Geomorphology of Gale Crater, Rock on! Options

[ngunn]

I'd like a discussion thread about the geology detatched from the time limits of current MSL threads. We had a 'Geomorphology of Cape York' thread that attracted a lot of interesting posts. How about 'Geomorphology of Gale Crater'? I have one or two ideas but many more questions, and I'd like to post them in a longer-running thread away from the day to day imaging discussion. Any other takers?

For starters, does anybody have a contour map of this place like the one at Meridiani with 5m intervals?


ADMIN: You have your wishes fulfilled on UMSF (sometimes)

[Don1]

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.

[Gerald]

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?

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.

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

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

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.

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?

When the atmosphere was rich in sulfur dioxide, sulphates were formed. More recently, another mineral, maybe carbonates was deposited.

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.

[Don1]

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.

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

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.