[meteorite-list] Alien Microbes Could Survive Crash-Landing

[Ron Baalke]

http://www.nature.com/news/2004/040830/full/040830-10.html

Alien microbes could survive crash-landing
Philip Ball
Nature
September 2, 2004

Tough bugs make interplanetary wanderings more plausible.

Bacteria could survive crash-landing on other planets, a British team 
has found. The result supports to the idea that Martian organisms could 
have fallen to Earth in meteorites and seeded life.

Bugs inside lumps of rock can survive impacts at speeds of more than 11 
kilometres per second, say the researchers [1]. The
work also shows that bacteria could survive crashing into icy surfaces 
such as Jupiter's moons Europa and Ganymede.

The possibility that Earth's first life came here inside space rocks - 
the panspermia hypothesis - was proposed in 1903 by the Swedish chemist 
Svante Arrhenius. But the painful landing has always been a stumbling 
block.

Mark Burchell and his colleagues at the University of Kent, Canterbury, 
have put panspermia to the test by firing lumps of porous ceramic 
infiltrated with bacteria into targets. During impact, the bacteria are 
crushed by up to a million times atmospheric pressure.

"A few years ago everyone said we were crazy," says Burchell. "They knew 
it wouldn't work." But in 2001 he and his colleagues showed that soil 
bacteria can survive a high-speed impact into soft gel [2].
Most of the microbes died, but enough survived to make panspermia 
possible, provided that the bugs don't have to travel too far: they 
would probably be sterilized by cosmic rays and UV radiation during a 
journey from another solar system.

Crushing blow

But the researchers didn't know whether the pressures generated in their 
experiment were comparable to those of a meteorite impact. Nor did they 
know how different microbial species would fare.

To find out, the team used a gas-powered gun to fire bits of ceramic, 
between 0.1 and 2 millimetres across, into targets of gel or ice. The 
projectiles were loaded with cells or spores of the soil bacteria 
Rhodococcus erythropolis or Bacillus subtilis.

At similar pressures to those that would be suffered inside a meteorite 
as it crashed, around one in every ten million R. erythropolis cells and 
a few in every hundred thousand B. subtilis survived when they hit the 
gel. A gram of terrestrial soil typically contains a billion bacterial 
cells.

The survival rate for an ice target was about ten times higher, so 
Burchell and colleagues think that it's not just Earth and Mars that 
could have swapped life. The icy moons of Jupiter, for instance, at 
least one of which, Europa, has a sub-surface ocean of water, could seed 
one another. Or a planet could re-seed itself if, as some have suggested 
might have happened on the early Earth, a massive impact wiped out all 
life.

References
  1.. Burchell M. J., Mann J. R. & Bunch A. W. Monthly Notices of the 
Royal Astronomical Society , 352. 1273 - 1278 (2004). 
  2.. Burchell M. J., Mann J. R., Bunch A. W. & Brandao P. F. B. Icarus, 
154. 545 - 547 (2001).