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Temperature and pressure at Gale, Suitable (for short periods) for liquid water?
Seryddwr
post Sep 30 2012, 03:23 PM
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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.
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ngunn
post Sep 30 2012, 03:33 PM
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690 Pa = 6.9 mbar
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Seryddwr
post Sep 30 2012, 03:40 PM
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6.9 - quite! 690 millibars would have been quite a discovery! ohmy.gif
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nprev
post Sep 30 2012, 03:51 PM
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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.


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Eyesonmars
post Sep 30 2012, 04:03 PM
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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
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djellison
post Sep 30 2012, 04:31 PM
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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.
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Eyesonmars
post Sep 30 2012, 04:50 PM
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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
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Eyesonmars
post Sep 30 2012, 08:04 PM
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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.
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djellison
post Sep 30 2012, 09:47 PM
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It would boil, would it not?
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udolein
post Sep 30 2012, 11:06 PM
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Liquid water is almost impossible under the current conditions:

Attached Image


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







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udolein
post Sep 30 2012, 11:19 PM
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BTW: This site has the current weather readings: marsweather.com

Udo


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djellison
post Sep 30 2012, 11:25 PM
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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.

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Eyesonmars
post Sep 30 2012, 11:37 PM
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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
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serpens
post Sep 30 2012, 11:40 PM
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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.
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udolein
post Sep 30 2012, 11:44 PM
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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


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