A THIRD of a tonne of extraterrestrial bacteria could be raining down on
Earth every day. The controversial claim comes from scientists who say they have
found microbes living more than 40 kilometres above the Earth鈥檚 surface.
Ever since the 1970s astronomers Fred Hoyle and Chandra Wickramasinghe have
backed the idea that Earth was seeded with life from space. They think microbes
hitchhike around the Universe on comets. Although the bacteria would spend much
of the time frozen, radiation from the Sun would warm a comet鈥檚 surface when it
got close enough, making it shed microbes that might just fall to Earth.
Now Wickramasinghe says he鈥檚 caught some extraterrestrial bacteria in the
act. In January, he and his colleagues at Cardiff University and the Indian
Space Research Organisation in Bangalore launched a balloon from Hyderabad in
southern India. Strapped to the balloon were several steel containers that were
sterilised and evacuated before launch.
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As the balloon rose through the atmosphere, the containers collected air
samples at four different altitudes over a range of 20 to 41 kilometres. When
the balloon landed, four containers with air samples and two unopened canisters
were sent to David Lloyd, a microbiologist at Cardiff University.
Lloyd says that no microbes had contaminated the unopened containers. But he
found that all four containers that were opened high in the atmosphere contained
clumps of bacteria. 鈥淲e found a mixture of alive and dead microbes,鈥� says
Lloyd.
The researchers found similar numbers of bacteria clumps at every height they
tested, Wickramasinghe told an astrobiology session at an engineering meeting
held in San Diego last week. 鈥淚f this was blown up from the ground, you鈥檇 get a
tail off at higher altitudes,鈥� he says.
He says finding bacteria above 16 kilometres is intriguing because of an
atmospheric 鈥渂arrier鈥� called the tropopause. Here, the atmosphere flips from
getting colder with height to getting warmer. This cuts off convection currents
that might otherwise carry microbes from the ground into the atmosphere.
鈥淚t acts like a lid,鈥� adds Ian Colbeck, an aerosols expert at Essex
University in Colchester. 鈥淭he only particles that can get near it are from
volcanic eruptions, and bacteria probably wouldn鈥檛 survive the heat. I can鈥檛 see
any way they鈥檇 have their origin on the ground.鈥�
Mark Burchell, a planetary scientist from the University of Kent at
Canterbury, is more cautious. 鈥淭here aren鈥檛 good models for how high bacteria
can be swept up,鈥� he says. 鈥淗e may have extended the range of life on Earth,
because 41 kilometres would be remarkable. But if it鈥檚 from space, it鈥檇 not just
be remarkable, it鈥檇 be astonishing.鈥�
Wickramasinghe estimates that if each clump of bacteria contains 100
bacteria, one third of a tonne of extraterrestrial bacteria could be falling on
Earth every day. He is now trying to grow the microbes he collected and identify
them.