you have a human mission, but we could do it on a Russian satellite," says Horneck.
        UV radiation from the Sun killed nearly all the spores, confirming that single bacteria would not survive long enough in space to travel from one planet to another. The same happened when the spores were behind a quartz window, so the researchers did the rest of their experiments with the spores confined under quartz.

        To test whether meteorites might protect the bacteria on their journey through space, Horneck and her colleagues mixed samples of 50 million spores with particles of clay, red sandstone, Martian meteorite or simulated Martian soil, to make small lumps a centimetre across. In most of the samples, between 10,000 and 100,000 spores of the original 50 million survived. And when mixed with red sandstone, nearly all survived. The results suggest that even meteorites as small as a centimetre in diameter could carry life from one planet to another, if they completed the journey within a few years.
        "Early in the history of Mars and Earth, there could have been an exchange of biological material between the two planets," agrees Benton Clark, a Mars exploration specialist at Lockheed Martin in Colorado.

Author: Anil Ananthaswamy
More at: Origins of Life and Evolution of the Biosphere (vol 31, p 527)

http://www.newscientist.com">New Scientist issue 12 January 2002


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