A question here, behaviour of water on Mars Options

[spdf]

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... )

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!

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  water2.pdf ( 134.75K ) Number of downloads: 1232
, 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.

[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]

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  water2.pdf ( 134.75K ) Number of downloads: 1232
, 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.

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.

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.