Temperature and pressure at Gale, Suitable (for short periods) for liquid water? Options

[Deimos]

The pressure plot is interesting because of the local minimum of pressure--like there will be at least two minimums a year, assuming it's due to the solar cycle and not some other factor.

Right now, northern polar CO2 is starting to sublimate. Within a couple months, it'll be all gone and condensation in the south will dominate until southern spring approaches and the Sun begins to rise on the Southern seasonal cap. Maybe a local maximum around 8.8-8.9, around 460 or so?

[jmknapp]

Any water pushed to the surface of Mars would "explode" into vapor.

Guess I don't follow--isn't the (low) pressure already accounted for in the phase diagram?

Seems like a lot of people have mentioned that the conditions on Mars are just on the cusp of the wet side of the triple point, like Making a Splash on Mars:

The air pressure is so low on Mars that even in the most favorable spots, where the pressure is higher than average, liquid water is restricted to the range 0 to +10 °C," says Bob Haberle of the NASA/Ames Research Center. ...

"First of all, you have to remember that the average atmospheric pressure on Mars is very close to the triple point of water," explains Richard Hoover, an astrobiologist at the Marshall Space Flight Center. "You only have to increase the pressure a little bit to make liquid water possible."

The 'triple point' is the combination of pressure (6.1 millibars) and temperature (0.01 °C) at which water can exist simultaneously in all three states: a solid, a liquid and a gas (see the 'phase diagram' below). ... On Mars the globally-averaged surface pressure of the planet's atmosphere is only slightly less than 6.1 millibars.

"That's the average," says Haberle, "so some places will have pressures that are higher than 6.1 millibars and others will be lower. If we look at sites on Mars where the pressure is a bit higher, that's where water can theoretically exist as a liquid."

It may evaporate, but necessarily "explode"? For example, the high temperatures for sols 100-120 were between 0 and 10 °C and the pressure about 8.5 mbar.

[Harder]

The term "explode" came to my mind from 1st hand experience working in an oil refinery: in a so-called vacuum tower, a huge distillation tower with trays (internals) designed to separate heavier oil fractions under medium vacuum conditions, it is customary to use steam as a "stripping medium" i.e. supply steam to the bottom of the tower to make sure that lighter components are maximally recovered from the asphalt-like bottom product of the vacuum tower.

A well-known pitfall during start-up of such a column is the situation where the stripping steam is not dry and superheated but "wet". When liquid water enters the vacuum tower through the stripping steam inlet it literally "explodes" due to its huge volumetric expansion from liquid to vapor, damaging the internals of the vacuum tower in the process. The analogy is in fact not to bad, because distillation towers also have other components (molecules) in the vapor phase, similar to N2, CO2 etc in the vapor phase (atmosphere) of Mars.

But on Mars the situation is much more extreme: the vacuum is much better that in my oil refinery! So I'm really confident that any liquid water being pushed to the surface of Mars will instantly vaporize. The only possible exception being water in the from of brines where the freezing point as well as absolute quantity of water in the brine may be so low that indeed "explode" is an over the top characterization. Sorry for that. Another factor that indeed makes "explode" less likely is that the very substantial heat of vaporization that is necessary to vaporize water is readily supplied in the distillation column (with devastating effects!) but not so on Mars. So when water vaporizes on the surface of Mars it has to withdraw the heat of vaporization from its environment which may slow (but not halt) the vaporization process.

I hope there are other chemical engrs in this forum who may chip in and corroborate, but the scientific bottom line is unfortunately firmly against any liquid water on the surface of Mars due to the extremely low pressure. The equilibrium equations for H2O simply dictate vapor phase, I'm sorry if I pour "cold water" (pun intended) on the hopes and aspirations of other forum members!

Regards,
Peter

[djellison]

the high temperatures for sols 100-120 were between 0 and 10 °C and the pressure about 8.5 mbar.

At those pressures, whilst we are above the triple point we are in the realm of a very narrow wedge for the fluid phase. The difference between melting point and boiling point at those pressures is less than 10 degrees. Indeed - those 'hotter' days are almost certainly well above the boiling point.

Hence why the occasional fluid flows seen in crater walls have been described as a boiling torrent. That's what they would look like.

For a crude playground of values...
http://www.trimen.pl/witek/calculators/wrzenie.html

As an example, you can put in the 760mm pressure and 100 degC for water then put in your own pressure.

8mm of pressure ( >10mb ) has a boiling point of only 4 degrees.

So - do we reach the melting point of ice. Yup. And almost every time we do, we probably go over the boiling point as well.

[Explorer1]

The lowest regions of the planet do have higher pressures; Hellas basin's bottom is at over 11 mb, and we know Martian summer days can get above freezing as Doug said (milder given the southern locale). Obviously Hirise hasn't noticed anything, and there has been no landing with something like REMS for ground truth. If there's ice hidden under the surface, like Phoenix saw, a trench at the right time might have results...

[djellison]

like Phoenix saw

Indeed - and the next time we looked, it was gone.

[jmknapp]

I hope there are other chemical engrs in this forum who may chip in and corroborate, but the scientific bottom line is unfortunately firmly against any liquid water on the surface of Mars due to the extremely low pressure. The equilibrium equations for H2O simply dictate vapor phase, I'm sorry if I pour "cold water" (pun intended) on the hopes and aspirations of other forum members!

More to the point, perhaps you are throwing cold water on the afore-quoted NASA scientists!

But how about an actual experiment--namely from this paper:

The Hunt for Liquid Water, Life and Landing Sites on the Surface of Mars Today

The authors note a possible ambiguity in interpreting the phase diagram:

A question frequently asked is whether the abscissa in Figure 3 is total pressure or partial pressure of water vapor. If the former, the pressure on Mars is frequently above the triple point. If the latter, the pressure would always be below it since the partial pressure of water vapor in the atmosphere is only a fraction of a millibar. This question will be addressed in the experimental methods section of this paper.

They simulated the Mars environment as follows:

Simulating Martian conditions in a bell jar was the objective of the experimental phase of this study. An ice cube in a glass funnel placed inside a bell jar containing Drierite (a desiccant), calibrated thermometers, and dry ice (to create a CO₂ atmosphere) was kept under Martian pressures by a vacuum pump.

Result:

At a mean ice temperature of 0°C, as seen in figure 8, liquid water was observed at pressures between 3 mb and 10 mb, Mars like conditions. This data demonstrates that liquid water can exist under these simulated Martian conditions. ...

The purpose of the bell jar experiment was to determine the feasibility of liquid water under Martian conditions. This condition was met. Additionally, we can conclude that total pressure drives the phase change of water, not the partial pressure of water vapor in the atmosphere.

After all, even though there's almost no water in the Mars atmosphere, there are still CO₂ molecules banging against the surface of any liquid. Maybe where you go wrong is focusing too much on the equilbrium state. If I put a dish of water out in my back yard, the equilibrium state is clearly the vapor phase, although it will take some time to get there.

[Explorer1]

That's the point I was making as well; unlike the 'typical' Mars conditions that Phoenix encountered at Vastita, subsurface ice at Hellas (if it exists) could melt if a trench exposed them one warm summer day, at least until nightfall.
This is all academic until someone actually does the experiment of course; none of Exomars' candidate landing sites include that area...

[Harder]

I stand corrected.. Where I went too fast was in assuming that ground ice does not exist because it is not stable in the present time and that liquid water could thus only feature in transient events - measured in hours (as I read in Wikipedia) - with thermodynamics inexorably determining the final outcome.

Many thanks for the reference, Joe, I've now also read the article "On the possibility of liquid water on present-day Mars" by Mr. Haberle et al. I recommend everybody interested in the subject to look it up and read it. It also makes a strong case for further experiments - as brought up by Explorer1.

[Explorer1]

I do wonder if the Exomars rover's drill would be suitable for such an examination. It's designed to go a lot deeper than any scoop, but it's a corer; so material wouldn't really be exposed per se. In any case, a low elevation is still the main requirement.

[serpens]

It is a little difficult to correlate the theories and simulation experiments regarding liquid water on Mars with the presence of kieserite. Either there is something about the stability field of kieserite under Martian conditions that we do not understand or the simulations/theories on water on Mars are missing something.

[jmknapp]

I've now also read the article "On the possibility of liquid water on present-day Mars" by Mr. Haberle et al. I recommend everybody interested in the subject to look it up and read it. It also makes a strong case for further experiments - as brought up by Explorer1.

Yes, that paper is written very clearly! If I read it right, they don't identify the Gale Crater region specifically as having the amount of "degree days" conducive to liquid water, but the date of it is 2001 and to date quite a bit more is known about the Mars climate and of course there's the ground truth provided by Curiosity.

[Gerald]

If, as it looks, perchlorates are more or less ubiquitious at the Martian surface, is it realistic to look for pure liquid water on Mars?
As perchlorates are highly hygroscopic, I'd expect water to be bound either as crystal water to perchlorates or to form a brine in the sense of this LSPC 2009 paper. Such brines may stay liquid down to -70°C.
So, I'd suggest it's worth to think about phase diagrams containing (perchlorate) brines and water bound in crystals under equilibrium conditions with water vapor partial pressure in the atmosphere. I'd guess, that the water vapor partial pressure needed for an equilibrium with a perchlorate brine at low temperature is much lower than for pure water at the triple point.
The Phoenix results about perchlorates weren't yet available in 2001. Therefore the paper looks a bit obsolete to me.

[djellison]

Such brines may stay liquid down to -70°C.

What's its boiling point?

Taking brine as an example - that can reduce the freezing point (at 1 atmos) to -21c. However, boiling point goes up just a couple of degrees.

So - frequent weather conditions on Mars would exceed the melting point of such brines...rendering liquid water possible on the surface.....and then rapidly exceed the boiling point ( which is likely to be single digit degC at Martian air pressure ) rendering the water transient at best

[Gerald]

At 10 mbar the boiling point of a saturated Mg(ClO4)2 solution is about 295K (using 10mbar partial pressure at 295K as estimate for the boiling point, according to the light brown cross in figure 3 of the LPSC paper).
So the evaporation of water is slowed down at day time by the brine, not stopped.
But at night the brine may form again due to deliquescence. So we may get a day/night cycle of salt/brine from air humidity, without need for water supply from underground.

If there is any chance for water to stay liquid at or near the Martian surface, then via perchlorates.
Without perchlorate the water freezes at night and evaporates at day, with minor chance to ever stay liquid.

Obviously the chances are better closer to the poles than near the equator, because freezing (below -70°C) is less a problem than evaporation.

Edit: According to figure 7 of this Phoenix paper the deliquescence threshold for Mg(ClO4)2 is near a relative humidity of 50% . At the same time the temperature needs to be above -70°C to form a brine.