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Seryddwr
Just a quick query from someone with no background in science. Obviously, MSL has AFAIK not returned evidence of recent (i.e. years/decades) liquid water in its vicinity; however, I was interested by the following graphs:

08.21.2012: First Pressure Readings on Mars

http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4501

08.21.2012: Taking Mars' Temperature

http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4502

The first indicates that the pressure between 15 Aug and 18 Aug never dropped below c. 690 millibars; the second shows that, for a period of a couple of hours on 16 Aug, the temperature rose above freezing. If water had been present on the surface, then, would it have been liquid during this brief period? The pressure and temperature seemed to satisfy the conditions for liquid water as I understand them (indeed, the pressure seems to be high enough (just) on a 24-hour basis to allow for the presence of liquid water). Thanks in advance for your opinions (corroborative or not!) on this.
ngunn
690 Pa = 6.9 mbar
Seryddwr
6.9 - quite! 690 millibars would have been quite a discovery! ohmy.gif
nprev
I doubt that the pressure on the surface ever exceeds 10 mb, and that would be at the bottom of Valles Marineris and some portions of the Hellas basin.

6.9 mb for Gale is probably about as good as it gets, plus or minus a few tenths...therefore, no possibility of sustaining liquid water.
Eyesonmars
QUOTE (Seryddwr @ Sep 30 2012, 03:23 PM) *
If water had been present on the surface, then, would it have been liquid during this brief period? The pressure and temperature seemed to satisfy the conditions for liquid water as I understand them (indeed, the pressure seems to be high enough (just) on a 24-hour basis to allow for the presence of liquid water). Thanks in advance for your opinions (corroborative or not!) on this.

In theory the answer is yes. But the thinking is that the strong evaporative cooling at what is essentially a 0% relative humidity associated with these brief high temperatures would make a liquid state difficult to maintain or achieve. However, imagine placing a pan of water at ground level in sun versus shade (insulated from the ground) at the MSL site. The pan in full noonday sun would still evaporate but solar heating might offset evaporative cooling enough to keep it liquid while it evaporated. Not so in the shade. Note that our pan of water is assumed to have an initial temperature just above freezing because the boiling point on mars even at the bottom of Hellas will never be more than about 5-10c above freezing. This last fact makes it even more difficult to keep liquid water from icing over
djellison
QUOTE (Eyesonmars @ Sep 30 2012, 09:03 AM) *
the boiling point on mars even at the bottom of Hellas will never be more than about 5c above freezing


This is the kicker. Are there conditions where liquid water could exist on the surface of Mars today. Yes. Two problems, where does it come from, and it would so very quickly evaporate away that its existence could only ever be transient.
Eyesonmars
QUOTE (nprev @ Sep 30 2012, 03:51 PM) *
I doubt that the pressure on the surface ever exceeds 10 mb, and that would be at the bottom of Valles Marineris and some portions of the Hellas basin.

6.9 mb for Gale is probably about as good as it gets, plus or minus a few tenths...therefore, no possibility of sustaining liquid water.

The maximum surface pressure on mars occurs just after southern summer solstice. This is still many months away. At the Viking 2 site the average pressure at this time was near 10.2mb. The MSL site is another 2km or so LOWER than this. Using a scale height of 11km for mars suggest we might see pressures on the order of 11+ mb at this time compared to our current 7mb. Plugging in the numbers for Hellas (-8km) you can see that pressures can be as high as ~14mb.
For those interested in a good introduction to the Martian atmosphere (with Viking data ) ......
"The Surface of Mars" by Michael J. Carr (1980) Yale University Press
Still one of the best in my opinion
Eyesonmars
QUOTE (djellison @ Sep 30 2012, 04:31 PM) *
.... and it would so very quickly evaporate away that its existence could only ever be transient.

An interesting ( at least for me) thought experiment is what would happen to a TALL glass of cold water at the bottom of Hellas under the mid day sun at southern summer solstice. Surprisingly the water would be stable for some time before evaporating ( or icing over). The trick is the small surface to volume ratio.
djellison
It would boil, would it not?
udolein
Liquid water is almost impossible under the current conditions:

Click to view attachment

The enclosed phase diagrams of water and carbon dioxid state clearly that the triple point of H2O is around 6 mbar and 0 deg Celsius, while for CO2 it is 5.1 bar (not mbar !) and -56.5 deg Celsius. This means: below 0 deg Celsius water is solid (ice) at 6 mbar and CO2 is a gas. At 7 deg Celsius as on sol 52 and 6.9 mbar air pressure water theoretically could be a liquid but it is most probably a gas. And CO2 is a gas always.

Cheers, Udo





udolein
BTW: This site has the current weather readings: marsweather.com

Udo
djellison
Quite- we''re dancing around a tiny tiny wedge at the low pressure end of the liquid part of the H2O phase diagram. Even with dramatic salt content, that end of the diagram doesn't change much.

Eyesonmars
QUOTE (djellison @ Sep 30 2012, 09:47 PM) *
It would boil, would it not?

As shown in one of my previous post, the surface pressure may be as high as 14mb. At that latitude (40 south) on mars at that season mid day near surface air temperatures are well above freezing. So temperature and pressure are high enough. Rapid evaporative cooling could ice over our glass of ice water in the extreme dryness, even with air temps above freezing. But solar radiation might offset the evaporative cooling. If this happens, our glass of ice water might slowly warm a few degrees before it completely evaporates but because the boiling point might be as high as 10c at 14mb the water would evaporate before it ever warmed to the boiling point.
Like I said, it is a fun little thought experiment. Small changes in parameters drastically change the outcome. For instance, if the glass was sitting on black sand dunes (albedo .05) the infrared warmth ( dune temp could easily be 35-40c) might warm the ice water quickly enough for it to begin boiling before it completely evaporates
serpens
Yep to djellison. And addressing reality rather than glasses of water on Mars, given the miniscule absolute humidity any overnight frost layer would be measured in microns. It would possibly sublimate while the temperature is below the freezing point and before it had a chance to change state to liquid.
udolein
QUOTE (Eyesonmars @ Oct 1 2012, 01:37 AM) *
For instance, if the glass was sitting on black sand dunes (albedo .05) the infrared warmth ( dune temp could easily be 35-40c) might warm the ice water quickly enough for it to begin boiling before it completely evaporates

The phase transition under the conditions mentioned would happen directly from ice to gas. It evaporates. There would be no boiling at all. At 40c a liquid phase is impossible at 6-10 mbar air pressure.
In my above phase diagram the phase transition would happen from point D to F. No liquid phase at all.

Udo
udolein
CO2 won't be a liquid at normal conditions due to the 5.1 bar triple point. This is the reason why dry ice sublimates directly to gaseous CO2. There would be no liquid phase in between. The phase transition would happen from D to F as well.

Udo
Eyesonmars
QUOTE (udolein @ Sep 30 2012, 11:19 PM) *
BTW: This site has the current weather readings: marsweather.com

Udo

They also have ground temperatures, true ?
It would be nice to see ground temperatures also.
One reason, for instance, is that the temperature of the bottom, sides and other parts of the rover facing the surface and not in direct sunlight is largely in radiative equilibrium with the ground. Heat conduction via the atmosphere is negligible regardless of air temp.
Eyesonmars
QUOTE (udolein @ Oct 1 2012, 12:44 AM) *
The phase transition under the conditions mentioned would happen directly from ice to gas. It evaporates. There would be no boiling at all. At 40c a liquid phase is impossible at 6-10 mbar air pressure.
In my above phase diagram the phase transition would happen from point D to F. No liquid phase at all.

Udo

Note that I am talking about magically placing a glass of ice water ( temp 1c or so) on the surface. Please see earlier post.
As I state, the 40c temp I mentioned is the temp of the sand, not the water. Radiative heating from the surrounding sand would probably be more important than conduction thru the bottom of our glass.

And I'm not clear on why you are referring to the phase diagram of CO2 in some of your post
Eyesonmars
QUOTE (djellison @ Sep 30 2012, 11:25 PM) *
Quite- we''re dancing around a tiny tiny wedge at the low pressure end of the liquid part of the H2O phase diagram. Even with dramatic salt content, that end of the diagram doesn't change much.

Are you looking at a phase diagram designed for engineers ( most of them are as is the one udolien is using). If so the area of interest to atmospheric scientist will always be a "tiny tiny wedge". On one designed for Martian use the "tiny wedge" will occupy a whole page.
nprev
Um. Let's put it this way, Eyes: We ain't gonna see any liquid water at Gale. We're a few billion years too late.

Let's deal with what is real.
djellison
QUOTE (Eyesonmars @ Sep 30 2012, 05:36 PM) *
On one designed for Martian use the "tiny wedge" will occupy a whole page.


Yes it would - and on that page you would see that at these very low pressures, the temperature difference between melting and boiling is very small.
Juramike
QUOTE (Eyesonmars @ Sep 30 2012, 07:33 PM) *
Note that I am talking about magically placing a glass of ice water ( temp 1c or so) on the surface.


Or take a glass of ice water and put it in a vacuum chamber and suddenly expose it to vacuum. [Done the equivalent lotsa times in a rotary evaporator].

Doug's right, it would boil. Bubbles of water vapor would form at imperfections in the glass of the surface and it would bloop up out of the glass and go everywhere. A combination of sudden cooling due to more evaporative surface area would make all the flung water droplets quickly turn to ice, which would then sublimate away.

[When this happens in the lab, it is due to the receiver trap bumping, and there is usually a "Dangit!" if any of the water drops land back in the previously dried compound].

(Water-->bloop--->freeze--------------------->sublimate)

The water-bloop-droplet-freeze sequence would happen on the order of a few seconds. If the glass has a rougher surface, it would foam and froth smoothly due to all the nucleation sites. If it was a really smooth surface, it would sit quietly for a second and then just release it in a massive spasm of boiling due to the fewer nucleation sites.
ElkGroveDan
QUOTE (Eyesonmars @ Sep 30 2012, 06:36 PM) *
Are you looking at a phase diagram designed for engineers ( most of them are as is the one udolien is using). If so the area of interest to atmospheric scientist will always be a "tiny tiny wedge". On one designed for Martian use the "tiny wedge" will occupy a whole page.

I can make one the size of a highway billboard and it won't change the state of matter anywhere in the universe. As Doug has repeated several times you are talking about a miniscule portion of the phase diagram in every practical sense. You might as well obsess over how many angels could break-dance in that portion of the diagram without bumping into each other.
Eyesonmars
Hey guys come on
Before you all pile on Please read my posts from the beginning.
I've never suggested that liquid water would be stable on mars in any realistic scenario.
I am addressing the question posed by the author of this thread.
In it I clearly state why liquid water is all but impossible on mars.
Then I offer some physical insight into my statement by offering a thought experiment.

I know it is so easy to take a single post out of context - who hasn't ?

(yet another Doug here)
Eyesonmars
QUOTE (Juramike @ Oct 1 2012, 02:01 PM) *
Or take a glass of ice water and put it in a vacuum chamber and suddenly expose it to vacuum. [Done the equivalent lotsa times in a rotary evaporator].

Doug's right, it would boil. Bubbles of water vapor would form at imperfections in the glass of the surface and it would bloop up out of the glass and go everywhere. A combination of sudden coo anling due to more evaporative surface area would make all the flung water droplets quickly turn to ice, which would then sublimate away.

[When this happens in the lab, it is due to the receiver trap bumping, and there is usually a "Dangit!" if any of the water drops land back in the previously dried compound].

(Water-->bloop--->freeze--------------------->sublimate)

The water-bloop-droplet-freeze sequence would happen on the order of a few seconds. If the glass has a rougher surface, it would foam and froth smoothly due to all the nucleation sites. If it was a really smooth surface, it would sit quietly for a second and then just release it in a massive spasm of boiling due to the fewer nucleation sites.

Juramike,
I have followed your posts for years and have learned a great deal from you, especially the thought experiments you pose.

In your opinion what would happen to a glass of water with a temperature of 4c put into that chamber at a pressure of say 12mb. ? ignore other variables for the moment
Eyesonmars
QUOTE (djellison @ Oct 1 2012, 04:13 AM) *
Yes it would - and on that page you would see that at these very low pressures, the temperature difference between melting and boiling is very small.

Yes
The temperatures and pressures we are used to dealing with on earth in engineering, meteorology, you name it,are rarely near the triple point of water. But on Mars we have an entire PLANET and atmosphere which is never very far from the triple point of H20. Small changes in conditions near the triple point can have major effects. I would imagine future residents of mars would see H20 behave in ways that would seem alien to us. Their very existence might depend on understanding how H20 behaves under Martian conditions.
marsophile
I think one can make a case for transient wetting under certain conditions. Suppose there is overnight frost concentrated , let's say, in a cold trap area. In the early morning while the air temperature is still well below freezing, the thin frost cover might produce a green house effect on the soil beneath, especially for an east-facing slope, so that it warms above freezing. So there might be brief wetting underneath a vanishing frost cover.

Obviously this can only happen at locations and times where there is overnight frost. The Opportunity rover has shown this can occur even in equatorial regions. It would be nice to have orbital surveys of where and when frost occurs.
Juramike
QUOTE (Eyesonmars @ Oct 1 2012, 12:20 PM) *
In your opinion what would happen to a glass of water with a temperature of 4c put into that chamber at a pressure of say 12mb. ? ignore other variables for the moment


(Assuming bp. of H2O at 12 mb is at 10 C)

It would probably behave similar to a low boiling liquid under terrestrial conditions. I'd use diethyl ether as an example (b.p. 35 C). It won't boil at a normal room temperature of 25 C, but it will evaporate very quickly.
As it evaporates, it will cool.

Water should do the same thing under vacuum, but water's melting point is pretty high, so it will evaporatively cool and then freeze. After it is frozen it will sublimate. Some wierdness might occur if the glass of water is deep enough, the water would freeze on top and maybe you'd get liquid water sealed up in the ice? (Sublimation will also suck heat out of the system, eventually the whole thing should freeze solid then sublimate.)

BTW, a very similar phenomenon is predicted for any Titan ponds of pure methane. They would evaporatively cool, freeze (from the bottom up), then the totally frozen methane pond would slowly sublimate.
Eyesonmars
QUOTE (Juramike @ Oct 1 2012, 06:53 PM) *
(Assuming bp. of H2O at 12 mb is at 10 C)

Water should do the same thing under vacuum, but water's melting point is pretty high, so it will evaporatively cool and then freeze. After it is frozen it will sublimate. Some wierdness might occur if the glass of water is deep enough, the water would freeze on top and maybe you'd get liquid water sealed up in the ice? (Sublimation will also suck heat out of the system, eventually the whole thing should freeze solid then sublimate.)

Do you think it is possible that by exposing the glass to simulated Martian sun the evaporative cooling could be offset enough to keep it liquid until it all evaporated ?
Juramike
Hmmm. That's a good question, but off the top of my head I'd guess no, it wouldn't. Local airspeed is probably a bigger factor, with faster winds increasing sublimation (and evaporation).
Eyesonmars
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.
mshell
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.

Click to view attachment

Click to view attachment

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:

Click to view attachment

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
djellison
Beautiful work mshell - that you very much ( and GREAT first post!! ) - that shows the tiny wedge so very well.

nprev
Indeed, I can only echo the Chairman of UMSF's words, Mark...a spectacularly informative & relevant post. Glad you're here! smile.gif
marsophile
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.
Harder
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.
abalone
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?
vikingmars
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
Explorer1
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.
serpens
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?
vikingmars
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
marsophile
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.
serpens
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.
Tom Dahl
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:
Click to view attachment
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
Eyesonmars
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 ...)
Blue Sky
It is interesting that in the NASA presentation of the fan outflow pattern last week, there was no discussion of where all that water came from in the first place. Based on the location of Gale crater, it would seem to have been from the northern ocean that was assumed to exist in the Noachian age. A long time ago.
abalone
An interesting paper
http://online.liebertpub.com/doi/pdfplus/1...9/ast.2011.0660
Liquid water could be stable as little as 100-200m below the surface
and
"At temperatures below 0C, liquid water can exist as either
a thin film or a brine with a freezing point < 0C. Including
salts to estimate ‘‘average Mars salinity water’’ expands the
regions where liquid water can occur. Modeling by Mellon
and Phillips (2001) showed that a concentration of 15–40% of
salts (sulfates, chlorides, bromides, carbonates, and nitrates)
is sufficient to allow the melting of ground ice (or to maintain
liquid water) in the top few meters of soil."
vikingmars
QUOTE (Eyesonmars @ Oct 2 2012, 08:09 PM) *
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

...The max pressure occurred really on Sol 277.34 but the Ls is 279.93... You are right, not far from this value wink.gif (see table herebelow as "pdf" file)
Click to view attachment
mshell
This is an update of my up-thread post here where I created phase diagrams for pure water and speculated (with input from Eyesonmars) on the maximum atmospheric pressures that we might see at Gale and on Mars as a whole. I spent a couple of blissful hours last night running down elevation figures, Viking Lander weather data, REMS data, and concepts such as “scale height”.

Here’s what I found …

1. Eyesonmars stated in his post here that the MSL site was “another 2km or so LOWER” than the Viking 2 landing site at Utopia Planitia. We can now update this based on the latest Mars Orbiter Laser Altimeter (MOLA) data. It was interesting to see that the original Viking elevation data was based on a different “reference ellipsoid” from what is now used for MOLA elevations. The MOLA elevations are based on a Mars geoid (an “areoid”) with a radius equal to the average equatorial radius of Mars and with a surface that has an equipotential gravitational field. This is just what we need to compare barometric pressures. It turns out that Curiosity’s MOLA elevation is virtually the SAME as the Viking 2 site:

Click to view attachment

The MOLA elevations are accurate to about +/- 1m. The references can be found on pp. 11-12, here.

I took the Curiosity elevations from the excellent profile map attached to this post by pgrindrod, which I am virtually positive are based on MOLA elevation data, since it seems to agree with other sources.

2. Since the elevations are virtually identical, it seems reasonable to expect that the barometric pressures at Gale will be similar to those at Viking 2. I found the weather data for Viking 2 here and the REMS weather data for MSL here for daily averages. You can press the “Data” button at the bottom of the weather display to access the (approximately) hourly data from Sols 9-12.

The Viking data was easy to cut and paste into a spreadsheet, but I had to type the REMS data in by hand. Does anyone know a place to get ALL of the REMS readings? Also, please contact me if you are interested in getting the weather data spreadsheet. The max average daily value for Viking 2 was indeed 8.20 mBar as Eyesonmars reported, and the single maximum pressure reading is indeed 10.72 mB as Vikingmars reported here.

I then plotted the average daily pressures from Viking against the Solar Longitude (a measure of the “season” on Mars) and overlaid Curiosity’s REMS average daily pressures:

Click to view attachment

The data are right on top of each other, which gives some credence to the concept that the atmospheric pressures at Gale through the summer are likely to be similar to what we have already seen with Viking 2 at Utopia Planitia. This leads me to estimate that we could very well see pressures above 10 mB at Gale.

3. With the updated elevations and using a “scale height” of 11.1 km (which varies a bit with temperature and is referenced here), we get a maximum pressure at the lowest point on Mars (an impact crater in Hellas Basin, see page 11 of this) of almost 15mB -- which is a bit more than the 14 mB that Eyesonmars had previously estimated. Although, given that Hellas is not in the “tropics” of Mars, the temperatures are likely to stay well below freezing year-round. Viking 2, for example, never got above -20 C.

Here is the updated phase diagram:

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4. The most interesting thing that I found during my investigation was this press release from the principal investigator of the REMS instrument, who says that “in the daytime, we could see temperatures high enough for liquid water on a regular basis” (my emphasis).

Maybe, just maybe, djellison’s “tiny tiny wedge” at the bottom of the phase diagram is just big enough …

Mark
Explorer1
Very impressive work Mark!
So the pressure will tick up and up until the start of summer, but what happens then? Would a gas sample in SAM see any difference?
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