A METEORITE that fell to Earth in Australia in 1969 may have played host to
microorganisms many times over the past 4.6 billion years, a NASA space
scientist claims. But the meteorite鈥檚 parent body was not the Earth, or even
Mars. 鈥淭his could have profound implications for our ideas about life in the
Solar System,鈥� says Richard Hoover of the Marshall Space Flight Center in
Alabama.
The meteorite is of a type rich in organic compounds known as a carbonaceous
chondrite. It fell near Murchison on 28 September 1969. Since the early 1960s,
there have been claims that some carbonaceous chondrites contain fossil
microorganisms, but they have never been widely accepted.
The main criticism of the claims was that meteorites are likely to have been
contaminated by terrestrial organisms after falling. Hoover says he can rule
this out for the Murchison meteorite, which was collected straight after it fell
and is the most pristine carbonaceous chondrite known.
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To minimise the chances of contamination, Hoover worked only with the
meteorite鈥檚 interior. 鈥淚 placed a sample in an environmental scanning electron
microscope within minutes of breaking it open to expose interior surfaces,鈥� he
says.
Hoover immediately found spherical structures in the meteorite measuring
between 1 and 15 micrometres across, including some with spikes. 鈥淎lthough most
are probably microchondrules of nonbiological origin, some populations have
distributions, chemical and morphological characteristics suggestive of coccoid
bacteria and cyanobacteria,鈥� he says.
Hoover has also discovered mushroom-shaped structures from 2 to 100
micrometres across. The largest of these resembles a slime mould, he says. Some
of the meteorite structures are similar to those in the famed Martian meteorite
which NASA scientists suggested contains fossils last year
(This Week, 17 August 1996, p 4).
If the Murchison structures are fossil organisms, their origins are
intriguing. Hoover suspects that they originally came from Earth, or possibly
Mars. Impacts of large bodies on Earth and Mars could have ejected rocks
containing living and dead microorganisms, which might have struck and
contaminated the parent body of Murchison, possibly the rocky core of a spent
comet. Hoover鈥檚 theory is that a chunk of the parent body eventually fell back
to Earth in 1969.
But many people are sceptical. 鈥淚 would be very cautious about interpreting
any microstructures in meteorites in terms of `organised elements鈥�,鈥� says Monica
Grady of the Natural History Museum in London. 鈥淎ll meteorites travel through
the atmosphere where they risk contamination.鈥�
John Kerridge of the University of California at San Diego insists that any
resemblance of the organic material to lifeforms is coincidence. 鈥淣aturally, it
squeezes between mineral grains and adopts all kinds of bizarre shapes,鈥� he
says. 鈥淎ll Hoover鈥檚 got is shapes. Like many people, he seems to believe that
minerals have crystal faces, and anything without nice crystal faces has to be
产颈辞濒辞驳测.鈥�