The weight of evidence accumulated to date strongly supports the potential for Martian biology. It is not definitive proof, however, and so long as there is no such proof, there will be naysayers whose primary objective is to assert a sterile Mars.

And so long as we accept sterility as a default position for Mars, those naysayers will carry the day. This is why, as I see it:

A sterile Mars is a positive assertion. I know it sounds like a negative: "There is no life on Mars." But that's not the actual assertion. The actual assertion is: "The surface of Mars is sterile." Any assertion to the contrary is therefore a "negative" in contrast to the positive assertion of Martian sterility.

You see the problem. There is no adequate way to prove the "negative" -- that the surface of Mars is not sterile.

And that's why I advocate reasserting Martian biology as the positive default, and asking for positive evidence that precludes biology. Let's just get rid of the "negative" altogether.

You cited what was until recently some of the best evidence precluding Martian biology: the atmospheric balance. But now we know that the atmosphere is faintly unbalanced, with transients like methane and formaldehyde now being detected. They have long been suspected.

These transients could well be the result of biological activity. But they don't prove it. All the indications of pack ice and snow packs and rivulets and gullies, and all the indications of some kind of intriguing behavior by those black spots and those curious streaks on the surface, and all the blanketing deposits of sulfates and iron concretions and so on could be connected in some way with biological activity. But they are not proof of it.

So long as proof of life on Mars is essentially a matter of proving a negative, nothing that is adduced to support a biological interpretation will be sufficient, especially if, as seems to be, any biology on the surface of Mars (and possibly below it) is tenuous at best by terrestrial standards.