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ngunn
An interesting article from Space Daily today: http://www.marsdaily.com/reports/Shallow_G...n_Mars_999.html
marsbug
How big would such an aquifer need to be to be picked up by SHARAD? We know there is a fair bit of relatively low lattitude ice a few meters beneath the martian surface, so wouldn't we expect to have seen some evidence if aquifers if this were happening in the present day?
ngunn
I'm curious about this too, and my ignorance of the workings of ground penetrating radar knows no bounds. Maybe someone can enlighten us. Can SHARAD and MARDI distinguish between ice and liquid water? (A seismic survey - now that would be another thing.)
marsbug
A very quick bit of reading tells me that SHARAD has avertical resolution of 15 to 20 meters. The electrical ( and hence radar) properties of water are different from those of ice so at a very very ill informed guess I'd say that SHARAD probably could distinguish between the two. So assuming my guess is near the mark the question is what is the vertical extent of these possible aquifers?

ngunn
We know that Mars has plenty of H2O, and that like any other world it gets warmer as you go down, so I think their must be liquid water at some depth on Mars (to be drilled for perhaps by future inhabitants). Of course when it has the temerity to appear at the surface it immediately freezes and pretty soon sublimes away, so we're looking at the dry rind of a wet planet.
How near the surface the liqiud phase gets and how often are great questions. I hope we get answers from the current generation of Mars explorers.
Den
QUOTE (ngunn @ Nov 27 2010, 11:14 PM) *
We know that Mars has plenty of H2O, and that like any other world it gets warmer as you go down, so I think their must be liquid water at some depth on Mars (to be drilled for perhaps by future inhabitants). Of course when it has the temerity to appear at the surface it immediately freezes and pretty soon sublimes away, so we're looking at the dry rind of a wet planet.
How near the surface the liqiud phase gets and how often are great questions.


If temp rises by ~30 C with every kilometer, then liquid water should be at about three kilometers down.
ZLD
Quoting the Wikipedia entry on Martian Climate:

QUOTE
Differing values have been reported for the average temperature on Mars, with a common value being −55 °C. Surface temperatures have been estimated from the Viking Orbiter Infrared Thermal Mapper data; this gives extremes from a warmest of 27 °C to −143 °C at the winter polar caps. Actual temperature measurements from the Viking landers range from −17.2 °C to −107 °C.


If that's true, then water could very possibly be just meters below the surface in some areas during daytime hours based on the lander estimates and could reside as liquid on the surface (if it was to get there) for extended periods according to the oribiter.

For comparison, Antarctica's climate ranges from -80°C at the pole to 15 °C on the coasts.
djellison
Den and ZLD - you're both forgetting one vital part of this puzzle. Air Pressure.

Surface conditions on Mars - at the very warmest - are playing around at the triple point of water. There would only be a tiny temperature window between ice melting, and evaporating away.

http://www.cims.nyu.edu/~gladish/teaching/...ase-diagram.jpg

There are times and places where liquid water could exist on the surface of Mars. Those times are uncommon, and brief.
ZLD
And most people forget that the ground has pressure as well. wink.gif

Edit: I guess I overlooked your last line. Yes, in this sense, it would be rare for liquid water to ever exist on the surface of Mars. However, honestly I have no idea what temperature water would need to be to exist as liquid. I will have to calculate that out.

Edit 2: Heres and interesting link. I found this on a quick search. Looks like a good read for this thread.
djellison
That paper is 10 years old, and many other studies since have suggested alternative mechanisms for those landslides - including dry dust avalanches, CO2 sublimation etc. The wet theory behind them is less popular than a decade ago.
Den
QUOTE (ZLD @ Dec 7 2010, 08:15 PM) *
Quoting the Wikipedia entry on Martian Climate:

If that's true, then water could very possibly be just meters below the surface in some areas during daytime hours


No, it can't. Subsurface temperature quickly - in less than 20 meters - settles to year-average. Thus, even on Mars equator the subsurface temperature is about -40 C IIRC.
Den
QUOTE (djellison @ Dec 7 2010, 08:26 PM) *
Den and ZLD - you're both forgetting one vital part of this puzzle. Air Pressure.

Surface conditions on Mars - at the very warmest - are playing around at the triple point of water. There would only be a tiny temperature window between ice melting, and evaporating away.


I never forget that. But with having positive Celsius temps a few *kilometers* down means than any vapor has ample time to re-freeze while it creeps through pores towards surface. I think over time this creates an extensive subsurface layer of permafrost, nearly airtight and impenetrable for water vapor from below.
Den
QUOTE (ZLD @ Dec 7 2010, 09:06 PM) *
And most people forget that the ground has pressure as well. wink.gif

Edit: I guess I overlooked your last line. Yes, in this sense, it would be rare for liquid water to ever exist on the surface of Mars. However, honestly I have no idea what temperature water would need to be to exist as liquid. I will have to calculate that out.


Water needs 0 C or 273 K to become liquid. Even on Mars. Melting temp depends only weakly on pressure.

QUOTE
Edit 2: Heres and interesting link. I found this on a quick search. Looks like a good read for this thread.


Here is an photo example of mini-gullies on Earth which definitely are not formed by water:
http://uploaded.fresh.co.il/2007/03/16/46040681.jpg

Click to view attachment

I know it is far from being scientific, but still...
ZLD
QUOTE (Den @ Dec 8 2010, 03:19 AM) *
Water needs 0 C or 273 K to become liquid. Even on Mars. Melting temp depends only weakly on pressure.


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. Similarly, aquifers could exist at somewhat shallow (>1km) depths under Martian soil so long as the pressure exists. What would need to be determined then is, at what depth does this pressure exist?

Secondly, I never brought up the gullies. I am fully aware of the current hypothesis surrounding them. I was at work when I posted that link and didn't have a chance to fully read it. I only posted it because it seemed to have some information that related to this topic and I apologize for it not suiting that goal entirely.
ElkGroveDan
QUOTE (Den @ Dec 7 2010, 06:19 PM) *
Melting temp depends only weakly on pressure.

That may be true but the boiling point of water depend quite a bit on pressure. And under most ambient Martian pressure conditions the boiling point is a fraction of a degree above and sometimes equal-to your melting point. As a result you are never going to have liquid water beyond a few fleeting moments under extremely rare conditions.

Best to leave your goldfish at home there's no stable liquid water on Mars.
Den
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).
ZLD
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.
Den
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.
machi
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).
sariondil
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.
Fran Ontanaya
Is brine lost so fast to evaporation in those conditions that a reasonable underground inflow wouldn't be able to counterbalance it?
ZLD
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.

Click to view attachment
ngunn
Thanks for the link. A good read indeed.
marsbug
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?
sariondil
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
ngunn
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?).
djellison
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.
ngunn
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.
helvick
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.
ElkGroveDan
I have to agree with Helvick.
imipak
A reminder of the the discussion of the notorious image of whatever-it-was on Phoenix' leg...
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