麻豆传媒

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
(麻豆传媒, 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鈥攖oo short a time to experience much
damage from deadly cosmic rays.

The Sun鈥檚 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. 鈥淵ou are not allowed to do that if you have a human
mission, but we could do it on a Russian satellite,鈥� 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.

鈥淓arly in the history of Mars and Earth, there could have been
an exchange of biological material between the two planets,鈥� agrees Benton
Clark, a Mars exploration specialist at Lockheed Martin in Colorado.