[meteorite-list] Invaders That Rock The World

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:48:13 2004
Message-ID: <200110251554.IAA29047_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/science/story/0,3605,580176,00.html

Invaders that rock the world

Are we really descendants of bacteria that rode on cosmic cannon balls, asks
Matthew Genge

The Guardian (United Kingdom)
October 25, 2001

In 1969 Michael Crichton wrote the Andromeda Strain in which a deadly
extraterrestrial virus was returned to Earth to infect the unsuspecting
populace. Films followed suit and in 1978 we watched as the spores of alien
body snatchers once again drifted down through the atmosphere and replaced
even the insomnia-ridden Donald Sutherland. Strangely, this notion that
extraterrestrial organisms can reach the Earth is a real scientific
possibility. It is known as panspermia.

Although a detailed theory of panspermia was proposed as early as 1900, it
was not until 1996, when structures resembling fossilised bacteria were
discovered by Nasa in a martian meteorite, that panspermia suddenly seemed
to be a real possibility. Virtually overnight a new and exciting field of
science, astrobiology, appeared.

The controversy over fossilised bacteria in martian rocks sparked research
into the transfer of organisms between planets on meteorites. Rocks could,
it appeared, be hurled into space from the surface of a planet on the impact
of asteroids and comets. Some of these rocks could even escape the enormous
heat and pressure generated when an asteroid, five kilometres in diameter
slams into a planet's surface at 20km/s. Could these rocks contain microbes
capable of colonising another world? In the case of terrestrial rocks, the
answer is probably yes. Take away the rocks, the oceans, and the atmosphere
and our planet ' s surface would be traced out in every detail in a
translucent layer of micro-organisms. Earth bacteria quite probably beat
humans into space by hundreds of millions of years.

Could microbes survive being cast from a planet at enormous speeds and
exposed to the harsh environment of space? Experiments suggest bacteria
certainly suffer little damage from acceleration and some multicellular
bacteria can even benefit from the white-knuckle launch into space since
they split into smaller units which increases their reproduction. Exposure
to radiation in space and typical journey times between planets of millions
of years are a more daunting challenge to wannabe microbial colonists. The
discovery in 250m-year-old salt crystals of viable bacteria spores, however,
suggests that in hibernation microbes can do a geologically significant Rip
Van Winkle impression. Microbial cells will nevertheless still be subject to
damage by radiation, with energetic particles ripping through their DNA like
cosmic cannon balls. Although some live bacteria, such as Deinococcus
radiodurans, can survive such irradiation by constantly repairing their
genetic material, spores will not have this ability. Only within rocks large
enough to shield their passengers from radiation are viable microbes likely
to survive.

So what are the chances that living organisms could arrive on Earth? We
already know of 15 martian meteorites which have landed on Earth in the last
2m years and the real number must be thousands of times higher than this
since finding meteorites is such a haphazard business.

What about microbes from outside our solar system? Jay Melosh from the
University of Arizona suggests that one rock from a planet in another
planetary system will, by chance, land on Earth once every 10bn years even
given the most favourable conditions. By contrast, the Earth itself is only
4.6bn years old. Panspermia between solar systems has long odds indeed.

The conclusive evidence for panspermia would be to find alien microbes on
Earth. Here there is one important lesson that has been learned in
astrobiology. Where microbes are concerned, contamination is difficult to
avoid. Every meteorite examined has been crawling with terrestrial bacteria
and fungi. These are after all the ultimate opportunists and have conquered
virtually every habitat on Earth, including those that occasionally fall
from space.

It is for this reason that the discovery of bacteria at 41km altitude in the
atmosphere announced by Chandra Wickramasinghe of Cardiff University is not
evidence for alien microbes. Prof Wickramasinghe and the late Sir Fred Hoyle
envisaged that microbial life may have evolved on comets and are delivered
on the 40,000 tonnes of comet dust that falls through the Earth's atmosphere
each year. They have even suggested that cometary microbes may have caused
influenza pandemics and BSE. Yet again provocative theories have made
panspermia controversial.

Comets do contain an intriguing mixture of organic chemicals synthesised
entirely in the absence of biology in space, which includes amino and
nucleic base acids - the basic building blocks DNA and proteins, and it is
possible that comet dust raining down on the early Earth may have provided a
ready-made "cake mix" for life. Why then are microbes not expected to be
present on comets? The answer lies in their icy nature. Comets consist of
ice and dust and liquid water cannot exist on these objects due to the low
pressures. The metabolic reactions which form the basic machinery of simple
living organisms all have one thing in common, they all occur in water. No
water, no life.

The discovery of bacteria in the high atmosphere is thus quite probably a
testament to the pioneering abilities of our own terrestrial microbes rather
than evidence for life from outer 9298"> en if alien microbes are falling
through our atmosphere, however, there is no cause for concern since they
will have been doing it for millions of years. Stockpiling antibiotics is
thus not necessary and if you should wake up and find your partner somehow
changed, then it's more likely to be a hangover than the invasion of the
body snatchers.

- Dr Matthew Genge is a meteorite scientist at the Natural History Museum.
Received on Thu 25 Oct 2001 11:54:55 AM PDT