Hello Barry -

Thanks for being civil - that has always differentiated you from the Mars Nuts who congregate here. Before you use the Labeled Release results again, you're going to have to adjust them for Mars radiation environment, that is if they can be adjusted. My current guess is that they can not, and were pretty faulty from the get go. Every one assumed an Earth-like Mars, and that includes Levin. My other guess is that Levin is responsible enough to undertake this revision himself.

One by one, then -

"I think there is enough evidence to support that Mars once had a much more substantial atmosphere."

Agreed. But with cosmic rays, nitrogen goes to C14, and then degrades to C12 and 2 neutrons. These 2 neutrons are important - the simply do not go away and disappear. Of particular concern is their later secondary interaction with other particles (radiation) streaming in from space.

"It was probably the Hellas impact event that destroyed the climate."

Until better crater counts are available from Jupiter's moons and elsewhere in our solar system, any date for the Hellas impact or any other Mars feature will be highly uncertain. The cooling of Mars core would have removed its magnetosphere and thus its radiation shielding - where this fits with the date of the Hellas impact is currently not known with any certainty.

"The point is, a substantial ozone may have persisted for hundreds of thousands to millions of years after the impact, acting as a shield against UV radiation and as it declined, Martian microorganisms evolved and adapted to their changing environment. Life will find a way...."

Ozone is useless against other forms of radiation, other particle streams -

I agree that there is a possibility that life evolved to deal with the new environment, but if the organic molecules are radioactive, then the DNA could not be complex (lengthy) - there would simply be too many breaks.

"Very small amounts of iron and manganese are needed to shield microbes from intense UV. If micorbes can survive in the Three Mile Island nuclear reactor, why do you think they would have any problem under a few microns of dust?"

Once again, more radiation hits Mars than simply UV.

If there is any thing living on Mars, then it will be nasty in the extreme.

ep