I still like the spring mound idea.

The rover is currently seeing a lot of rocks which look spongy and porous. What if there is a thick layer of such rock underlying Gale Crater? In wet, high atmospheric pressure climates these rocks would fill up with water, creating a large aquifer.

Then the atmospheric pressure drops quickly, due to carbon dioxide freezing out at the poles.

The drop in pressure reduces the boiling point of water, and the water in the aquifer starts to boil. The porous beds slope upwards towards the center of the crater, so the warmer less dense fluids migrate in that direction. They erupt from Mt Sharp, leaving behind an evaporite deposit.

The chemistry of the evaporite depends on the chemistry of Martian water and the atmosphere at the time. When the atmosphere was rich in sulfur dioxide, sulphates were formed. More recently, another mineral, maybe carbonates was deposited. Martian winds have eroded Mt Sharp over time, giving the deposits an aeolian appearance.

The lowest clay bearing layers might be old lakebed deposits which were covered and protected from erosion by later materials.

Mt Sharp could be the result of a long history of oscillations in atmospheric pressure which alternately filled an aquifer and then dropped the pressure enough to boil it.

I dunno; sounds like a bit of a reach to me.

Meh; we'll know a LOT more about Gale in a couple of years, certainly enough to constrain these hypotheses based on actual data.

Maybe. But despite Curiosioty's impressive capability compared to the MER she is still pretty much constrained to analysing the immediate surface. Translating findings to the macro environment of the far past may be a bit of an ask.

Didn't say 'solve'; just constrain.

Oh yeah. Gotcha. Duuh - put it down to a senior moment.

As currently conceived, scientific value vis-a-vis the structure of Mt. Sharp is that the most interesting stuff is the oldest materials which are at the bottom. First Curiosity has to get there. Then, as Curiosity ventures higher, it will basically be visiting more recent areas in martian history and perhaps arrive at the same location/era that typified Meridiani - wet but acidic. This is a bit less interesting for several reasons, not least of which that Opportunity already spent years exploring it (with a poorer set of instruments), and that acidic water is in various ways less earthlike and perhaps depleted in other interesting dynamics. Additionally, the structure of Mt. Sharp appears to have much, much thicker layers representing more recent layers, so even given a constant speed of march in terms of terrain, the rate of march into more recent martian history will slow dramatically; in essence, the upper layers appear to be less diverse than the lowest layers.

All of that, is of course based on the best speculation. There's no guarantee that the most interesting single rock on Mars isn't perched high on Mt. Sharp. But rational planning will be based on weighing the expectations with the effort and the risk.

This is all simply to say that when (if we are fortunate enough for all to proceed with success for decades) Curiosity reaches a certain high location on Mt. Sharp, there will probably be a desire to bring it back down, and that will probably be slowed by terrain.

So if I had to place my bets, it'll be that we'll have a wait for the most interesting stuff, then we'll have a long bonanza of peak interest followed by diminishing returns before Curiosity reaches a peak altitude and the decision is made to bring it down to explore the lower altitudes laterally. While layers are emplaced according to chronology, this arrangement is "patchy"; whichever route it takes up, there'll be other units on other paths. Anderson and Bell describe two ascent routes with similar but non-identical attractions. I think we'll have to wait through a relatively boring descent, before a "second coming" when Curiosity gets back down to the layers of primary interest and finds some of the things it missed on the way up.

And of course, this is only an educated guess. The most interesting thing(s) Curiosity finds may come at any time and in any place. That's why it's exploration.

'Relatively boring descent' is relative, of course. The roads to Victoria and Endeavor certainly weren't!

I think, the spongy-looking surface of those rocks may be explained by conglomerates similar to those at Bradbury Landing. Easily weatherable rounded stones might be embedded in a more resistant material. As soon as the conglomerate is exposed to the acidic and oxidizing environment, embedded stones fall out of their holes or weather rapidely.
To an explanation of the embedded stones being more weatherable might contribute acidity: Embedded stones are older than embedding rock. So they probably will be more basic (alkaline) due to increasing acidity of the Marsian surface over time; they might be more basic, if they are of magmatic or plutonic origin (basalt), as well. Alkaline rocks will tend to weather more easily today than acidic ones.


Water will freeze out first, before carbon dioxide. Freezing produces warmth. So a runaway freezing at the poles looks to me rather unlikely.


Some water might evaporate or sublimate; boiling might have occurred in the context of vulcanism. Capillar forces are too weak to drive water upward more than a few hundred meters, I think. Pressure from shrinking rocks will erupt surface water at most once, thereafter the pores will allow less water contents. Repeated formation of new pores by solvents probably leads to a net shrinkage of the mountain. The only way, I can imagine, able to change this may be periodic hot vulcanism. The other question is: Why doesn't the water flow sideward as ground water on a layer of clay and form springs at the laterals of Mt. Sharp?


Normally carbonates will tend to be more alkaline than sulfates. So I guess, that carbonates might have formed in the Noachian, i.e. early in Marsian history, together with clay minerals. Later, in the Hesperian, sulfur oxides might have transformed some of the carbonates and clay minerals to sulfates or sulfites.
Many sulfates are more water-solvable than the corresponding carbonates or clay minerals. So acidic weathering sounds rather plausible to me.

Acidic weathering, together with acidic deposites in riverbeds, might also contribute to the inverted river and pool beds, because acidic beds within more alkaline surrounding rock will tend to be more resistant under the present acidic conditions. Same with reduced stuff under oxidizing conditions.

I could imagine an ice cap or permafrost helping prevent Mt. Sharp from fast erosion, much the same as mountains on Earth.

I like the idea of acidic weathering being responsible for some of the spongy rocks, but I don't know if the present environment is acidic. The soil at the Phoenix landing site was alkaline, so recent Martian conditions might be more suitable for forming carbonates. I think Glenelg makes most sense if viewed as a big stack of magnesium/iron carbonates with a variety of concretions. For earth examples of a carbonate terrain, see 'Concretions and nodules of North Dakota' .

A result from the Grail mission caught my eye, which was that the crust of the moon is about 12% void to a depth of several km below the surface due to it being fractured by impact. If the ancient Martian crust is similar, then at one time there should have been a huge amount of water in subsurface aquifers. At past Martian surface pressures, hydrothermal is going to mean something different from what is found on earth. At 60mb pressure, water will boil at 36C, so you don't need a lot of volcanic heat to drive a hydrothermal system.

Drop the pressure to 10mb, and water boils at 7C. Previously stable aquifers will boil until they cool below 7C. For a mixture of 90% rock and 10% water, 14% of the water will turn to vapor, if the system starts out at 36C.

An interesting question is what happens if the pressure falls below the triple point pressure of 6mb. If a cup of water starts out at a little above 0C, I think 12% of the water will end up as vapor and the rest will turn to ice.

How much vapor do you get if you start with 1 cubic km of aquifer with a 10% void fraction and turn 10% of the water in the voids to steam over 100 years? That works out to 3kg/s of steam, which should give you a small geyser.




True, water will freeze first, and water is a greenhouse gas. The result is a dryer and cooler planet, so I think that runaway freezing at the poles is quite possible. The present Martian atmosphere varies by about 25% in mass over the course of a year, so significant changes may be possible over a 100 year period.

Thanks for sharing the idea of acidic weathering of some of the spongy rocks!
I like the paper 'Concretions and nodules of North Dakota', you pointed to. Several features look rather similar to features near Yellowknife Bay. I had been looking for some paper of that kind, because it may explain the "bubbles" and more.
I can duplicate your calculations, under the given assumptions.

Nevertheless, several things are not quite conclusive to me. Still open is especially: How is the water forced to the mountain top, although there will be needed a hydrostatic pressure of more than 100 bar at the foot of the mountain in porous material? I'd expected a fountaine there, at the foot.

I think you've found the flaw in my scheme. I don't have the pressure to get water to the top of the peak. I can get steam out of the top, which could condense to water or ice when it hits the cold air. This could provide enough moisture to cement the Martian dust into a layer that won't blow away. Or I can entrain some droplets of moisture and salt particles into the gas flow if the velocity is high enough. There should be quite a lot of nitrogen and CO2 in addition to steam because dissolved gases will come out of solution when the pressure drops.

As a non-geologist I wonder if sedimentary rocks can tell MSL anything about their compression history?" Would they have a different signal depending on either a history of being overlain by a couple of kilometers of sediment for a billion years or so OR a more recent formation, and therefore a less deeply buried history. I understand that this entails the assumption of Gale crater being significantly buried.

Edit I did some research and discovered some better terminology, lithification & metamorphism, but can the extent of lithification or metamorphism, due to pressure, be indicated directly or indirectly by MSL. Some of these rocks look pretty weak!

Metamorphim, of course, by CheMin, because metamorphism changes crystal structure. That is well detectable by X-ray diffraction, I'm almost shure.
I cannot give a unique answer to the determination of the degree of lithification, because weathering might make things ambiguous, imho, probably the reason, why some rocks look weak.

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