A question here, behaviour of water on Mars Options

[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]

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]

...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]

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]

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]

...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]

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]

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]

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]

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)