sterility at all? Of course the paradigm of sterility helps organize study and helps direct exploration. If the Sterile Mars paradigm is the guiding factor, then hypotheses don't need to be tested. They are self-evidently true. What you do is build on these trusted findings. Ultimately you may 'discover' life on Mars, but it won't be for centuries, perhaps. Any such discovery implies a paradigm shift, but it also may require a paradigm shift.

To assert that "life is far more extraordinary than any inorganic reaction" strikes me as a kind of rigid denial that has little basis in actual, testable fact. That's not to say that this or that formation is the consequence of biological activity. It is to suggest that biology cannot necessarily be ruled out in every case. But if you accept that Mars is sterile from the get go, then actually biology is ruled out in every case until and unless there is a demonstration to the contrary. In other words, the student must prove the negative. Anything the student tries is a priori inadequate.

In essence, Carl Sagan asserted that Martian surface biology was an "extraordinary" claim and so it became one by consensus. As I've said, prior to 1964, it wasn't considered "extraordinary" at all, and it only became "extraordinary" when those lunar-like craters were exposed by Mariner 4, and confirmed by Mariners 6 & 7. Our knowledge of surface conditions did not significantly shift -- those conditions were fairly well worked out before spacecraft arrived at Mars -- but the appearance of the surface was "shocking" to planetary scientists who expected something else again, a fundamentally more active (and eroded) surface than was revealed. When Mariner 9 showed a remarkable surface variety and plenty of indications of volcanic and other activity, all of the new revelations (which would have had a powerful effect had they been discovered in 1964) had to be integrated into the pre-existing paradigm of the Lunar Like Mars.

But the Lunar Like Mars paradigm was fundamentally in error and should never have been accepted in the first place. Yet it was accepted and some scientists have been trying to work their way out of it ever since.

Mars is not like the Moon and it is not like the Earth. Thus, the sterility of the lunar surface should have nothing whatever to do with the presence or absence of biology on Mars. Yet it still is the touchstone for comparison. On the other hand, the Ancient Wet Mars (and oh, so Earthlike!) assertions of a while back are probably wildly off the mark as well. The more likely scenario is that the surface of the planet has always been cold (by comparison to the surface of the Earth), probably colder in the past than it is now, and that many of the surface features attributed to water are probably the result of glacial action, that surface ice and glaciers are still apparent and possibly active today, that water is not necessarily the most widely operative fluid, that subsurface conditions may be (and may always have been) quite a bit more conducive to the development and flourishing of biology.

After all, there appear to be deposits of what could well be Meridiani-type sulfates and salts revealed kilometers below the current upper surface in Valles Marineris, as well as extensive deposits in craters and elsewhere (just where is yet to be mapped) that imply the action of subsurface fluids (including "warm" water) over vast stretches of time.

The presence of these deposits (and they are very extensive, possibly as extensive as the postulated "Wet Mars" carbonate deposits -- which apparently are completely absent) suggests that conditions suitable to a very distinctive Martian biology -- which may not even be immediately recognizable as such to us -- have been present throughout much of Mars' history, and continue to be present.

If biology ever got a foothold on Mars, it is probably still there -- for there is nothing to preclude biology on Mars. There may well be little evidence of active biology on the surface of Mars (my guess is that the only biology routinely present on the surface is something spore-like -- but it is only a guess). Subsurface, though -- and perhaps very near the surface -- there could well be a vast and complex biosphere that can only be faintly imagined by us now.

On the other hand, maybe it isn't there!

We just don't know. And what is so frustrating to this observer is that hypotheses asserted in support of a sterile Mars are not tested on the planet. To me, it is perverse. To somebody else, it's prudent.

Cheers,