Water vapor on Mars?

jgarzik · 2012-09-11 13:35:18

What does the literature say about water vapor on Mars?

An ESA page noted possible water vapor 25km above the planet's surface.

Is there any water vapor in the air at night?

Koeng · 2012-09-11 16:26:32

Hum... Nice question. I would love to know the answer. That would make it very possible for Bacterial life to exist there. Last time I checked... No. Please prove me wrong

-Koeng

Last edited by Koeng (2012-09-11 16:27:13)

Koeng · 2012-09-11 18:36:40

Yah, I couldn't find anything that said "liquid" water only frozen

-Koeng

louis · 2012-09-12 12:07:29

Koeng wrote:

Yah, I couldn't find anything that said "liquid" water only frozen
-Koeng

So what was that drop of liquid on the Phoenix lander in the freezing north?

RobertDyck · 2012-09-12 13:29:30

There is lots of evidence of liquid water on Mars. MGS found rivulets down the side of gullies and "valllis". There are many examples where they took two images separated by time, and the rivulets changed. That indicates liquid water, due to permafrost melting. That only happens near the equator, only in the afternoon, only in direct unlight, and Mars has a lot of salt in the soil so any liquid water is brine. That water soaks into dry soil quickly, and evaporates. Mars air is very dry, but it's so thin that liquid water evaporates slowly.

Surface pressure is low, but it is above the triple point. Mars Pathfinder recorded pressure between 6.77 and 7.08 millibars. Liquid water can exist above 6.12 millibars. Below that it will sublimate like dry ice, but only the tops of mountains have pressure below the triple point. Pressure can change freezing temperature, but at Mars pressure (other than mountains) it will change by less than 1°C. High temperature for Mars in the first year MGS collected data was +24°C, but that was right at the surface, and at the equator, and that was the plantary high.

There are pictures of dark spots that appear to be pools of liquid brine on the surface.

As for humidity, atmospheric gas was analyzed by Viking landers. Viking 2 recorded 0.03% water vapour.

GW Johnson · 2012-09-12 17:30:37

There's a common error I keep seeing regarding stability of water in near-vacuums like Mars. That 6.1 mbar triple point pressure is a water vapor partial pressure, not a total atmospheric pressure, and it refers to stable equilibrium conditions relative to a liquid pool at ) C. Look in any standard steam table, or ask user Midoshi, he will confirm what I am saying. That pressure in the steam tables is specifically labeled as "water vapor partial pressure".

The sum of all the partial pressures (water vapor and all the dry gases) is the total atmospheric pressure, which in turn is what you measure with a transducer,, gauge, or manometer. At higher liquid pool temperatures, the equilibrium water vapor partial pressure that is required is even higher. That's exactly the numbers in the steam table.

That being said, liquid water can indeed exist temporarily on Mars at 6-7 mbar total atmospheric pressure. It's just not in equilibrium, and it will eventually evaporate/boil away. The more there is, the longer it can last, and the longer a stream channel it can make. But it does finally "go away".

It's the same for the exposed ice (which we have already seen with the holes dug by the Phoenix lander): ice requires 6.1 mbar water vapor pressure in the atmosphere for 0 C ice to be in equilibrium. Colder ice temperatures require less water vapor partial pressure above it to be in equilibrium. Anything less than the equilibrium partial pressure of water vapor, and the ice sublimes. We've already seen it in action at Mars (Phoenix).

There is something about sufficient regolith cover that isolates buried ice from the lack of water vapor partial pressure in the (exceedingly-dry) Martian atmosphere above that regolith. We've already seen that, too, in the Phoenix holes. Apparently, it only takes a couple of inches (around 5 cm) to have that effect on Mars at 0.38 gee gravity. The regolith density ought to look just about like that of sand or dirt here: around 105 lb/ft3 (about 1683 kg/m3) effective mass density (there's considerable void volume due to the spaces between the grains).

BTW, a recorded humidity (relative) of 0.03% is pretty darn close to being equivalent to a 0.03% volume fraction (0.0003 without the % sign) of water vapor, which is 0.03% of a Martian atmosphere's worth of partial pressure. The calculated Martian partial pressure of water vapor at 6 mbar is then 0.0018 mbar. At 7 mbar atmospheric pressure, the water vapor partial pressure is about 0.0021 mbar.

Yep, that atmosphere is very, very, very, very dry.

GW

Koeng · 2012-09-12 18:10:04

Very interesting....

-Koeng

jgarzik · 2012-09-12 18:30:14

For higher up in the atmosphere, this article appears in searches, from Sept 2011:

ESA orbiter discovers water supersaturation in the Martian atmosphere
http://sci.esa.int/science-e/www/object … ctid=49342

And also interesting was the "Mars Water Project" spanning 1988 - 1997, http://www.lpl.arizona.edu/~sprague/planatmos/mars.html

But the most interesting was http://web.ics.purdue.edu/~nowack/geos1 … ture14.htm which notes

Since the atmosphere of Mars is so cold and thin, it can hold very little water vapor. In this sense, it is dry. However, it holds about the maximum it can at its pressure and temperature. So, “relative” humidity on Mars is very high. It is thus common to see water vapor fogs to form at low elevations at night.

The lecture continues and covers the subject well, including a review of the general behavior of water at these atmospheric pressures and temperatures.

Given all these caveats, nonetheless it would be interesting to know what a "low elevation" is, and typical pressures and temperatures one would encounter at night.

Last edited by jgarzik (2012-09-12 18:30:50)

louis · 2012-09-12 18:35:00

Some interesting photos:

http://www.universetoday.com/17854/clum … nder-legs/

RobertDyck · 2012-09-13 08:52:33

GW Johnson, I respect your knowledge greatly, but I have to respectfully disagree. Total atmospheric pressure affects the boiling and freezing temperature of liquids. Each liquid behaves differently, for example CO2 forms a liquid at very high pressure, but at normal atmospheric pressure on Earth it cannot form. Dry ice at 1 atmosphere pressure will sublimate, which means a direct transformation from solid to gas. By the way, what you see is water vapour in the air condensed by cold CO2 gas, not the CO2 itself. Similarly water requires certain pressure to form a liquid. If total atmospheric pressure is below a certain point, it cannot form a liquid. In the hard vacuum of space, water ice will sublimate, it will not melt. However, pressure on Mars is sufficient that water ice will actually melt, not sublimate.

That said, humidity on Mars is extremely dry. Any exposed liquid water will evaporate quickly. Relatively quickly, the thin atmosphere of Mars cannot hold a lot of water, so a pool of water will still take some time to evaporate.

Here is an image taken by the European orbiter called Mars Express. It shows a lake of blue ice, water ice, in a crater on Mars. Note frost on the crater rim. This is near the north pole of Mars.

This HRSC image provides a perspective view of residual water ice on the floor of Vastitas Borealis Crater on Mars.
The image is centred at 70.17º North latitude and 103.21º East longitude.

Last edited by RobertDyck (2012-09-13 08:56:49)

RGClark · 2012-09-13 11:00:27

Perhaps the disagreement is coming from two different uses of the word "stable". One use of the word is whether or not water will boil away, another is whether or not it will evaporate away. Even on Earth where surface conditions won't have water boil away, the evaporation rate can be quite rapid on, say, a hot Summers day. Would you say water is unstable during Summer days on Earth because a puddle of water will evaporate away within hours to even minutes?
The now famous gullies on Mars led Mars scientists to further understand the conditions under which water can appear on Mars. They use the term "metastable" to describe liquid water on Mars. By this they mean water won't necessarily boil away, but it will be subject to a rather rapid, comparatively, evaporation rate. Note that by "rapid", this does not mean instantaneous.
Do a Google search on "metastable", Mars, and "liquid water" for more on this.

Bob Clark

jgarzik · 2012-09-15 07:31:13

This is the reply received from a University of Arizona researcher, when asked the question "how much water vapor might be found on ground level at night?"

I get about 1.3e11 cm-3 for a nighttime temp of -120 C and a pressure of 2.7e-4 Pascals (the vapor pressure of water at that temperature).

Midoshi · 2012-09-15 18:42:30

After carefully reading the posts of both RobertDyck and GW Johnson, virtually everything you have both said is technically correct. I'm not sure why there is a perception of disagreement?

The reason you get stable water ice near the Martian poles, as in Robert Dyck's last post, is because it is very, very cold. The ice is in some ways more like rock than our mundane concept of water ice (and rocks tend not to sublimate easily). The relative humidity at the poles is also quite high, often approaching 100% (low relative humidity drives sublimation/evaporation). Of course, the specific humidity, i.e. the actual amount of water in the air, is still very low because it is so cold. Near the equator things are warmer, but the relative humidity is very low (as is the specific humidity, again). This is because the Martian poles are the only major source of water to the global atmosphere, and whatever specific humidity you pull off the poles is the maximum you have to work with on the rest of the planet.

One challenge for free liquid water at the surface of Mars is that while the atmosphere does have enough pressure to support liquid water across much of the surface, if you raise the temperature just a little above freezing, say to 3°C, the water will boil. This is a very narrow region of (potential) stability, and to simultaneously get a relative humidity high enough to prevent rapid evaporation is almost impossible (at least under the current climate; things may have been different in just the past few million years). There are a lot of different effects conspiring to make liquid water unstable at the surface.

The reason water ice can be stable in the subsurface at midlatitudes (and metastable in tropical glaciers) is partly because daily temperature swings are moderated so that the temperature, and thus vapor pressure, stays low during the afternoon, and partly because of the barrier to diffusion out to the atmosphere that the overlying layer of regolith provides.

The evidence for flows of material associated with water on Mars are thought to be due to very, very thin layers of "liquid-like" water, possibly only a few molecules thick, that form on regolith grains. This can occur at temperatures far below freezing because interactions with the grain surface "loosen up" the water molecules, which all have easy access to the surface to be influenced (it's only a few molecules thick, remember). But this is enough to lubricate the grains and allow flow behavior; or so the reasoning goes. Attempts to spectroscopically detect water in the flow regions has failed, which confirms that there is very, very little water involved.

Koeng · 2012-09-15 20:19:13

Interesting! First good conversation on water on mars since 2011 !

thanks jgarzik for making this and thanks all posters!

-Koeng

RGClark · 2012-09-16 00:34:02

jgarzik wrote:

This is the reply received from a University of Arizona researcher, when asked the question "how much water vapor might be found on ground level at night?"
I get about 1.3e11 cm-3 for a nighttime temp of -120 C and a pressure of 2.7e-4 Pascals (the vapor pressure of water at that temperature).

That description looks like a calculation rather than a measurement. The water vapor content measured by previous Mars missions in absolute terms was only in the range of .03%:

http://en.wikipedia.org/wiki/Atmosphere_of_Mars

Bob Clark

RobertDyck · 2012-09-16 06:58:11

jgarzik wrote:

This is the reply received from a University of Arizona researcher, when asked the question "how much water vapor might be found on ground level at night?"
I get about 1.3e11 cm-3 for a nighttime temp of -120 C and a pressure of 2.7e-4 Pascals (the vapor pressure of water at that temperature).

Night time temperature of -120°C only occurs at the poles, and only when that pole has winter. Temperature measured by Pathfinder was -77°C, and Phoenix was around -80°C. Phoenix landed near the north pole, but that -80°C temp was in summer. And the question of "water vapour at night" has to include condensation. So how much water during the day? Then how quickly will that condense out at night? Remember Pathfinder recorded -8°C in the day.

Viking 2 recorded 0.03% of the atmosphere was water vapour, and 7.5 millibar total pressure (750 Pascals). That should be 0.00225 millibar partial pressure water vapour (0.225 Pascals). Again that was not a pole during winter, so temperature was not -120°C.

Viking 2 recorded temperatures for more than a Martian year. The low it recorded was -111°C. That was at night, in the depths of winter at that latitude.

GW Johnson · 2012-09-18 16:39:08

I guess what I have been trying to say is that the triple point of water is but one point in the standard steam tables. If you read in the thermo textbooks what it says about those tables, then you understand that what is tabulated there is values of pressure and temperature at which condensed and vapor phases of a PURE SUBSTANCE can coexist IN EQUILIBRIUM, with both (or all 3) phases at the same temperature. (Caps are for emphasis only, I'm not shouting, please don't misunderstand!)

They make an assumption in these books of a simplified model to use these in the presence of a SECOND SUBSTANCE, i.e., the gases in an atmosphere that can get humidified, up to a maximum, by the water vapor. That simplified model says (1) the atmosphere is a mixture of ideal gases (far from triple point), and (2) no gases dissolve into the water or the ice. There is a 3rd thing, but I forget now what it was. I'm getting old, so please excuse my failing memory. I first studied this subject over 4 decades ago.

But, everything is assumed to be at equilibrium. The gas laws say the total atmospheric pressure is the sum of the partial pressures of the water vapor and all of the ideal gas mixture components. Equilibrium says the water vapor pressure from the steam tables is the water vapor partial pressure in the atmosphere. If it cannot meet this ncondition, it cannot be in equilibrium: it can still exist, but it cannot persist.

We typically mis-use this model here on Earth to compute a water vapor pressure above a pool of liquid water, with a different air temperature. The vapor will mix into the air, and the gases will dominate the mixed gas-phase temperature, so that the vapor and liquid are not at the same temperature. Yet, it is still pretty accurate to do it this way. Weathermen do it all the time, and quite successfully, I might add.

The "hard" part is equilibrium, which is never approached very closely in the real world, either here or on Mars. The low vs high humidity disparity drives evaporation speed, yes, which is an inherently nonequilibrium process. Here, because pressures are near 500+ mbar even on the mountains, there is usually plenty of "room" in the total pressure for 100% relative humidity, regardless of the liquid pool temperature, except for very frigid air temperatures. That gets limited by the max absolute humidity.

Having "enough room" for triple-point levels of partial vapor pressure is not ever true on Mars. At the 0 C triple point, 0 C liquid water AT EQUILIBRIUM would require 6.1 mbar water vapor partial pressure above it not to evaporate, perhaps violently. The total atmospheric pressure on Mars is only about 7 mbar all over the lowlands. That's only 0.9 mbar CO2 pressure, something we know cannot exist on the lowlands, because we already know the Martian atmosphere is very dry (0.03% relative humidity typical), and measures about 7 mbar total. You might violate that in a transient very locally, but as the winds mix it into the atmosphere generally, it cannot persist with that much water vapor.

That CO2 cannot hold very much water vapor without water condensation, even if it were as hot as our air. Same is true here with our air. That's the dew point, and it sets a limit on how much water vapor partial pressure you can really have at equilibrium. The colder the gas, the smaller the absolute humidity (water vapor partial pressure) you can have, right down to the triple point.

If your max allowable-by-humidity water vapor partial pressure is under 6.1 mbar, then even a 0C pool of liquid evaporates by boiling, no matter what the total atmospheric pressure is. Those conditions exist today on Mars. You can temporarily have a stream or river by any conceivable means, but it is not "stable" in the sense that it is boiling away (some of that vapor likely falls as snow nearby, but even that sublimes later). Eventually it is gone, unless you have some means to replenish it at or above the loss rate.

Same is true for 0 C ice: if the water vapor partial pressure is under 6.1 mbar, the ice sublimes. The vapor mixes into the far more massive atmosphere (thin as it is), and essentially is 0% of the total. That's why all the ice the landers have dug up has sublimed away in a matter of hours to a day or so. In that sense, water ice is not stable when exposed to the Martian atmosphere. Any exposed ice we see there has either been uncovered recently, or was very much larger in size in the past. You cannot get away from that picture.

Sometimes, in some places, it gets above 0C air temperature on Mars. (Actually, the surface regolith will warm more reliably in the sunlight than the air, because that air is so very thin.) In any event, a piece of ice might melt and form a puddle of water. Temporarily. That puddle will evaporate, and rather quickly. I think it will boil, and may actually freeze in the act of boiling, just like a dish of water in a bell jar that you pump the air out of. Then, left in near-vacuum inside that bell jar a while, the ice sublimes. You may get frost elsewhere in the bell jar as this happens, due to absolute humidity limits. Same happens on Mars, except it's all out in the open.

To make exposed water or ice stable on Mars, the CO2 atmosphere must be very much thicker. The total pressure must be high enough so that the dew point absolute humidity corresponds to a minimum of 6.1 mbar water vapor partial pressure. I don't have a hygroscopic chart for a CO2 atmosphere, mine is for air. But it's exactly the same idea.

That being said, the Martian air has indeed been very much thicker in the past. Perhaps only a few million years ago, certainly about 3-4 billion years ago. How else could there be lake and ocean shorelines and dendritic river channel systems preserved on its surface?

I think we're seeing transient phenomena associated with both water and CO2 on Mars. That's clouds and snow. Not much in the way of liquid water other than some very transitory dewdrops, and most certainly no liquid CO2.

Just offering my humble opinions. But I do know my classical thermodynamics. Even if I forgot that 3rd thing above.

GW

Vincent · 2012-09-19 12:47:53

I am more of a "show me the baby" type of guy. Clouds are a visible accumulation of vapor on condensation nuclei. We know it is H2O because of the instumentation onboard Phoenix. This is a "natural color" image of the clouds from Phoenix. We don't use the term, "true color" any more.

This is one of my favorite images from "Hortonherdawho" of "water ice clouds" drifting overhead at high noon.

The total water(H2O) vapor content of the Martian Atmosphere is .03% if I'm not mistaken. Numbers can lie, and liars figure.

Vincent

clouds 2 by dfrank39, on Flickr

Last edited by Vincent (2012-09-19 12:54:45)

Koeng · 2012-09-19 16:22:28

I wonder if bacteria could live in those clouds, much like earth, and that is where they get their water from...

-Koeng

SpaceNut · 2014-12-24 17:54:19

Russian scientists from the Space Research Institute of the Russian Academy of Sciences and the Moscow Institute of Physics and Technology (MIPT), together with their French and American colleagues, have created a 'map' of the distribution of water vapour in Mars' atmosphere.

Russian scientists 'map' water vapor in Martian atmosphere using data collected over ten years by the Russian-French SPICAM spectrometer aboard the Mars Express orbiter.

latitudinal-distribution-humidity-mars-atmosphere-through-year-spicaminfrared-lg.jpg

Conditions on Mars -- low temperatures and low atmospheric pressure -- do not allow water to exist in liquid form in open reservoirs as it would on Earth. However, on Mars, there is a powerful layer of permafrost, with large reserves of frozen water concentrated at the polar caps. There is water vapour in the atmosphere, although at very low levels compared to the quantities experienced here on Earth.
If the entire volume of water in the atmosphere was to be spread evenly over the surface of the planet, the thickness of the water layer would not exceed 10-20 microns, while on Earth such a layer would be thousands of times thicker.

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Water vapor on Mars?

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