-----Original Message-----
From: Jacobus, John (NIH/OD/ORS) [mailto:jacobusj@ors.od.nih.gov]
Sent: Monday November 04, 2002 11:37 AM
To: RADSAFE
Subject: RE: " we're evolved for radiation " - geophysicist on pole revers als
Jaro,
Thanks for putting a "date" on the last magnetic field reversal. I always
assumed that the atmosphere was the source of shielding for life on earth..
That is why any life on Mars would be primitive at best, and why single cell
creatures were the only life form on Earth for so long..
-- John
Sorry, but I don't follow your reasoning.
Over millions of years, the main agent of the mutations responsible for biological evolution is radiation (the last polarity flip was less than a million years ago).
So life is liable to remain primitive in the absence of radiation, not its presence ("we wouldn't be here if it weren't for radiation").
What the atmosphere shields us from is UV radiation, mainly by the ozone component.
The atmosphere of Mars is very thin and lacks the UV-protective ozone layer, thus killing any life on the surface.
However, according to some estimates, even on the Earth, most of the total biomass of the planet resides underground - including deep underground, up to several kilometers - in the form of single-celled thermophilic life forms. That those haven't evolved (much, except for the amazing radiodurans bacteria) is something of a curiosity, because over long periods (thousands of years, when in the dormant state) they will get a huge cumulative dose from geological NORMs.
I haven't seen any data on Martian polarity reversals. Much more surface exploration will have to be done before such data is obtained (may not exist if Mars has no plate tectonics), or information about possible subsurface microbial life. It is known however, that early in its history Mars had large oceans of water, similar to the earth.
There is also a high probability (read certainty) that even if indigenous life did not evolve on Mars, microbial life from earth would have contaminated its neighbour by way of meteoritic impact ejecta transfer (and vice-versa, from Mars to Earth - we could be descendants of Martians :-).
Why were single cell creatures the only life form on Earth for so long ? Good question. But I don't see an answer in any of the above. I believe that current exobilogical theory holds that its just a matter of extremely low probability for evolving multicellular life.
Jaro