RE: "You presuppose there was a "wet Mars". Because the Sun is a main sequence star, we know the surface luminosity experienced by Mars has never been brighter than it is today. The only tenable theory for a "wet Mars" is one based upon a green house effect. If Mars had a denser atmosphere then where is that atmosphere today? Where are the carbonates that would be the relic of that ancient atmosphere? There should be mountains of it like the Dolomite Mountains in Northern Italy. Only recently have tiny quantites of carbonates been detected."

I'm presupposing a wet Mars as a null hypothesis counter to your contention there wasn't. You base this on no carbonates and I'm saying we can't be certain we could detect the carbonates that could explain what happened to an early dense Martian atmosphere. Mountains are formed by plate tectonics and volcanoes. There are no plate tectonics on Mars and the volcanoes are part of a possible explanation.

RE: "There's detectable CH4 in the Earth's upper atmosphere. That CH4 is
coexisting with atomic oxygen. How is this possible? Answer: The CH4
is being regenerated faster than it is being decomposed. Are you
proposing that the martian soil is more oxidizing that the Earth's upper
atmosphere? If so, why do you believe that life could survive in such an
environment?"

Please note your reply involved a dynamic explanation. My reply is that all you need to miss indicators of subsurface life is a rate of CH4 production lower than its destruction rate. I have no doubt that Martian life is far less common than Earth life and will produce CH4 at a far lower rate. I don't presuppose any relative rates of reaction in comparing the upper atmosphere of Earth to Martian soil, just that you can't rule out that CH4 production could be lower than it's destruction on Mars based on any data you have presented. The habitable zone I think is possible is not the same as the oxidative zone either. It is deep under the surface wherever liquid water exists. The recent Altacama results indicate Mars' surface properties are in part due to it's dryness. The same conditions obviously do not occur in an underground aquifer.

RE: "I'm reluctant to disagree with Nick. However it is my understanding that
none of the martian volcanoes are active and Mars maybe geologically dead.
I believe(?) Nick is open to the concept of "deep dark life", e.g. hydrogen
metabolism micro organisms. The concept of deep dark life on Mars can not
be dismissed although the lack of detectable methane weighs against it. I'm
skeptical that biological genesis can occur on a world without oceans. The
theory that Mars once had oceans is no longer accepted. If Mars once had
oceans enabling biological genesis, then it would be credible that some
life forms could evolve to live deep underground. However I am skeptical
that life originated deep underground (I believe Nick might disagree with
me on this last point). To recap, my point of disagreement with Nick might
be the necessity of an ocean for biological genesis. Other readers should
note that I am only a humble engineer and Nick does geology for a living."

The current status of Martian volcanoes is irrelevant to past life. We know they were active in the past. Keep in mind my yes was to past life. Your arguements and some of Nick's lead me to lean away from current life, but not to rule it out. I am also skeptical that life can evolve without surface water, but have addressed that possibility in comments above. But again, I can't rule out an undergound genesis. As long as these questions are open I think NASA's plans are scientifically valid.

Regards,
Mark