before it can be accepted as an impact structure. I'm not saying it definitely isn't an impact basin, but there is very little evidence at this point that it is one. The authors have, I believe, jumped the gun. But that's OK; it gets some thought processes going.

I like to point to Lyot as the most likely and most recent example of an impact into fluid on Mars, but when I do, I get slapped around by people who insist that Lyot is ever so ancient, and couldn't have formed in fluid in any case. Nevertheless, Lyot and its massive ejecta blanket has all the diagnostics. And Lyot is doubtless an impact structure, whereas this putative basin in Meridiani is dubious at best.

Was Meridiani a sea at any time? I suspect not. Not only is it highland terrain -- which doesn't by itself preclude standing water/acid -- but it has few signs of ever having been open water. There are some isolated areas of Meridiani which look very much like they are fluid filled now, but we must remember that Mars is the planet of mimicry. And there are other areas that appear to be heavily eroded by water/acid action, but this erosion need not have been due to lakes or seas.

Long ago, it was speculated that Meridiani was a site of water rising to the surface, water that flowed underground from the base of the north polar cap. The odd terrain and the strange readings from Meridiani (showing, for example, a saturated atmosphere above it) could be accounted for if there were subsurface channels which could allow polar melt to flow toward the equator. As Meridiani straddles the equator and is warmed by the sun, water/acid might be drawn toward the surface here and eventually sublime into the atmosphere. It is a rather simple, and to my mind elegant, explanation of previous findings, and there is nothing inconsistent with that explanation in current findings. The only missing element is any recognized current site of water/acid rising to the surface (this is, after all, an ongoing process) and sublimating into the atmosphere, or (why not?) pooling in lakes and ponds and so forth. This may be a failure of observation and equipment rather than an actual absence of evidence. We must await further developments.

I think we need to be very careful about asserting a fully "marine" environment on Mars at any time in its past (there certainly is none now). There might have been lakes and seas -- by Martian standards -- but those lakes and seas may well have been very different than those we're used to, and might even be unrecognizable to us if we were to see them. Kind of like fossils on Mars. If there are any at the surface of Mars, we've may well have seen them; but how could we recognize them? If all we have to compare them with is terrestrial fossils, and Mars has never really been terrestrial, how would we know whether what we've seen is the result of biology or not? If we had nothing Martian that we could confidently claim was biological -- and we don't -- then we'd have nothing to do a rational comparison with.

Just so with Martian seas and lakes and so on. If the active fluid on Mars isn't and has never been water as we know it, but has usually or always been something else (sulfuric acid, carbon dioxide, or whatever) then we need some way to understand how these and other active fluids might behave under Martian conditions, and it's only been recently that serious exploration and experimentation has been done with regard to water under Martian conditions (showing, as you have amply demonstrated, that liquid water can exist at the surface of Mars under current conditions for far longer and in far greater abundance than previously believed. Yet that knowledge, as important as it is, still isn't generally understood. How often are you still authoritatively informed that liquid water cannot exist at the surface of Mars? Every day, right? Why? Well, because you assert evidence that conflicts with standard models, is not widely known, and doesn't fit with what "everybody knows."

Long drawn out process of education...

Cheers, BR