<blockquite>OTOH, I do like models where whatever happens on Mars, does so in conditions not very different from today's. One observation is that the Meridiani acid fluids would be metastable on Modern Mars. Although they would lose H2O to the thin atmosphere, that actually helps, since the chemical reaction with rock that deposits sulphates also lowers the acidity and raises the freezing point as it makes a more dilute solution, so there's a battle between freezing, boiling, and reacting, implying a dynamic environment with recharge of acid and then environmental processes that concentrate the acid.

Certainly a theoretical case can be made that Martian surface conditions today are pretty much what they have been for aeons, with localized exceptions (volcanics of various kinds, impacts and the like) and such a model does seem to work reasonably well for Meridiani. I especially like your description of the potential interactions between the acids, H2O, the sulfate deposits, the atmosphere and so on. It is a rational explanation of what we see, and yet....

Do we see it elsewhere on Mars? Or is Meridiani unique?

Further, where does the aqueous solution come from? Is Meridiani a sink, a la the Humboldt Sink in Nevada? If we're dealing with a sabkha like formation (of course modified by those Martian conditions) whence the water table essentially at the surface? What is charging the "aquifer?" Is it still being recharged?

Let's say the surface is always dry and cold much as we see it now, and there has been no ice cover in this area over time. Then how do we get a water table essentially at the surface in equatorial latitudes?

At one time I believe it was Michael Carr who speculated that there was a subsurface water circulation regime by which liquid water melted from the base of the North Polar deposit was circulated through subsurface channels to areas near and at the equator (Meridiani in particular) where it rose to the surface and sublimed into the atmosphere and was returned to the pole to repeat the process endlessly...

Of course who at the time would have thought we would be dealing on Mars with sulfuric acid (H2SO4), that quinessential Venusian solvent?

The surface of Meridiani looks quite fresh, that is freshly eroded from who knows how ancient an original surface. There's no substantial dust deposit anywhere, which is somewhat odd given the proximity to dust-laden Arabia. We may be looking at primary erosion rather than the detritus of repeated episodes of deposition and erosion. There appears to be some sand. I saw someone claim the sand was found between layers of sulfate, but I'm not convinced of it. But the surface our dauntless little robot is traversing is essentially made up of an enormous number of blueberries and decomposed sulfate deposit. Since the balls are found to grow in the sulfate, it's reasonable to propose that some tens or hundreds of meters of deposit have weathered away (the mesas and buttes found elsewhere in Meridiani essentially prove the case anyway) -- but there is no evidence (is there?) that Meridiani has been subject to repeated episodes of deposition and deflation...

So I'm not seeing a lot of evidence that a wind regime has contributed substantially to the layering of the deposits we see. Most of the layers should be the result of "wet" deposition... But then, if hundreds of meters have eroded away, how recent was the most recent deposition?
There seems to be some agreement that the deposits we see are quite ancient.

Hmmm...