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Martian Hydrology
Den
post Dec 8 2010, 05:19 AM
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QUOTE (ZLD @ Dec 8 2010, 04:47 AM) *
That isn't really all that true. Pressure can play a large part in the melting point, especially when you consider other factors such as water purity. Seawater for instance, freezes at -2C which is quite a big difference. Also, as pressure increases, melting points definitely do decrease, which is what allows Lake Vostok to remain (likely) in liquid form.


I know. *In the context of current discussion*, though, +/- 2-3 degrees Celsius are not a big deal. Martian subsurface, even at equator, is at least -40 C, which is a deep freeze for water at any possible pressure or salinity.

(BTW, I arrived at the figure of -40 C by googling up various data on internet, picking the highest one, and adding some margin, - I want to err on a safe side in this argument. The real figure may easily be -60 C).
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ZLD
post Dec 8 2010, 05:53 AM
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I remember a while back spirit recording a temperature above 0C. Again, much of the atmosphere is too light to allow for water to exist on the surface. However given a deep enough crater/cavern/ravine or under the soil far enough where the pressure will be above the triple point, water could very possibly exist as liquid. Mars is extremely cold but it isn't always as such. Also, the difference between completely frozen over and completely liquid can waiver entirely on those couple of degrees that water impurities can provide.
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Den
post Dec 8 2010, 11:21 AM
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QUOTE (ZLD @ Dec 8 2010, 06:53 AM) *
I remember a while back spirit recording a temperature above 0C.


That's the maximum detected temperature. What was the detected minimum? And what that tells you about average temperature?

Viking landers detected temps in -17...-107 C range. IOW: they never saw temperatures approaching positive Celsius in more than 6 years of operation.
For reference: Viking 1 landed at ~22 N latitude and Viking 2 landed at ~48 N latitude.

Viking Orbiters detected (with remote sensing) temps in 27...−143 C range over the entire globe.
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machi
post Dec 8 2010, 03:44 PM
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Phase transitions (for example melting) of all chemical substances are dependent on pressure and temperature.
You can found phase from phase diagram. Some informations for instance here http://www.chemguide.co.uk/physical/phasee...phasediags.html or on wikipedia.
Some regions on Mars (Hellas Planitia) have conditions (temperature, pressure) close to so called triple point of water, thus liquid water on Mars is theoretically possible (at least for some time in the Martian year).
But amount of water vapour in Martian atmosphere is low and we haven't still accurate informations about subsurface water in equatorial regions of Mars (we need some hi-res SHARAD).

" Also, as pressure increases, melting points definitely do decrease, which is what allows Lake Vostok to remain (likely) in liquid form."

This isn't only because of pressure, but also heat from Earth's interior play role. Amount of interior heat in Martian case is minimal (compared to Earth).
And with antifreezing substances, water can be liquid even at much lower temperature (with ammoniac and so on).


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sariondil
post Dec 8 2010, 04:15 PM
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For those with access to 'Icarus':

Article in press on cryobrines on Mars with melting points for various brines and phase diagrams. At first glance I see nothing on boiling points, but generally they should be at higher temperatures than for pure water.
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Fran Ontanaya
post Dec 8 2010, 06:00 PM
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Is brine lost so fast to evaporation in those conditions that a reasonable underground inflow wouldn't be able to counterbalance it?


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ZLD
post Dec 8 2010, 07:48 PM
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QUOTE
Conclusions - The presence of salts on and in the surface of Mars and of water vapour (up to saturation) in the near-surface martian atmosphere indicate the possible existence of at least temporarily present liquid brines at temperatures far below 0° C. Evaporation (in warm early time) and (later) sublimation must have dried out salt lakes on the surface of Mars, which might have existed in the Noachian and early Hesperian. Salt lakes without sub-surface supply of water might be possible on present Mars in closed sub-surface cavities only.


This was a pretty interesting read. Thanks for pointing it out.

Attached File  Properties_of_cryobrines_on_Mars.pdf ( 573.07K ) Number of downloads: 893
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ngunn
post Dec 8 2010, 08:32 PM
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Thanks for the link. A good read indeed.
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marsbug
post Dec 9 2010, 01:38 AM
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Thanks for that, its a good resource for anyone trying to understand the many water on Mars questions. I hope you don't mind if I link people to this thread and that post in future?


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sariondil
post Dec 9 2010, 09:23 PM
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Another paper on modelling of subsurface water. Again, Elsevier wants you to pay for it, and the pdf is too large to post here (1.5 MB).

QUOTE
The thickness of the liquid subsurface layer depends primarily on the crustal heat flux, the thermal conductivity of the overburden sediments, and the surface temperature. A subsurface liquid water layer of 1 to 2 km can be achieved under present day surface temperature conditions and a crustal heat flux of 15–30 mW m− 2


With some simplifications (steady state, no hydrostatic pressure... ) one can calculate temperature and pressure at a given depth as follows. Density and thermal conductivity for some materials is here. Then look up p and T in the phase diagram of your favourite substance smile.gif
Attached thumbnail(s)
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ngunn
post Dec 9 2010, 09:45 PM
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QUOTE (sariondil @ Dec 9 2010, 09:23 PM) *
the pdf is too large to post here


Ha! I'm glad you'd be willing to if it weren't. How about the first half? (and then maybe the second half?).
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djellison
post Dec 9 2010, 09:48 PM
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QUOTE (sariondil @ Dec 9 2010, 01:23 PM) *
. Again, Elsevier wants you to pay for it, and the pdf is too large to post here (1.5 MB).


Then you would be breaching the T's and C's of Elsevier to share it here.
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ngunn
post Dec 10 2010, 12:11 AM
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The way we are just now I supose censorhip is an integral part of the dissemination of knowledge from the point of view of organisations like Elsevier. How far we go along with that is your call, but I know you are a champion of free information, and I doubt that the unbridled persuit of knowledge on this forum represents a commercial threat to Elsevier.
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helvick
post Dec 10 2010, 12:43 AM
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Much as I might be rabidly opposed to Elsevier's approach to firewalling off access to scientific research papers\data the problem here is that even if the presence of such material here posed no material threat to their business* it is still a breach of the terms under which it was originally provided to someone and we can't host such material here. They have the right to decide how their content is disseminated and can get quite nasty about it, we've run into similar issues in the past and have had to remove material. Much as I'd love to read it myself it shouldn't be uploaded here.

*And their business is to control access and extract a price for it - I may not like that but they have a perfectly legitimate business to defend and I'd be surprised if they would allow something like this to remain on a relatively high value indexed site.
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ElkGroveDan
post Dec 10 2010, 01:19 AM
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I have to agree with Helvick.


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If Occam had heard my theory, things would be very different now.
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