News: Meteorite experiment deals blow to 'bugs from space' theory

Meteorite experiment deals blow to 'bugs from space' theory

A novel experiment has dealt a setback to a theory that life on Earth was
kickstarted by bacteria that hitched a ride on space rocks.

The "pan-spermia" hypothesis is that cells were transported to the infant
Earth on rocks that were bumped off other planets or even came from another
star system.

The theory gained a boost in 1996 when a group of US scientists proposed
that a famous meteorite found in Antarctica may have held traces of
fossilised bacteria that once lived on Mars.

Seeking to find out more, European scientists have devised "artificial
meteorites" to see what happens when rocks bearing fossil traces and living
bacteria are exposed to the fiery heat of entering Earth's atmosophere.

In research to be unveiled on Thursday, they attached small rocks two
centimetres (0.8 of an inch) thick to a Russian unmanned Foton M3 capsule
that was launched in September 2007 and returned to Earth 12 days later.

The samples were imbedded on the capsule's heat shield, which reached a peak
velocity of 7.6 kilometres (4.75 miles) per second (17,000 miles, or 27,200
kms, per hour) during the controlled descent.

One sample comprised a 3.5-billion-year piece of sedimentary rock from
Pilbara, Australia, that contained carbonaceous microfossils.

The other was a piece of lake sedimentary rock from the Orkney Islands,
Scotland, containing chemical traces of past organisms.

The back of both rocks was smeared with a living bacterium called
Chroococcidiopsis -- a hardy, primitive species that lives on the underside
of stones in the desert, surviving on tiny droplets of moisture.

Some scientists have considered it, or a relative of it, to be a good
candidate for a Martian germ.

Recovered and analysed after the return, the Pilbara sample was found to be
covered with a creamy-white fusion crust about half a millimetre (0.02 of an
inch) thick but, underneath, its microfossils were intact.

The Orkney samples lost nearly a third of its mass, but otherwise survived,
as did its biomolecules.

But there was bad news for the Chroococcidiopsis. The bugs were burnt to a
crisp, although their carbonised outline remained intact.

"The STONE-6 experiment suggests that, if Martian sedimentary meteorites
carry traces of past life, these traces could be safely transported to
Earth," said investigator Frances Westall, of the Centre of Molecular
Biophysics in Orleans, France.

"However, the results are more problematic when applied to pan-spermia," she
said a press release.

"STONE-6 showed at least two centimetres (0.8 of an inch) of rock is not
sufficient to protect the organisms during [atmospheric] entry."

The study was scheduled to be presented on Thursday at the European
Planetary Science Congress in Muenster, western Germany.

So far 39 meteorites have been found on Earth that have been attributed,
through their chemical signature, to a Martian origin.

The notion is that they were knocked off the planet in the distant past by
an asteroid impact. They then wandered in space before landing here.

But all of these meteorites are of basalt, or volcanic origin.

None is sedimentary, a term for rocks that are laid down in beds or strata
as a result of wind, water or gravity. This has perplexed scientists, as
there is abundant evidence for sediments on the Red Planet.

The outcome of the STONE-6 experiment, though, shows that Martian
sedimentary rocks could survive entry through Earth's atmosphere.

The Foton capsule generated temperatures of around 1,700 degrees Celsius
(927 degrees Fahrenheit), although its speed was somewhat slower than that
of a meteorite.

Meteorites normally attain a velocity of 12-15 kms per second (26,800-33,500
mph, 42,800-53,600 kph) depending on their angle of descent.

A third piece of rock, a control sample of basalt, was lost during the
descent.

© 2008 AFP
http://www.physorg.com/news141539594.html

Posted by
Robert Karl Stonjek


.

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