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Temperature and pressure at Gale, Suitable (for short periods) for liquid water?
Eyesonmars
post Oct 1 2012, 08:00 PM
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An interesting experiment I've had students do is to use a syringe to boil water at room temperature and then refrigerate the syringe and redo the experiment.
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mshell
post Oct 1 2012, 09:04 PM
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I searched the web and couldn't find a phase diagram for water that showed the “area of interest” in much more detail than the one that Udo posted up-thread, so I created my own using Excel and the equations found here:

http://www.iapws.org/relguide/MeltSub2011.pdf (Eqns 1 and 6)

and here:

http://www.iapws.org/relguide/IF97-Rev.pdf (Eqns 29b and 30)

I have attached a couple of phase diagrams for water over the different ranges of temperatures and pressures that have been discussed.

Attached Image


Attached Image


I used the “max pressures” as estimated by Eyesonmars for Hellas and Gale at summer solstice. As djellison said, we are indeed “dancing around a tiny tiny wedge.” I have also attached the values in tabular format:

Attached Image


As Udo noted, if the pressure is below the triple-point of water (6.11657 mBar = 6.11657 hPa = 611.657 Pa), then it doesn't matter the temperature, there can be NO pure liquid water on the surface.

Of course, this all is based on pure water. Impurities (e.g., salts) change both the boiling point (usually elevated) and freezing point (usually depressed) at a given pressure. I couldn't find any easily accessible (and understandable to me) information on how the phase diagram changes with molality, particularly at these low pressures. Maybe someone else can find some Mars-specific info. At Earth pressures, very salty water (think Dead Sea) doesn't freeze until -20 C or lower (from the CRC Handbook of Chemistry and Physics for a 4.6 M solution of NaCl). The boiling point is less affected -- it elevates about +2 C for a 4 M solution.

So, as others have said, liquid water on Mars today seems POSSIBLE, but probably short-lived, if it exists at all.

I’m not going to worry too much more about the fate of glasses of pure water on Mars -- I’m going to put on my brand new, VERY cool, red-cyan clip-on, flip-up glasses and go see if I can talk anyone else in the family into imagining what the waterfalls and rushing waters of Gale Crater could have been like a couple of billion years ago.

Mark
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djellison
post Oct 1 2012, 09:09 PM
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Beautiful work mshell - that you very much ( and GREAT first post!! ) - that shows the tiny wedge so very well.

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nprev
post Oct 2 2012, 12:08 AM
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Indeed, I can only echo the Chairman of UMSF's words, Mark...a spectacularly informative & relevant post. Glad you're here! smile.gif


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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marsophile
post Oct 2 2012, 06:23 AM
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That is a fine statement of first-order behavior, but it does not necessarily rule out possible second-order effects provided by such things as surface tension or capillary action in smaller-scale contexts.
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Harder
post Oct 2 2012, 07:31 AM
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As mshell rightly mentions, the phase diagram is for pure water only. The implication is that in order to assess the equilibrium condition you have to use the partial pressure (for water) at Mars, rather than the absolute pressure. This is linear (every molecule has the same volume) so for agument's sake 1% H2O content in the Mars atmosphere means you have to read the phase diagram at 1% of the prevailing pressure to see what can exist in equilibrium. I think this definitely rules out liquid water.
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abalone
post Oct 2 2012, 07:46 AM
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What would also be interesting is the depth below ground where the temperature is constantly above Zero and what the pressure would be at this depth. Its possible that only 100s of metres below the surface any water could be permanently liquid. Does anyone have any data in this?
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vikingmars
post Oct 2 2012, 07:59 AM
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QUOTE (mshell @ Oct 1 2012, 11:04 PM) *
I used the “max pressures” as estimated by Eyesonmars for Hellas and Gale at summer solstice. As djellison said, we are indeed “dancing around a tiny tiny wedge.” I have also attached the values in tabular format:

GREAT post Mshell !
For your info, the max pressure measured by VL2 for its entire mission was 10.72 mb on Sol 277...
=> Could you, please, adjust your last (and nice) table ?
==> Besides, what would be the max pressure at Hellas at Winter solstice ?
(I guess your 14 mb figure is valid for the lowest part of Hellas at -8530m altitude)
Warm welcome and thanks again ! smile.gif smile.gif smile.gif
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Explorer1
post Oct 2 2012, 08:10 AM
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On a related note, did anything ever come of the 'brine droplets' on Phoenix's landing legs? I never heard anything about them after the mission ended; if they were salty water than that would be relevant to this discussion.
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serpens
post Oct 2 2012, 08:20 AM
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QUOTE (vikingmars @ Oct 2 2012, 08:59 AM) *
GREAT post Mshell !
For your info, the max pressure measured by VL2 for its entire mission was 10.72 mb on Sol 277...


Weren't these readings attributed to the diurnal heating and consequential expansion of gas in the Tavis pressure transducers used for the Vikings (and Pathfinder I think), which were assessed as jammed with dust during the landing process? Wasn't this why the Phoenix pressure sensors were not activated until the landing dust had settled?
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vikingmars
post Oct 2 2012, 09:31 AM
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QUOTE (serpens @ Oct 2 2012, 10:20 AM) *
"...which were assessed as jammed with dust during the landing process?


Well... I was not aware of those being "jammed" nor people doing both experiments for VLs and MPF.

The problem of dust pollution was well foreseen by scientists when designing their instruments before their integration within the landers.
The VL pressure sensors could not be "jammed" by dust, because they were protected from engine exhauts during the landing inside an housing located 1 meter above ground. they were released 5 mn after landing at the end of the meteo boom. And the 1st imaging sequence show that the dust took less than a minute to settle down.
The MPF pressure sensors (derived from the VLs) were protected from dust because they were packed inside the folded petals of the lander, itself protected inside the airbags during its landing on Mars...
So I think that both measurements are perfectly valid, like most scientists involved in both missions. Please, see link :
http://www-k12.atmos.washington.edu/k12/re...e_overview.html
This is why I think that this "10.72 mb" value is real.
Enjoy ! smile.gif
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marsophile
post Oct 2 2012, 03:10 PM
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QUOTE (Harder @ Oct 1 2012, 11:31 PM) *
... partial pressure (for water) at Mars, rather than the absolute pressure ...


The partial pressure determines whether evaporation occurs, i.e., vaporization of molecules from the surface of a liquid. The total pressure governs boiling, i.e., vaporization of molecules from the interior of a liquid.

Even on Earth, the partial pressure of water is below the triple point most of the time. Water is generally unstable on Earth's surface, which is why hanging wet clothes out to dry is a reasonable thing to do.

The issue is not whether water is stable (it is not), but whether it is replenished, and how long it can persist. The latter is affected, for example, by the area of the exposed surface, which is why water in a bottle with a narrow top evaporates more slowly than wet clothes.
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serpens
post Oct 2 2012, 04:31 PM
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QUOTE (vikingmars @ Oct 2 2012, 10:31 AM) *
This is why I think that this "10.72 mb" value is real.
Enjoy ! smile.gif


Thanks. Can't argue with that. On reflection I think my misconception came from a Mars Society presentation a few years ago. I should be more careful in filtering information sources.
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Tom Dahl
post Oct 2 2012, 04:50 PM
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QUOTE (vikingmars @ Oct 2 2012, 05:31 AM) *
The VL pressure sensors could not be "jammed" by dust, because they were protected from engine exhauts during the landing inside an housing located 1 meter above ground. they were released 5 mn after landing at the end of the meteo boom. And the 1st imaging sequence show that the dust took less than a minute to settle down.

Please forgive me, but I don't think the Viking lander pressure sensor was part of the Meteorology Sensor Assembly on the met boom. The MSA had detectors for temperature, wind speed, and wind direction only.

The lander pressure sensor was located inside the lander body mounted to an interior bracket near leg 2. That Tavis sensor was fed via a tube passing through the lander body to a Kiel Probe located a bit below the lower edge of the body sidebeam. The general arrangement of the sensor and probe are indicated in the following diagram at center-right:
Attached Image

Here is a photo of the Kiel Probe on the Proof Test Capsule in the Smithsonian NASM. (A few other photos of the probe can be seen via the next and prev PicasaWeb image widgets.) The Flight Capsule 3 (backup) body in the Seattle Museum of Flight has the tube but not the Kiel Probe itself.

The location of the Keil Probe opening below the lander (and the angle of the probe's cylindrical shroud) was deliberately chosen to enable measurement during the rocket-borne final phase of descent (after aeroshell jettison), as well as while on the surface. Whether the probe opening was susceptible to dust during touchdown may be debatable but it does seem like a possibility. Edited to add: nevertheless, I have no reason to doubt the results obtained.
-- Tom
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Eyesonmars
post Oct 2 2012, 06:09 PM
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QUOTE (vikingmars @ Oct 2 2012, 08:59 AM) *
GREAT post Mshell !
For your info, the max pressure measured by VL2 for its entire mission was 10.72 mb on Sol 277...

==> Besides, what would be the max pressure at Hellas at Winter solstice ?

As mshell stated, he took the value of 10.2mb from one my earlier posts. I believe i stated that this is representative of the maximum DAILY MEAN surface pressure at the Viking 2 site As the subsequent discussion has shown there is still some uncertainty in what the EXTREME max pressure might have been.
As Vikingmars points out this occurred around LS 280 at the VK2 site. ( I think it was LS and not SOL, as you stated. Correct me if I'm wrong. The max pressure occurred around SOL277 at Vk1 and LS277 at VK2 ) blink.gif

My purpose was to show how difficult it is for liquid H2O to exist ANYWHERE on Mars, let alone at the MSL site.
So using the only long term meteorological surface data we have I estimated what the pressures might have been at Gale and Hellas on that day. ( I even rounded off the scale ht to 11km)

I know this is getting off topic admins so if you think a new topic is warranted I understand..

But back to the Viking data. It is interesting that the max pressure (and wind speed) at the VK2 site occured as a global dust storm engulfed the lander. While this is very pronounced at the VK2 site it is a hardly noticible at the VK1 site.
In addition, the diurnal temperature fluctuations drop to almost nothing by Martian standards ( 10-15c).
Given that the surface can disappear as seen from orbit during a major storm I will go out on a limb and predict that Curiosity will no longer be able to see Mt. Sharp if we have a major/global storm - and for many months afterwords..
If so, our image experts Might have to make due for a while ( not to mention the loss of HiRise imaging ...)
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