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A question here

There are signs that in the past there was liquid water on Mars. So lets assume thats true.
Since the gravity on Mars is much lower than on Earth, so how does water (waves) behave on Mars compared to Earth?
Someone did say, that waves would have been much higher but also much slower. Is this true? Does anyone have an animation where you can see a waive on Earth in comparsion to a wave on Mars?

Thanks
tuvas
This is just a guess, but I would guess waves would be higher and faster on Mars, but without the tidal affects, at least, not quite so much... The wind speed on Mars is much greater than on Earth. The lower gravity would allow the waves to go somewhat higher. But again, this is only a guess.
ugordan
I believe the higher-and-slower waves is the correct assumption. Wind may be the driving factor of waves, but it's gravity that dictates the wave propagation.
nprev
Another major variable would have been atmospheric density during a hypothetical past era. Mars' current winds are indeed much faster than Earth's, but the average surface pressure is only 6.1 mb vs. 1017 mb for Earth at sea level; not nearly as much mass there, so correspondingly less energy transfer between wind & water. IIRC, it would take at least 10 mb of surface pressure to sustain liquid water on the surface anyhow.
dvandorn
Actually, liquid water on Mars would behave consistently at nearly any atmospheric pressure we can imagine at which water can exist as a liquid. At least, at pretty much any point in a range from 10mb to 1b. The effect of winds would be different, but the speed at which water rises and falls is entirely defined by gravity. And since Mars is at roughly 1/3 G, water will fall roughly 1/3 more slowly.

I call as evidence Buzz Aldrin's description of the flow pattern of the wine he poured when he took Communion on the lunar surface. He said the wine poured in slow motion and moved about in the tiny chalice he used as it would on Earth, but more slowly and more with exaggerated motions. It was easier to make the liquid slosh and try to rise up over the lip of the cup than it would have been on Earth -- as he poured, the wine curled right up to the opposite lip of the cup. (Leave it to Aldrin to carefully examine the physics of fluid motion in 1/6 G while taking Communion... smile.gif )

Roughly double the gravity (damping down the motions somewhat) and scale up a tiny chalice to rushing floodwaters and lakes, if not seas, and you get an idea of the semi-slow-motion, vertically exaggerated waves you'd likely have seen on Mars.

-the other Doug
AndyG
...and add bigger drops. Surface tension will play a somewhat bigger role when spray's in the air longer, and atmospheric pressure is (presumably) less than a bar.

Andy G
nprev
Great story, oDoug! smile.gif

Yeah, I should have been more specific when talking about atmospheric density effects. What I meant was that, even though the winds on Mars can really shriek, they still don't pack much punch so I doubt that they'd raise very impressive waves in and of themselves.
Juramike
While there would be no really big tides on a Martian sea, there might be some significant effects from weather.

Storm surges caused by weather could temporarily (and dramatically) raise sea levels on Mars more than on Earth.

(there was a random set of waves that topped 10 feet that hit Daytona Beach on a calm night many years ago. It trashed several cars parked on the sand. It was thought to have resulted from a storm far offshore that pushed a wall of water in front of the storm front. The wind speed matched the wave propogation speed nicely and the additive effect built up an impressive set of waves that persisted after the storm dissipated.)

[For a really cool powerpoint presentation on Tsunamis vs. Rogue Waves: click here (Warning: long download)


Also, low pressure centers can allow a water column to get sucked up (actually, it's due to "not being pushed down"). IIRC, on Earth a 100 mb (10%) drop seen in the center of a Hurricane can cause a water column to rise 8-10 feet. (This is only one of the factors of the devastating storm surge when hurricanes make landfall).

Could a similar drop in a low pressure cell on Mars (10%) cause a similar surge? Would the lower gravity make it even more sensitive to pressure effects?

(uh-oh, sounds like a lot of math will be needed to figure this out)

-Mike
helvick
On earth the average atmospheric pressure of ~101kPa is a pretty powerful hydrodynamic pump - it is equivalent to a 10m water column. The martian atmosphere is only 1% of that - even accounting for the lower gravity there the equivalent water column on Mars is only 23cm (at most). Large storm surges like the one you describe require a low pressure zone surrounded by a high pressure zone all over the same body of water. Say we had such an item (e.g. the hypothetical northern Martian ocean) then even an extreme hurricane like storm with a 20% internal pressure drop would only be capable of pushing a surge of a handful of centimeters.

It must be said though that _if_ there was an ocean like that then the atmosphereic pressure would have had to be significantly higher - at these sort of pressures it would just boil away. If that was close to Earth like pressures then the storm surges would be massive - 2-3x what we see on Earth from a similar storm.
marsbug
I have a question I've not been able to resolve and here seems the best place to ask it: The atmospheric pressure of mars is around the triple point pressure of water, hence on a day of high atmospheric pressure and a temperature above freezing (or with the right impurities) a puddle of liquid water could remain on the surface as liquid , but with a very reduced boiling point compared to earth? That is what a great many articles I have recently read seem to assert or imply. However a chemist friend of mine has just argued to me that the triple point pressure on the phase diagram is the partial pressure of water vapour, and since on mars a H2O partial pressure above 6.1 millibars is almost impossible to build, liquid water is indeed impossible on mars! He is quite convinced, but I find it difficult to accept that so many journals and articles have been mistaken over such a fundamental fact. In fact I have read papers on experiments showing that brines at least can remain stable and liquid at mars atmospheric pressure Click to view attachment, but I am at a loss to explain how this is possible to my friend!

EDIT:So you all know I'm not just being lazy, I've googled and wikipediad and gone to the university library, and although I've found both versions I've not found anything that clarifies the difference, or addresses the question directly. smile.gif smile.gif
Greg Hullender
Here's a couple of useful comments from a NASA site:

"According to the model, the highest surface pressure, 12.4 millibars, occurs at the bottom of the Hellas Basin (a low-lying area created by an ancient asteroid strike). The problem is that the boiling temperature there is only +10 C. It can't get very hot or the water will boil away."

"There are 5 five distinct regions where we might sometimes find surface water: in the Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. Together they comprise about 30% of the planet's surface. That's not to say that liquid water really does exist in those places, just that it could."

Here's the link for the whole thing:

http://science.msfc.nasa.gov/headlines/y2000/ast29jun_1m.htm

--Greg
djellison
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.

Doug
Juramike
Both theory and experiment agree that cold brine solutions could exist for some time on Mars.

(IIRC, the triple point curves will need to be determined for the the exact concentration and salt type. The triple point curve for fresh pure deionized water can only be used as a rough guide. Salt concentration, ion type, and other impurities present will mess with the graph.)

And the paper does a pretty good job of demonstrating that the evaporation rate is low. From Table 1 in the article you cited, for a -20 C solution of 29.8% aqueous CaCl2 at 7 mbar, the evaporation rate on Mars is predicted to be 0.1 mm/h.

For comparison, the evaporation rate of ocean water at Earth's equator is 0.2 mm/h.
Source (and more than you ever wanted to know about ocean dynamics): http://ams.allenpress.com/archive/1520-044...442-1-9-841.pdf

So if you had a brine solution and magically transported it to Mars. It could hang out for a while.

[Quick back-of-the-envelope calculation and assuming no change in evap rate on concentration, a gobal 20 m deep brine ocean would evaporate (and redeposit as polar ice?) in 22 earth years.]

As Doug mentioned, you'd need a recharge mechanism to keep the water/brine/atmosphere system going.

But as long as they prepared it themselves, future astronauts could still do a quick, cold Sitz bath at the end of a long day.

-Mike
dburt
QUOTE (marsbug @ Nov 15 2007, 11:14 AM) *
I have a question I've not been able to resolve and here seems the best place to ask it: The atmospheric pressure of mars is around the triple point pressure of water, hence on a day of high atmospheric pressure and a temperature above freezing (or with the right impurities) a puddle of liquid water could remain on the surface as liquid , but with a very reduced boiling point compared to earth? That is what a great many articles I have recently read seem to assert or imply. However a chemist friend of mine has just argued to me that the triple point pressure on the phase diagram is the partial pressure of water vapour, and since on mars a H2O partial pressure above 6.1 millibars is almost impossible to build, liquid water is indeed impossible on mars! He is quite convinced, but I find it difficult to accept that so many journals and articles have been mistaken over such a fundamental fact. In fact I have read papers on experiments showing that brines at least can remain stable and liquid at mars atmospheric pressure Click to view attachment, but I am at a loss to explain how this is possible to my friend!

EDIT:So you all know I'm not just being lazy, I've googled and wikipediad and gone to the university library, and although I've found both versions I've not found anything that clarifies the difference, or addresses the question directly. smile.gif smile.gif


Marsbug - An excellent (and not uncommon) question. The phase diagram for water, with its triple point at 6.1 millibars of pressure, is for the one component system H2O. For that simple system, liquid water is transiently possible at pressures higher than this, initially for a ridiculously small range of temperatures of only a few degrees C, with an increasing range possible at higher pressures, up to the entire 100 degree range between freezing and boiling possible at terrestrial atmospheric pressure. At low martian pressures, this means that at lower elevations you could melt ice to liquid, heat it a degree or two, and it would begin to boil. That is, liquid water might be stable at low elevations, but not very.

Consider an atmosphere with constituents other than steam, however, and the picture gets more complex. The atmosphere on Mars is mainly dry CO2. Unless it is actively snowing, or condensing frost, the humidity of this atmosphere is less than 100%, so liquid water (or even ice) will be metastable (transient), like it is in Phoenix, Arizona in June. That is, it won't boil, but it will tend to evaporate (for liquid) or sublimate/melt (for ice). This isn't to say people in Phoenix can't maintain swimming pools (or cold drinks by the poolside) in June, but they have to keep replacing the evaporative losses (or ice in the drinks). At the extremely cold temperatures of Mars, metastable ice can persist for rather a long time, and liquid water, if it were not actively boiling, might also. But your chemist friend is correct, liquid water would not be stable, just metastable.

Next consider the effect of dissolving salts, especially chloride salts, in liquid water. At any pressure, this simultaneously lowers the freezing point ("freezing point depression" familiar to those who salt down their icy sidewalks in winter) and raises the boiling point (one reason why most cooking recipes call for adding salt to the water BEFORE boiling). To thermodynamicists, dissolved salts do this by greatly lowering the activity of H2O in the liquid. Some salt mixtures, particularly those rich in calcium chloride, can lower freezing temperatures to more than 50 degrees C, to temperatures commonly found on Mars. Under these conditions, liquid brine would be stable, or at the very least could persist for extremely long times, despite low humidity (concentrated brines are hygroscopic, meaning they can actually suck moisture out of air). Paul Knauth and I wrote a couple of papers on this topic in 2002 and 2003, in which we suggested that the "young gullies" on Mars could have been carved by such multicomponent chloride brines (so-called eutectic brines), which normally would be expected beneath permafrost or ice layers. I just submitted an impact-related update to this idea to the Mars gullies workshop upcoming in Houston on Feb 4-5.

To your friend the chemist, everything is either stable or unstable. Geologists consider that most of the world we observe is actually metastable (like diamond jewelry is, if you understand the phase diagram for carbon, or read old Superman comic books). And I fear that, despite my best intentions, I've been as clear as mud. smile.gif

Edit: minor change to text as per following post by Gsnorgathon about too salty food.

-- HDP Don
Gsnorgathon
Not to rain (metastably or otherwise) on anyone's parade (least of all dburt's after such a wonderfully detailed post), but 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.

I'd guess that an amount of salt sufficient to raise the boiling point would be a tad much for most people's taste.
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
QUOTE (djellison @ Nov 15 2007, 02:12 PM) *
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
QUOTE (rlorenz @ Nov 16 2007, 01:47 AM) *
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
QUOTE (nprev @ Nov 15 2007, 06:06 PM) *
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
QUOTE (Gsnorgathon @ Nov 15 2007, 08:41 PM) *
.... 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
QUOTE (dburt @ Nov 16 2007, 12:27 PM) *
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! smile.gif 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
QUOTE (nprev @ Nov 16 2007, 06:17 PM) *
Thanks! smile.gif 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. smile.gif

-- HDP Don
nprev
They don't call ya Herr Doktor Professor for nothin'! tongue.gif Thanks, Don; most interesting and informative, as always.
dburt
QUOTE (nprev @ Nov 16 2007, 06:50 PM) *
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
QUOTE (Juramike @ Nov 16 2007, 12:57 PM) *
[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


ohmy.gif ...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
QUOTE (nprev @ Nov 17 2007, 10:49 AM) *
ohmy.gif ...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! smile.gif smile.gif smile.gif
ElkGroveDan
QUOTE (Juramike @ Nov 17 2007, 09:02 AM) *
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... sad.gif wink.gif
Juramike
Space.com article says that the crust of Mars is colder (and thicker) than previously thought.

From the article:

"Unexpectedly, the radar scans also revealed the massive weight of the ice cap does not deform any underlying sediment. This implies the crust beneath the cap is strong more than 180 miles thick (300 km).

To have such a thick crust, "Mars might be colder than we thought," Phillips told SPACE.com. As a result, any liquid water that might be underground has to be buried even deeper than once speculated. "If one thought that liquid water was 5 kilometers deep (3 miles), it's now at least 30 percent deeper than that," he said."

(The article to be published in the May 15 issue of Science is not yet available.)


I have a really ignorant question, here: What is the ductile strength of sediment with interstitial ice? Is it stronger or weaker than normal sediment?

-Mike
dburt
QUOTE (Juramike @ May 15 2008, 12:03 PM) *
Space.com article says that the crust of Mars is colder (and thicker) than previously thought.

From the article:

"Unexpectedly, the radar scans also revealed the massive weight of the ice cap does not deform any underlying sediment. This implies the crust beneath the cap is strong more than 180 miles thick (300 km).
..."

I have a really ignorant question, here: What is the ductile strength of sediment with interstitial ice? Is it stronger or weaker than normal sediment?

Mike - So here's an ignorant answer. If by "normal sediment" you mean unconsolidated sediment (i.e., loose particles) where the pore space is filled with liquid water (below the water table) or air (above the water table), presumably ice-cemented sediment should be somewhat stronger, because ice is a solid. However, ice expands as it freezes, and can move sedimentary particles around (e.g., in ice polygons inferred on the Martian surface), so that ice-cemented sediment could be weaker than an actual sedimentary rock (if the particles in the rock were cemented by something stronger than ice).

That ambiguous answer may be irrelevant, though, because the space.com quote appears to be inadvertently misleading. They are not really talking about deformation of a thin, weak sedimentary veneer, but about deformation of the much stronger and thicker underlying igneous (metamorphic?) crust and uppermost olivine-rich mantle (i.e., what geophysicists call the lithosphere on Earth). The colder the underlying solid rock, the less easily deformable it is. So what they are mainly saying, if I am guessing correctly, is simply that Mars is somewhat more rigid (and therefore colder by inference) inside than was formerly modeled (at least beneath the poles). Calling this cold, rigid layer "the crust" appears to be PR-speak for "cold and rigid lithosphere". The present-day lack of plate tectonics on Mars (i.e., the fact that Mars is a one-plate planet) already implies that Mars has a very thick, rigid, non-deformable lithosphere. The lack of deformation owing to the weight of polar ice caps strengthens (pardon the double-entendre) this inference. Again, just my ignorant answer - I'm not a geophysicist and haven't read more than what you quoted. (I do know enough to state that a seismic network on Mars could provide badly-needed data about the martian interior.) Hope this clarifies rather than confuses.

-- HDP Don
dvandorn
QUOTE (dburt @ May 16 2008, 08:14 PM) *
...I do know enough to state that a seismic network on Mars could provide badly-needed data about the martian interior...

Oh, we are *so* in agreement! If there are two sets of data I dearly want from Mars, one is from a sustained seismic network, and the other is from a carefully designed heat flow network.

Those two sets of data could seriously constrain a lot of the current theories of Martian history, IMHO.

-the other Doug
dvandorn
As a general comment to the "discovery" that Mars' crust (for want of a better term) is thick and firm, I thought that had been "discovered" back in the early- to mid-70s when it was found that the Tharsis Plateau was sitting on top of the original crust, which hadn't deformed to a really significant degree. The planet is roughly spherical with a significant bulge above the mean where Tharsis sits.

If the planet can vomit trillions of tons of lava onto a quarter of its surface and the crust doesn't deform a tremendous amount, I can't imagine the lack of polar compression is all that surprising.

BTW -- yes, I know that Tharsis is surrounded by rift valleys formed by compression of the crust under Tharsis. I didn't say there was zero compression. But the mass of lava that makes up the bulge is far greater than the mass of either permanent polar cap, and, unlike the seasonal polar caps, once emplaced the lavas didn't come and go seasonally. The point I recall from Mariner 9 and Viking orbiter data is that Mars is quite significantly out-of-round because a vast majority of the height of the Tharsis lava pile has not been pulled back down to mean over several billion years, which led the scientists of the day to conclude Mars' crust must be very thick and solid...

-the other Doug
Juramike
QUOTE (dburt @ May 16 2008, 08:14 PM) *
Hope this clarifies rather than confuses.
-- HDP Don


Yup. I asked the wrong question by using the wrong terms.
Hopefully, now I can ask better questions:

What is a good estimate for the water content of Martian lithosphere compared to Earth?
In deep crustal rocks, would this be as "free" water or would most of it be incorporated into hydrated minerals ?
How would the lithospheric water (free or hydrated minerals) content affect rigidity? Does it matter what form it would be in (liquid or frozen?)?

-Mike





Juramike
QUOTE (Juramike @ May 15 2008, 02:03 PM) *
From the article:

"Unexpectedly, the radar scans also revealed the massive weight of the ice cap does not deform any underlying sediment. This implies the crust beneath the cap is strong more than 180 miles thick (300 km).

To have such a thick crust, "Mars might be colder than we thought," Phillips told SPACE.com. As a result, any liquid water that might be underground has to be buried even deeper than once speculated. "If one thought that liquid water was 5 kilometers deep (3 miles), it's now at least 30 percent deeper than that," he said."



PIA10652 is a recently released SHARAD and MOLA combo which gives a cross-sectional view of the layers of the polar cap (neat tree-ring effect), also showing the contact between the ice cap and the surface.

There is no crustal sag evident at all.

-Mike


dburt
QUOTE (Juramike @ May 17 2008, 05:53 AM) *
...What is a good estimate for the water content of Martian lithosphere compared to Earth?
In deep crustal rocks, would this be as "free" water or would most of it be incorporated into hydrated minerals ?
How would the lithospheric water (free or hydrated minerals) content affect rigidity? Does it matter what form it would be in (liquid or frozen?)?

Mike - I'm writing this from home, so this is completely off the top of my head, but I'm not sure there's any really good basis at this point for estimating the water content of the martian lithosphere. You'd need lots of well-characterized samples, and all we have now is a few martian meteorites from undocumented sources. My impression is that the deep lithosphere, given that melting it generally produced (produces?) olivine-rich basalt, was/is relatively dry, even more so than on Earth. This is consistent with the very localized hydration seen along fractures in the Martian meteorites - evidence of local, relatively brief exposure to brines, at least in those samples. No wholesale hydration (as in greenschist- or greenstone-type, or serpentinite-type metamorphic rocks formed on Earth via wholesale hydration of basalt and mantle rock, respectively) has yet been detected, AFAIK. Hydrated minerals commonly include phyllosilicates (layer-structured clays and micas), which tend to be somewhat slippery (e.g., talc or "soapstone") and so wholesale hydration would be expected to produce a mechanically weakened rock (unless it consisted of say, higher temperature amphibolite, a stronger rock).

As for the state of water in the deep lithosphere, the big question would be how deep? Where the crust is below or close to the freezing temperature of water, the amount of hydration would be small, owing to kinetic limitations, even if much free water or ice or concentrated brine were present in fractures. Once things got "hot" or even "warm" hydration should be more common, if abundant liquid (or supercritical fluid) were present. The apparent strength of the martian lithosphere might then imply that it is very cold, very dry, or both (and probably not very highly hydrated). Given that ice is much weaker than most rocks except rock salt (as implied by, e.g., its flowing in glaciers) the deep martian lithosphere, even if it is sufficiently cold, is probably not particularly rich in ice (let alone liquid water). The prevailing opinion is that if there's a lot of water on Mars, it probably lies mainly near the surface, in the highly broken regolith that resulted from meteorite bombardment, mainly as ice (with perhaps some highly concentrated brine). The low atmospheric pressure currently keeps it from appearing at the very surface, except ephemerally or at the poles. Again, just off the top of my non-geophysicist head - corrections welcomed.

And dvandorn - that's was a good observation about the Tharsis pile of volcanic rocks. I completely neglected to mention that. Thanks.

-- HDP Don
silylene
QUOTE (spdf @ May 24 2007, 12:38 AM) *
A question here

There are signs that in the past there was liquid water on Mars. So lets assume thats true.
Since the gravity on Mars is much lower than on Earth, so how does water (waves) behave on Mars compared to Earth?
Someone did say, that waves would have been much higher but also much slower. Is this true? Does anyone have an animation where you can see a waive on Earth in comparsion to a wave on Mars?

Thanks


Many chemists use a piece of equipment called a 'rotary evoporator', more commonly called a 'rotovap', to quickly evaporate solvents out of a round bottom flask to leave behind a concentrated liquor or salts. In a rotovap, the flask containing the liquid is rotated at an adjustable speed (which can be a quite fast rpm) + an adjustable vacuum is applied to suck off the solvent + an adjustable optional heating is applied to the flask exterior via a heating bath, which can be heated up to 100C.I n this setup, you can also induce sloshing of the liquid by jarring the equipment. Sometimes it is useful to cause sloshing to prevent 'bumping' which is rapid nucleation and boil-over from a super-saturated condition.

So basically you can watch the behavior of evaporating liquid at various g-forces (centrifugal spinning) of 1 or higher, at various vacuum forces (1 atm to near zero), with applied external heating or not.

If you do this using a rotovap, and induce sloshing, then you can see the behavior of waves as a function of vacuum or as a function of gravity (g=1 or greater). So I tried with water.
It seems that slosh-induced waves settle down the same speed regardless of applied vacuum. (i.e. vacuum has no obvious effect on wave height).
It seems that slosh-induced waves are smaller and settle down faster as the centrifugal force increases. (i.e. increasing gravity greater than earth's gives smaller waves).

From this I would infer on Mars that wave heights would be higher due to a lower surface gravity, and the reduced atmosperic pressure would not be a significant effect on waves.
Juramike
A rotovap simulation experiment!

That...is...so...brilliant!

-Mike

[I can't wait to try it in the lab on Tuesday! So basically the extra G forces are also preventing bumping? This should be easy to check. Put in an organic solvent, place the flask in the bath, and pull a gentle vacuum until a few bubbles start being visible. Then slowly increase the rpm: the bubbles should stop, all things being equal.]

(I used to spin the heck out of things when I was worried about bumping, but I just figured I was smearing the solution around the flask and making more surface area)

The things you learn here....wow.

rlorenz
QUOTE (silylene @ May 23 2008, 11:21 PM) *
If you do this using a rotovap, and induce sloshing, then you can see the behavior of waves as a function of vacuum or as a function of gravity (g=1 or greater). So I tried with water.
It seems that slosh-induced waves settle down the same speed regardless of applied vacuum. (i.e. vacuum has no obvious effect on wave height).
It seems that slosh-induced waves are smaller and settle down faster as the centrifugal force increases. (i.e. increasing gravity greater than earth's gives smaller waves).

From this I would infer on Mars that wave heights would be higher due to a lower surface gravity, and the reduced atmosperic pressure would not be a significant effect on waves.


More of this kind of experiment needs to be done!

Some related wind-tunnel experiments I did are at
http://www.lpl.arizona.edu/~rlorenz/marswit.pdf

Note that while atmospheric pressure doesnt affect waves directly, it appears to strongly influence
how effectively momentum is coupled from wind into the liquid.
I wonder if the sloshing in this rotovap thingy relates to the centrifugal force due to the mechanical
configuration. I'd imagine that these waves are small enough that they are capillary waves (controlled
by surface tension) rather than gravity waves...
Juramike
QUOTE (rlorenz @ May 24 2008, 09:27 AM) *
More of this kind of experiment needs to be done!

Some related wind-tunnel experiments I did are at
http://www.lpl.arizona.edu/~rlorenz/marswit.pdf

I wonder if the sloshing in this rotovap thingy relates to the centrifugal force due to the mechanical
configuration. I'd imagine that these waves are small enough that they are capillary waves (controlled
by surface tension) rather than gravity waves...


Thanks for the paper!


I think the sloshing in the rotovap (image and "semi-accurate" description here - [I can't imagine any chemist adding boiling chips to a solution to be rotovapped]) begins as the rotation of the flask is initiated. The fluid at the bottom of the flask in contact with the glass wall of the now rotating flask gets pulled along with the glass wall due to friction. The solution up higher, closer to the surface finds itself with the bottom pulled out from under it, so it drops due to gravity. I think this is responsible for the intitial slosh. Larger volumes have less of an effect on rotovap startup. If you crank up the rotation immediately you get a big slosh, if you gently increase rotation (preferred) you get less then slosh and prevent potential bumping (violent boiling).

Since you have a flask that is rotating, and flasks are rarely perfectly symmetrical at certain rotation speeds you can get resonate effects that can cause a standing waves to occur in the solution due to the back and forth shimmy of the rotating flask. Some of these could get to the point of being controlled by gravity pulling the peaks back down.

[Capillary waves vs. gravity waves: http://en.wikipedia.org/wiki/Capillary_wave]

Since most smaller rotary evaporators hold the flask at an angle, you can usually get a nice swirl in the surface of the solution due to differential rotating effects. (the "equator" of the flask is rotating faster than the "mid latitudes" of the neck.)


I wonder if a secret probe cam inserted down the neck of rotovap (and sealed on the back end with epoxy) could provide useful information?

(Rotovaps are relatively expensive, so this wouldn't be something to do unless you had surplus rotovap equipment.)

-Mike
silylene
I agree that the problem with resonance waves makes interpretation qualitative.

The flask was about 20% full, and at a shallow angle, which is necessary as was mentioned. Jarring was intentional. AsI tried to indicate, this was just a quick, simple experiment.

How to improve?
I think that the best way would be to use a round flask which has small internal baffles. This would force the liquid to pass over the baffles as the flask turned, and set up turbulence waves. The turbulence height of the induced wave as the liquid passes over the baffles could be measured as a function of vacuum, turn rate, and temperature.
SickNick
QUOTE (Juramike @ May 17 2008, 10:53 PM) *
Yup. I asked the wrong question by using the wrong terms.
Hopefully, now I can ask better questions:

What is a good estimate for the water content of Martian lithosphere compared to Earth?
In deep crustal rocks, would this be as "free" water or would most of it be incorporated into hydrated minerals ?
How would the lithospheric water (free or hydrated minerals) content affect rigidity? Does it matter what form it would be in (liquid or frozen?)?

-Mike


Water content - nil

ICE content - quite a bit...

The melting depth is 12-20 km on Mars.

of course, since the lithosphere extends to 300+ km, there's plenty of "wet" lithosphere, but down at those depth there's not much porosity. A few % of H2O, and probably very similar to Earth rocks under the same pressure (38% of the depth). As to mineral incorporation, this is temperature-dependent and this will also scale with heatflow on Mars (much less than gravity on Mars, compared to Earth)
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