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Temperature and pressure at Gale, Suitable (for short periods) for liquid water?
HSchirmer
post May 12 2017, 09:57 PM
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Agreed,

QUOTE (atomoid @ May 11 2017, 10:03 PM) *
at near zero humidity and pressure over geologic timescales


Well, not entirely zero-humidity.
Low absolute, but IIRC the rover has seen 75% or higher relative humidity.

    https://www.hou.usra.edu/meetings/lpsc2017/pdf/2972.pdf
    ATMOSPHERE-REGOLITH INTERACTIONS THROUGH DELIQUESCENCE AS SUGGESTED BY
    THE PHOENIX LANDER AND THE MARS SCIENCE LABORATORY

    Lunar and Planetary Science XLVIII (2017)
    Conclusions:
    Our results suggest that water vapor can be stored within the regolith via deliquescence of calcium perchlorate.
    In the top, few meters of soil, the stored water vapor would be released by sublimation after the solution transitions to the ice phase. Deeper in the regolith, though, the attenuation of the diurnal temperature permits solution activity for longer periods until the ERH is met. At these depths, water vapor would instantly be release d, and would diffuse through ~15cm of regolith.
    ...
    Therefore, this process may play an important role in the near-surface
    water cycle on present-day Mars.
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serpens
post May 14 2017, 12:12 AM
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[quote name='HSchirmer' date='May 12 2017, 09:57 PM' post='235777']
Agreed, Well, not entirely zero-humidity. Low absolute, but IIRC the rover has seen 75% or higher relative humidity.......
.....Our results suggest that water vapor can be stored within the regolith via deliquescence of calcium perchlorate.....
....Therefore, this process may play an important role in the near-surface water cycle on present-day Mars.
quote]

With atmospheric water content varying between some 10 to 60 ppm at Gale Cater it is difficult to see any significant effect in this locality.
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HSchirmer
post May 15 2017, 02:41 AM
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QUOTE (serpens)
QUOTE (HSchirmer)

Agreed, Well, not entirely zero-humidity. Low absolute, but IIRC the rover has seen 75% or higher relative humidity.......
.....Our results suggest that water vapor can be stored within the regolith via deliquescence of calcium perchlorate.....
....Therefore, this process may play an important role in the near-surface water cycle on present-day Mars.


With atmospheric water content varying between some 10 to 60 ppm at Gale Cater it is difficult to see any significant effect in this locality.


QUOTE
https://www.hou.usra.edu/meetings/lpsc2017/pdf/2972.pdf
Mars Science Laboratory.
...
Our results suggest that water vapor can be stored within the regolith via deliquescence of calcium perchlorate.
In the top, few meters of soil, the stored water vapor would be released by sublimation after the solution transitions
to the ice phase.
However, in the deeper subsurface, the attenuated temperature and resulting relative humidity,
avoid the transition to the ice phase. In this scenario, a solution is present for several hours...


Eh, liquid water on mars, for several hours each day.
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Gerald
post May 15 2017, 04:32 PM
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QUOTE (HSchirmer @ May 15 2017, 03:41 AM) *
...Eh, liquid water on mars, for several hours each day.

We discussed these possibilities a few years ago in this thread about temperature and pressure in Gale, see this post, or the posts starting here.
I'd suggest to continue the discussion about the possibility of liquid water in Gale, including brines, with new results getting available, in the dedicated thread.
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serpens
post May 16 2017, 02:37 AM
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Indeed. Perhaps one of our overworked moderators could transfer the relevant posts?
There is an hypothesis that brines could form through deliquescence. However there are a number of empirical findings that mitigate against this. First is the presence of kieserite which indicates absolute desiccation. Secondly the Thermal and Electrical Conductivity Probe (TECP) deployed by the Phoenix lander did not detect any indication of water vapour or liquid in the regolith despite a much more likely environment than Gale. Thirdly no indications of frost have been identified by Curiosity although to be fair any such would be limited to a micron or so. Finally it is absolute humidity that is the important measure for atmospheric water. The actual amount of water available rather than relative humidity which is simply the water vapour measured as a percentage of what the atmosphere could hold at a specific temperature. We need an adjective more intense than nanoscopic to describe 10 to 60 ppm.
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HSchirmer
post May 17 2017, 02:05 AM
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QUOTE (serpens @ May 16 2017, 02:37 AM) *
First is the presence of kieserite which indicates absolute desiccation.


Ok, interesting point, but,
isn't the presence of MgSO4*H2O, i.e. a hydrated magnesium sulfate, evidence of some water?
Wouldn't MgSO4 without any H20 at all be absolute dessication?
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serpens
post May 17 2017, 04:38 AM
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On Mars, kieserite is the end state dehydration product for the MgSO4 hydrates. Water is tightly bound to the crystal structure in monohydrates and it would take temperatures over 300 C to force the water out of kieserite. However kieserite is unstable with respect to hydration and is very sensitive to humidity. Unless there is something we don't understand about the stability field of kieserite in the Martian environment then the presence of kieserite implies absolute dessication of the environment (not of the kieserite).
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Gerald
post May 17 2017, 12:54 PM
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When comparing the stability diagram of Mg(ClO4)2 brine in figure 2 of this paper with the stability diagram of kieserite in figure 3 of this paper, I presume the possibiliy of an Mg(ClO4)2 brine in the presence of kieserite. It would form above -50C at a relative humidity near 50%, and stay metastable down to a relative humidity of 20%. For temperature above -50C, kieserite is stable according to the first paper. However, according to other papers, e.g. this one, kieserite may transform less easily into hexahydrate and epsomite, such that we may lower the temperature further, where a Mg(ClO4)2 brine can form easier due to the higher relative humidity at low temperatures, but only weak dependency of the lower bound of relative humidity for brine formation on temperature.
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HSchirmer
post May 17 2017, 08:27 PM
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QUOTE (serpens @ May 17 2017, 05:38 AM) *
However kieserite is unstable with respect to hydration and is very sensitive to humidity.
Unless there is something we don't understand about the stability field of kieserite in the Martian environment
then the presence of kieserite implies absolute dessication of the environment (not of the kieserite).


QUOTE
http://www.indiana.edu/~geosci/bish/Vanima...20on%20Mars.pdf
Under colder conditions on Mars (220 K equatorial average) and at typical Mars surface pressure
(5 torr) and diurnal RH ranging from , 1% to 100% (summer equatorial average 50% RH),
...
experiments in the environmental cell show that [kieserite] is easily hydrated on exposure to elevated humidity
(55% RH) where it converts to hexahydrite and then epsomite.


Eh, perhaps this area hasn't see 55% RH recently.
Or, perhaps re-hydration doesn't happen, because it is thermodynamically favorable, but kinetically slow.

QUOTE (http://www.planetary.brown.edu/pdfs/3913.pdf)
Vaniman et al. [2006] have shown that under Martian temperature conditions (constant 243 K trials)
but ~1 bar pressure, kieserite hydrates to hexahydrite in ~1000 h at 100% RH.


So, kiersite proves that this site hasn't seen a biblical flood, underwater for 40 days and 40 nights (~1000 hours), anytime recently.

QUOTE
At 100% RH in lower temperatures (193 K), the water vapor does not hydrate kieserite,
but accumulates as surface frost on the mineral [Vaniman et al., 2006].


So, adding water to kiersite for a few hours per day might not result in hydrates, only a wet rock.
The twice daily temperature swings, ice-liquid-vapor, might effectively limit the time that liquid water is chemically available,
which would effectively prevent rehydration.

PS

http://meteorites.wustl.edu/abstracts/lpsc37/a_l06w01.pdf

In a separate study of phase transformations among the hydrated Mg sulfates [7],
we found no evidence of any rehydration of the kieserite (MgSO4H2O) at 30% RH
and 50C within the first 24 hours.
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ngunn
post May 17 2017, 11:39 PM
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I just want to say thank you to all involved for having this informative discussion here. It's exactly the sort of thing (besides the excellent image work) that I come here for.
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serpens
post May 18 2017, 12:58 AM
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Hydration of kieserite under Martian conditions would indeed be kinetically slow. Sluggish is the term used in some experimental results. However we are considering extraordinarily long timescales. If the kieserite was formed as a primary evaporate then the current slow erosion rate would have resulted in long exposure times at or near the surface. 75% humidity has been recorded at Gale and the thing is that under current Martian conditions, over Martian surface timescales kieserite could rehydrate, possibly to an amorphous state provided that there was a sufficiently high absolute humidity. The most likely end product would be starkeyite which is stable in this environment. But 7 to 9.6 mbar and 100% RH with 10 to 60 ppm water hardly meets the definition of a biblical flood and if all water in the atmospheric column precipitated there would be just a few microns on the surface.
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HSchirmer
post May 18 2017, 01:49 AM
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QUOTE
Hydration of kieserite under Martian conditions would indeed be kinetically slow.
...
However we are considering extraordinarily long timescales.
...
If the kieserite was formed as a primary evaporate then the current slow erosion rate
would have resulted in long exposure times at or near the surface.


That part is where it gets interesting.
Exposure at or near the surface could be what keeps it dry.

I think of crossing Mar\'s dunes as similar to humans walking across hot sand at the beach.
Think of the microclimate of desert sands, superheated air and shimmering mirages.

With a thin atmosphere, convection will be much less efficient at cooling.
With fluffy aeolian dust, conduction will be much less efficient.
Of course any surface soaking up the sun all day is going to be (relatively) \"hot\" and \"bone dry\".

So, yes, there are salts with long exposure times, literally geologic timescales...

QUOTE (Experimental stability of magnesium sulfate hydrates that may be present on Mars)
http://www.sciencedirect.com/science/artic...016703706020965
Several of the hydrates also show significant metastable extensions, such that phase boundaries can only be approximated. For example, kieserite, which has been reported on Mars from OMEGA data, in addition to having a distinct stability region,
is resistant to transformation and persists throughout temperature-RH space until very high relative humidities are achieved.


But the daily variations in surface temperature and RH seem to ensures that the visible surface
constantly over or undershoots the combination of temperature and RH needed for hydration reactions at the visible surface.

You\'d probably get water adsorbed onto the salt, but it wouldn\'t be there long enough to incorporate.
There\'s plenty of time, but not enough time where the right combinations for weathering occur simultaneously.

EDIT-
Ah, that may be a threshold issue-
MgSO4-H20 +6H2O => MgSO4-7H2O is the thermodynamically favored reaction. (at least at Earth pressure and temp).
So, it requires about 70% by weight of water to form the next stable hydrate.
If you don\'t reach 70% weight by water, then hydration may be reversible.

QUOTE (Experimental studies of the mechanism and kinetics of hydration reactions- Energy Procedia 48 ( 2014 ) 394 – 404)
http://www.sciencedirect.com/science/artic...003087-main.pdf

though not yet fully understood, it appears plausible that diffusive water transport across
a barrier product layer formed at the reaction interface may often be the rate limiting step.

The water uptake curves show that below the DRH of MgSO4∙H2O (red curves) a partial hydration and formation
of MgSO4∙6H2O takes place. The thermodynamically stable hydration product under these conditions is MgSO4∙7H2O.
Full hydration to MgSO4∙6H2O was not achieved within 60 hours (only the first 25 hours are shown in Fig. 2).
Above the DRH of MgSO4∙H2O the hydration rate is significantly increased and the formation of MgSO4∙6H2O is complete within 25 hours (blue curves).


So, kieserite may adsorb water molecules into a non-reactive film, which blocks anything from happening until there is an actual liquid phase. Think of the protective patina that forms on copper; or even better, aluminum, which is, paradoxically so-reactive with oxygen that it instantly forms a non-reactive patina... But the fact is that pure aluminum reacts so readily with water that, according to the laws of chemistry, the aluminum shell of an airplane should actually dissolve in the rain.

However, dig down into a dune, and should be cooler, and more humid, and much more interesting...
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serpens
post May 18 2017, 04:06 AM
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Just as an aside, you may like to consider the interactions between the hypothesised brine, kieserite and other hydrated sulphates.
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Gerald
post May 18 2017, 09:16 AM
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Pure H2SO4 is so hygroscopic, that it pulls the water out of dry sugar, leaving carbon.
Although perchlorates arent't quite as hygroscopic, brines near the eutectic point may still bind water molecules strong enough, that they aren't available for reaction with kieserite. I didn't find information whether in such a settings, a sulfate-perchlorate double salt would form, or whether double salts like Mg(SO4)(ClO4)nH2O even exist.

We can find out, whether films of brines are possible in Gale. But how can we learn, whether they are actually there?
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serpens
post May 19 2017, 12:27 AM
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Analysis of Phoenix WLC results led to the conclusion that there was between 0.4 to 0.6 % by weight of perchlorate in the regolith with SAM indicating a similar outcome for Rocknest. With only 10 to 60 ppm atmospheric water content at Gale and low RH I suspect that any brine that did form would consist of really minuscule, isolated droplets rather than extended films or pools of liquid.

While the detection of ephemeral hydrated salts in some recent RSL has been nominated as proof of brines by some, I question whether the phase change observed is not a function of the exposure of previously protected hydrated salts to the atmosphere as a result of a dry slip. Such an effect was recorded by Spirit at Tyrone. Regardless I suspect that there will need to be boots on the ground before the question of brines can be resolved.
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