A question here, behaviour of water on Mars Options

[nprev]

Mike or HDP Don, how different are the curves for sulfur salts from chlorides? (Hope that wasn't a faux pas; afraid I've forgotten most of my basic chemistry). Martian brines, if any, might be considerably different chemically then their terrestrial counterparts, and the crust is certainly sulfur-rich.

[rlorenz]

And of course, the fact that water can exist, at some times, in some places - doesn't mean it does. It is a transient thing and would boil away quite easily - thus it would have to be replenished in some way.

Good thread. I remember I got asked about this one by NPR when the Mars Gullies story first came out. 'Isnt
Mars too cold for liquid water?' Well, yes, I answered, and Earth is too cold for liquid rock - doesnt mean it
never happens.....

Another data point - when I was at the Mars wind tunnel at NASA Ames doing wave generation experiments,
I noticed they have a big jar of water in the chamber just inside the window to the control room (wind tunnel is open
circuit - inside a large room that gets pumped down). As the pressure drops, the water starts to bump and boil,
but then stops, while still liquid. (i.e. it boils until the evaporative cooling brings the temperature well below
that at which the saturation vapor pressure equals ambient). We had a big tray of water on which we were
hoping to generate waves - we could see on the video link that it bumped once or twice, I guess with bubbles
of dissolved air coming out. Then we turned the airflow on in the hope of making waves at 10mbar or so, and saw
the water glaze over - the enhancement in evaporative cooling by the airflow was enough to freeze it.

Pure water has 6mb vapor pressure at 0 C (and 20mb at 20C - I remember it as 20:20)

When I lived in Arizona, this issue of the metastability of water on Earth's surface was rather evident - spill
water in the kitchen, no problem, it'll dry up by itself in 3 minutes. At the DPS conference in Monterey (2003?)
I raised the question in connection with Titan (known to have 50% or so relative humidity, so where are the
oceans?) - I pointed out that Earth is 60% covered in water, and yet we can hang out laundry to dry. Clearly
this wouldnt work unless the relative humidity were much lower than 100% (because of circulation to higher
altitudes, which dries the air..)

Anyway, it remains a subtle issue. I'd urge people read Mike Hecht's work on the topic (he has a rather
fresh perspective). And I think Titan (where ethane takes the role of salt, in lowering the saturation vapor
pressure of the volatile component in a solution) will be very instructive in comparisons with Mars.

So - Doug's words 'transient' and 'replenished' are key - if the system is out of equilibrium, then lots of
things are possible. And I guess I am learning over the years that disequlibrium isnt that hard to generate...

[ngunn]

I'd urge people read Mike Hecht's work on the topic

Thanks for that post. Can you direct us to Mike Hecht's work? Is there a book or linkable abstract that would be a good place to start?

[dburt]

Mike or HDP Don, how different are the curves for sulfur salts from chlorides? (Hope that wasn't a faux pas; afraid I've forgotten most of my basic chemistry). Martian brines, if any, might be considerably different chemically then their terrestrial counterparts, and the crust is certainly sulfur-rich.

Very different, in that common sulfates (e.g., of Mg) can't depress freezing the point more than about 5 degrees C, whereas NaCl (sodium chloride table salt) depresses it about 20 C, MgCl2 about 34 C, and CaCl2 about 50 C. Chloride salt mixtures gain several extra degrees below that (so-called eutectic freezing). Therefore any low temperature brines on or in Mars would have to be dominated by chlorides, a group of salts that can't normally be detected by infrared spectroscopy (TES and THEMIS from orbit, and Mini-TES on the rovers). That is, Mars could be chloride rich, and the salts would be difficult to detect. In this regard, their greater solubility and greater tendency to be frost leached (via freezing point depression) suggests that chloride salts should be less persistent than sulfate salts at the martian surface.

Nevertheless, chloride-rich areas on Mars have recently been inferred by this very lack of an IR signal - they look something like a "black hole" to IR spectrometers. See, e.g., Fall AGU abstract P13D-1563 by M.M. Osterloo et al.
http://www.agu.org/cgi-bin/SFgate/SFgate?&...P13D-1563"
Several of these inferred chloride-rich areas were suggested as possible landing sites for the Mars Science Lander (MSL), but didn't make the semi-final cut a couple of weeks ago.

Gsnorgathon, with regard to cooking with salt, I stand corrected. I perhaps should have said, "among other reasons" or "one reason" and not "the reason". (Adding salt is commonly recommended even for the simplest of recipies such as boiling an egg, where the taste of the salt might be undetectable.)

Ngunn, with regard to Mike Hecht, you could start here:
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1364.pdf
although he also published longer papers later. Personally, I enjoyed the exciting description of metastable water in an active outflow channel in the novel "Red Mars".

-- HDP Don

[ngunn]

Thanks for that Hecht link. Definitely some counter-intuitive things going on, and a subject well worth getting one's head round properly.

[Juramike]

.... I strongly suspect that most recipes call for salt to be added before boiling so that whatever's being boiled will absorb the salt, and thus enhance its flavor.

Actually, the advice is more a safety issue. If you add salt (or any other powder) to water that is boiling or very near boiling, the addition of so many new nucleation sites (rough edges and pits from salt xtals) could cause sudden boilover.

[There was a Mythbusters episode where they confirmed that adding sugar to water that was zapped to superheating could cause the water to violently boilover. (Season 1, Episode 4)]
Mythbusters index: http://mythbustersresults.com/

[Very cool video of adding coffee powder to superheated water (WARNING: DO NOT TRY THIS AT HOME OR IN MY LAB): http://www.phys.unsw.edu.au/~jw/superheating.html]

-Mike

[nprev]

Very different, in that common sulfates (e.g., of Mg) can't depress freezing the point more than about 5 degrees C, whereas NaCl (sodium chloride table salt) depresses it about 20 C, MgCl2 about 34 C, and CaCl2 about 50 C. Chloride salt mixtures gain several extra degrees below that (so-called eutectic freezing).

Thanks! That's really a dramatic difference. I presume those values are referenced to terrestrial standard temperature & pressure? Has anyone crunched the numbers for average Martian STP (if they've derived that in any meaningful form yet, that is)?

[dburt]

Thanks! That's really a dramatic difference. I presume those values are referenced to terrestrial standard temperature & pressure? Has anyone crunched the numbers for average Martian STP (if they've derived that in any meaningful form yet, that is)?

Inasmuch as brine, salt, and ice are all condensed (non-gaseous) phases, changes to pressure should have very little effect until the pressure gets so low that the brine boils or the ice sublimates (definitely a consideration for Mars - but less of a concern for chloride brines, owing to the tremendous lowering of the activity of H2O in such brines, as mentioned above). The salts might lose waters of hydration too, although this doesn't affect the basic argument about freezing point depression. A lot of basic data is given in a 1980 Icarus paper by Brass, "Stability of brines on Mars" that Knauth and I cited in our 2002 and 2003 papers.

And Juramike - thanks for the added insight on the safety of adding salt to water before boiling it - yet another reason to do so, and undoubtedly the most important one. I guess you can tell this professor doesn't cook much.

-- HDP Don

[nprev]

They don't call ya Herr Doktor Professor for nothin'! Thanks, Don; most interesting and informative, as always.

[dburt]

They don't call ya Herr Doktor Professor for nothin'!

Actually, no one calls me that but me, AFAIK (a form of self-mockery). Actual Herr Doktor Professors I've met over the years can NEVER make a mistake, practically by definition. That pretty much rules me out as the real thing, I'm happy to say.

-- HDP Don

[nprev]

[Very cool video of adding coffee powder to superheated water (WARNING: DO NOT TRY THIS AT HOME OR IN MY LAB): http://www.phys.unsw.edu.au/~jw/superheating.html]

-Mike

...son of a <clink!!!> Rest assured, I'll never try this at home or anywhere else, period, and thanks for the public service announcement!

What I'm getting here is that soluables usually depress the freezing point but accelerate the boiling point. Is that a fair generalization?

[Juramike]

...son of a <clink!!!> Rest assured, I'll never try this at home or anywhere else, period, and thanks for the public service announcement!

What I'm getting here is that soluables usually depress the freezing point but accelerate the boiling point. Is that a fair generalization?

The effect on the water boil-over was due to nucleation. If you added sand (insoluble) it would have done the exact same thing and boiled over.

Perfectly smooth surfaces prevent boiling and crystallization. (No good nucleation sites or crystal defects to start the phase change). So you can get superheating and supercooling. Add a rough surface (or scratch the glass vessel) and bingo! you get nucleation or crystallization. (Scratching the inside of a flask is a great way to start crystals growing). So if you go against my advice and try this at home, a brand new coffee cup with no interior dings will work best.

Same deal could happen on Mars or Earth. You have a fluid which goes through a smooth "pipe" without any nucleation sites, and you could get fluid at temperatures/pressures outside where it would normally be in the phase diagram. Throw in a defect and you get a geyser, or it freezes up, depending on just where it is in the phase diagram. This is applicable to either pure materials or brines. (Although the vapor phase from a geyser would be pretty much pure water, and the first phase to freeze out of a brine would likely be water if it is not at it's eutectic.)

-Mike

[marsbug]

Thank you very much one and all, I can look foward to a very interesting conversation ensuing on monday when we see each other again!

[ElkGroveDan]

The effect on the water boil-over was due to nucleation. If you added sand (insoluble) it would have done the exact same thing and boiled over.

A similar process happens with super-cooled liquids. When I was in college I lived in a very dry, high-altitude town (that nprev is familiar with). I used to keep a bottle of club soda (carbonated water) in my small dorm refrigerator (as I was a Scotch drinker even back then). Every now and then the cheap refrigerator would go out of whack and I'd find everything inside frozen solid, except for the sealed bottle of soda water (occasionally it would be Diet Coke). So I would call over some of my neighbors to demonstrate what happens when a crystal of just about anything was dropped into this supercooled liquid. It's probably best to do this with a plastic bottle, as I'm not sure what might happen with a glass container when the entire contents go "thud" and turn to ice in a millisecond.

(For those of you still in college this works best on Freshmen engineering majors, but not the ladies. )

[nprev]

Rats...knew I shoulda gone to EGD's alma mater. In addition to learning about freezing stuff instantaneously, it's only 90 miles from home, so Mom could've still done my laundry...