Life hitching a ride to Earth: Bugs could travel to Earth in comfort
aboard Martian meteorites
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.
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Author: Anil Ananthaswamy
More at: Origins of Life and Evolution of the Biosphere (vol 31, p 527)
New Scientist issue 12 January 2002
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