Thanks, Alex. I love a good logic problem, and found Levin's "Scientific logic for life on Mars" meaty reading. His Twelve Point Plan for Earth-Mars Panspermia is very reminiscent of Drake's Equation.

When I hit points 10 and 11....

10. Terrestrial microorganisms can grow under Martian conditions.



11. Microorganisms are capable of adapting to or evolving to spread over wide regions of Mars.



...I perceived a disconnect with point 1:

1. Microorganisms are widely distributed on Earth.

Much of Levin's argument centers on the discovery of extremophiles in marginal environments, which are rare by definition (otherwise we'd have found the beasties long before now--how's that for scientific logic?). Levin's catalog includes ". Living populations of microorganisms have been found in active ecosystems in supercooled droplets in clouds above the Austrian Alps[17], deep within ancient permafrost[18], in very high salinity[19], in sub-freezing ice on the South Polar Cap[20], beneath a high Arctic glacier[21]...Even the deep trenches in the seas have been reported to be teeming with organisms surviving extreme heat, cold, enormous pressure and lightlessness" I would observe that while there may be wide diversity in the niches extremophiles occupy, they're not widespread in actual numbers. What are the odds?

One factor in the equation that I haven't seen mentioned is that panspermia in a system of planets aligned roughly to the ecliptic would depend on near-equatorial impacts. Higher-latitude impacts, I'd think, would impart an out-of-ecliptic vector. The resulting inclined orbits might make the ejecta inaccessible, or at least prolong the time before capture (playing against "6. Microorganisms can survive the ionizing radiation of space." and calculations of survival versus exposure time). If the orbital calculations don't already include this, the odds go down again.

But wait, there's more: What kind of extremophiles do you find at the equator? Levin's catalog describes extremophiles mostly at high latitudes or in the very deep ocean trenches.

I hate myself for this. I'd rather the odds go up, but what can I do? I think that perhaps Dr. Levin was a tad optimistic in assigning a value of 1 to so many probabilities in the chain.

What's your take, Alex? Is this just a parlor game like the Drake Equation?

--Robert