Gary,

Good arguments! I would love for the warm-wet model to be true, it would sure be exciting, but it looks like it is in deep trouble. I don't know that I buy the hardcore version of the White Mars theories that the role of liquid water was negligible and CO2 carved everything, but even with the new models where things are basically frozen but you get very brief periods of impact-generated rainfall are not at all warm or wet -- not very life friendly at all. That's what this non-specialist takes away at least, though I guess we can't rule out the possibility that I've simply been tricked by Nick's superior persuasive ability.

There's always the hydrothermal possibility, that hasn't been ruled out.

But without an ocean or at least very widespread hydrothermal systems, can life spread? If you do get that magic chemical transformation in a geothermal area, how does it spread elsewhere if such centers are rare? On earth, whatever evolved around oceanic vents could quickly be transported elsewhere, and there would have been a lot of such vents back in the Archeon era.

What does seem true though, is that we're quickly getting to the point that IF there is evidence of life to be found on Mars, it is fossilized, not living; it will not be found in widespread oceanic or lacustrine sediments (since they don't seem to exist); if it is in hydrothermal deposits, they are rare and/or hard to locate remotely.

The implication is that finding fossils won't be easy. At best, sounds like we need to send a bunch of rovers to turn over a whole lot of rocks. That gets expensive and demoralizing. Nope, not there. Nope, not there either.

So if our program is centered around "finding life", what do we do when that seems unlikely, or at best an expensive crapshoot? We stay home, that's what.

The real question is, could we generate enough support for a Mars program without life being so central? It would be more difficult, but I think not impossible; maybe I'm just letting my space hobby cloud my professional political judgement.

Anyway, we could still argue that if we want to send humans *somewhere*, Mars is the only real option in the next fifty years, and that justifies study.

We could also still have a research program that is strongly influenced by astrobiology, if not by digging for fossils. I would imagine there are all sorts of questions about the evolution of planetary bodies and atmospheres that would be relevant to the likelihood of life elsewhere, that Mars could shed light upon.

On the other hand, if we're not likely to hit the jackpot on Mars, you could also wonder whether scientifically it makes more sense to focus on other bodies. If it's only about geology/planetology, why Mars and not Venus, Io, Titan, comets, etc?

Even for astrobiology, Venus has important lessons. If plate tectonics makes planets more hosiptable to advanced life (creating landmasses, chemical recycling, regulating heat transfer and climate), then it's important to know whether plate tectonics works on Earth and not Venus because of our thin lithosphere due to the rare circumstances of the Moon's formation (Nick is a booster of that idea), or, if plate tectonics doesn't work on Venus because its lithosphere is so dry, in which case it (and other worlds) might have had plate tectonics until the water was lost.

Personally, I'm happy to focus on the geology of Mars because I think we can make progress unravelling it on a time scale that's meaningful to me (Venus rovers are a tough engineering problem!), but I agree that the hunt for Martians is likely to leave us frustrated and to underutilize the resources being put into these missions.