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Basaltic Sediments, rethinking Mars - again?
Gerald
post Oct 24 2014, 11:41 PM
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QUOTE (ngunn @ Oct 24 2014, 11:17 PM) *
We know there's ice in contact with the rocks in many places. Are basalt minerals completely immune from hydration as long as the water remains frozen? A 'Yes' would help here.

Let me pick out this question. The answer is an "almost".
As a general background chemical reactions tend to slow down by a factor of 2 every 10 K as a rule of thumb.
Another factor of the reaction rate is the concentration, or the partial pressure, in this case of water vapor.
According to this NOAA calculator, vapor pressure is 9.92 hPa at 280 K, but only 0.04 hPa at 220 K. Hence the number of available molecules for chemical reactions decreases rapidly with temperature. Water vapor is of course already much less reactive than liquid water at the same temperature.

A 1990 LPSC paper about basalt weathering in Antarctica and on Mars even neglects possible weathering of basalt by vapor, and only considers temporary wetting.

Ice will lack direct contact with basalt, after a thin weathering crust has formed, as long as there doesn't occur additional physical weathering exposing unweathered basalt.
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dburt
post Oct 25 2014, 12:11 AM
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QUOTE (djellison @ Oct 24 2014, 04:26 PM) *
For those who want to read tens of thousands of words by Don on that issue - read this thread http://www.unmannedspaceflight.com/index.php?showtopic=4308

Thanks for providing the link. Warning! If that thread seems too long to read that's because it really got too far long to read, so evidently nobody read it and I had to answer the same questions again and again. The moderators and I have NO desire to repeat that experience. For information and updates I therefore recommend instead searching and linking on our relevant publications, especially LPSC abstracts and other abstracts, although they tend to be somewhat abbreviated and technical, or contact me directly.

Don Burt
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serpens
post Oct 25 2014, 10:16 AM
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In all this discussion we need to differentiate between hypothesis and empirical evidence. For example, despite the faint young sun hypothesis we know there was long lasting liquid water on both Earth and Mars. Empirical measurement trumps hypothesis every time. Reaction between acidic water and mafic rocks commonly yields alkaline ground water and significant quantities of hydrogen so the atmosphere of early mars could well have had a water CO2 sink, a high H2 component and a carbon dioxide/sulphur dioxide/hydrogen sulphide component from volcanic release. This mix would fit empirical measurements at Meridiani including deposition of hematite concretions in a mixing interface in a basalt buffered environment. A high H2 content atmosphere would fit water/atmosphere loss and correlate to the measured deuterium/hydrogen ratio although correlation does not necessarily imply causation. Another point is that while basalt weathering at the earths surface in a wet, oxygen rich environment is indeed swift by geological yardsticks, on the ocean floor it is much, much slower. Even in a highly oxygenated environment like Papakolea beach, olivine resists change. The polar cap water reserve is no indication of the amount of water available to early Mars, nor can we use a Terran based yardstick to measure Martian environmental history.
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SFJCody
post Oct 25 2014, 10:27 AM
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Some exciting ideas so far! Sorry for the ridiculous CO2 suggestion earlier. Rolled my eyes at my own stupidity when I was reminded of the pressure necessary for a stable body of liquid.
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Julius
post Oct 25 2014, 12:00 PM
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How long in terms of time scale are we talking here when you refer to chemical weathering of deep ocean rocks takes longer than weathering of surface rocks in an oxygen rich earth atmosphere? Still my impression remains that water contact with basaltic rocks has been brief in most instances probably due to water instability on Mars surface.
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dvandorn
post Oct 25 2014, 12:57 PM
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QUOTE (serpens @ Oct 25 2014, 05:16 AM) *
...empirical measurement trumps hypothesis every time.


Thank you, serpens. I've been saying this for many years, just in regards the photogeology of the Martian surface vs. what the geologists focusing solely on surface-level measurements keep trying to say.

In essence, even before the chemical cues you mention were confirmed, mapping as early as Mariner 9 clearly showed vast catastrophic flow plains, well-developed river and delta patterns -- many, many macroscopic evidences of liquid water carving the surface at some time in the (as now is known, quite distant) past. And once these extremely convincing pieces of evidence for liquid water became known, it seems like a branch of planetary geology became fixated on proving that these aqueous erosional features just had to have an explanation that didn't require large quantities of liquid water, solely on the basis that since liquid water cannot exist on the surface now, we must assume that it could never have existed there.

But, over and over, we see that in both additional orbital imagery and data from surface assets, the empirical evidence points to the presence of abundant liquid water in ancient times. And the theorists step back, very reluctantly and with great inertia holding them back, with new theories that, well, maybe it was some other fluid carving channels... or maybe there was a little tiny bit of water that only existed on the surface for hours, not thousands or millions of years.... or maybe, just maybe, if you push us really hard, we'll admit that in a very few places some water persisted for longer than a few days... And yet, the empirical evidence keeps pointing to relatively long-lived streams, rivers, pools and lakes. Long enough lived to have created rounded and sorted populations of pebbles, long enough to create conglomerate rocks with clay-like matrices... et cetera, et cetera.

Maybe it is the theorists' jobs to creep very slowly back from their best-understood hypotheses as new empirical data comes in to to challenge them. But at some point, you just have to admit defeat and start to look for a new hypothesis that actually fit the empirical data. Heck, even Harold Urey was able to do so (if not with terribly good graces) when the first lunar samples were returned by Apollo. Urey, a cold-Mooner, had predicted that the Moon and its rocks would be primitive and chondritic, without any volcanic transformation. But when the well-described rocks were initially analyzed, one of Urey's staffers came in to tell him the results, and Urey, in a very irritated voice, cut him off and said "Don't tell me, I know, they're basalts, right?" The staffer confirmed that, and Urey just grunted, seeing his entire set of theories of lunar origin and composition fly completely out the window with the first truly empirical results.

I think we still need that moment to pass through the planetary science community in re Mars, and I'll be interested to see what finally tips them over the edge...

-the other Doug (With my shield, not yet upon it)


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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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serpens
post Oct 25 2014, 01:30 PM
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Subduction zones have a way of terminating the ocean floor weathering process however an example of timescale would be basaltic clasts with weathering rinds recovered from the Iberia Abyssal Plain in early Cretaceous sediments (say 135 Ma ago) and reported in the Proceedings of the Ocean Drilling Program. Interestingly enough given Emily's comment re potassium enrichment, these basalt clasts were enriched in K2O, attributed to low temperature alteration.
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Gerald
post Oct 25 2014, 04:32 PM
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QUOTE (ngunn @ Oct 24 2014, 11:17 PM) *
Martian water verus terrestrial water: Could the former be less efficient than the latter at hydrating basalt minerals? Absence of some crucial ingredient peculiar to Earth??

Three possible approaches:
1. Lower atmospheric pressure on Mars: Due to lower gravity the weight of the atmosphere is lower, even with the same mass per area. This tends to reduce the overall temperature for water being liquid, hence generally slows down chemical reactions with water relative to Earth, since water vapor and ice are less efficient in weathering than liquid water.
2. Less energetic transport processes: Lower gravity on Mars reduces the overall abrasion between grains, since mechanical processes are less energetic on Mars for otherwise the same conditions as on Earth. Abrasion and transport is necessary for fast weathering, otherwise we get a chemical equilibrium preventing from further chemical weathering.
3. Atmospheric hydrogen escape: Olivine is alkaline (e.g. increases pH of water during serpentinization by releasing Mg(OH)2 and Ca(OH)2) and needs acids to weather. Water and additional CO2 or SO2 can act as acid; pure water is amphoteric, hence can act as an acid, too. An acid is by one of its definitions a proton-donor. Due to the lower gravity and the weaker magnetic field on Mars, hydrogen escapes faster from Mars than from Earth, resulting in less hydrogen abundancy. Protons are hydrogen nuclei; less hydrogen should lessen the proton supply, hence result in higher pH. Higher pH means less acids to weather olivine. Olivine is hard and prevents basalt from abrasion.
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serpens
post Oct 25 2014, 08:57 PM
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QUOTE (Gerald @ Oct 25 2014, 04:32 PM) *
2. Less energetic transport processes: Lower gravity on Mars reduces the overall abrasion between grains, since mechanical processes are less energetic on Mars for otherwise the same conditions as on Earth.


To the contrary. While Mars today is an extremely benign environment, before the atmosphere thinned and when the sedimentary features that Curiosity is investigating were formed it would have been an very energetic environment. Low gravity plus thick atmosphere would have resulted in a pretty impressive sand blasting effect with an energy contribution from the elliptical orbit and axial tilt variations.
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Gerald
post Oct 25 2014, 11:14 PM
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Changing too many parameters at the same time makes things difficult to understand.
So here two examples with just the gravity as variable:
- A river, the same in all properties besides gravity, is flowing slower in lower gravity (Mars) than in higher gravity (Earth), since the viscosity of the liquid - hence the friction within the liquid and between river bed and liquid - remains the same, but the force downhill is reduced in the lower-gravity environment due to the reduced weight of the same mass. Therefore the kinetic energy released on grain collisions is lower.
- Lifting a grain to a given height difference is propotional to the gravity. The release of energy after free falling from the same height is lower in lower gravity, hence less abrasive.

That way the same energy can result in more impressive structures in a lower-gravity environment.

Ancient Martian weather systems are rather hard to estimate, since we don't know pretty much about composition and density of the atmosphere. Even then, weather models are inherently complicated.
Different climatic and atmospheric conditions may have resulted in very different weathering products. The question has been about water-rich conditions favorable to leave basalt chemically less altered. High-energy aeolian events in the context of basalt weathering become only relevant in the presence of water.
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dburt
post Oct 26 2014, 12:00 AM
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QUOTE (serpens @ Oct 25 2014, 03:16 AM) *
In all this discussion we need to differentiate between hypothesis and empirical evidence... Empirical measurement trumps hypothesis every time...

Thanks. I might phrase this that in science we need to differentiate between actual observations and various interpretations (or testable hypotheses) based on them. Measurement cannot "trump" hypotheses because scientific hypotheses are based on those very measurements. You seem to be confused about the essential relation between the two. The first yields the second, so the second cannot be "trumped" by the first.

Some people start to do science with simple models (e.g., "warm, wet Mars" or "cold, dry Mars"), and then insist that all of the observations must agree with their models, but such simple models should not be confused with testable, competing scientific hypotheses based on actual measurements or observations.

Sorry for my delay in responding - I was leading a field trip to an alluvial fan complex earlier today.

Don Burt
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jmknapp
post Oct 26 2014, 01:34 AM
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In terms of fixed, time-honored ideas about Mars, hasn't the notion of early and widespread water and habitability been the prevalent one (notably Lowell) and only very grudgingly amended as ground truth comes in (Mariner, etc.)?


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MrNatural
post Oct 26 2014, 02:43 AM
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Speaking of empirical evidence, how does MSL's DAN's findings work with these theories? As we know, DAN is detecting water (in hydrated minerals?) in the subsurface, with a patchy distribution, but I am not a geologist/geochemist and am not qualified to interpret these readings. Can anyone here speak to these findings?

http://www.spaceflight101.com/dan-science-reports.html
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Don1
post Oct 26 2014, 06:08 AM
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I think the most likely scenario is not that Mars was dry, but that the water chemistry was not suitable for weathering basalt. Geological waters are never really pure, but contain a variety of dissolved gases and salts which change the chemistry quite a lot. Articles on basalt state that it weathers by oxidation, but that won't happen in an oxygen free environment.

Some articles on early atmospheres speculate on the presence of ammonia, NH3. Ammonia is extremely soluble in water, and tends to produce alkaline solutions, which may be less likely to weather basalt. It also depresses the freezing point. 25% ammonia in water freezes at -58C.
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Gerald
post Oct 26 2014, 12:23 PM
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QUOTE (MrNatural @ Oct 26 2014, 04:43 AM) *
Speaking of empirical evidence, how does MSL's DAN's findings work with these theories? As we know, DAN is detecting water (in hydrated minerals?) in the subsurface, with a patchy distribution, but I am not a geologist/geochemist and am not qualified to interpret these readings. Can anyone here speak to these findings?

http://www.spaceflight101.com/dan-science-reports.html

Clay minerals at Yellowknife Bay have been confirmed by CheMin:
QUOTE
The abundance of clay minerals was estimated separately, using the program FULLPAT, at ~22 +/- 11 weight% of the total sample mass.

They can embed water into their crystal structure, and they usually form from basalt in the presence of more or less neutral water (hydrolysis).
Bassanite has also been found. It's one of the most abundant minerals in the veins.
More discussion in e.g. this paper.

But that has been a different unit. The unsolved puzzle is mainly about the dark-toned capping unit.
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