Martian surface water
Published on Dec 8, 2006 at 2:02 pm.
8 Comments.
Filed under Mars.
On my last post, Ed posted a comment with a link to his web site showing photos from the Spirit rover that seem to show a drop of water. (Ed links to Charles Shults page, where the photos originated, but the link there to the pictures is broken as of this writing). You should take a look. This seems most interesting. It does look like a drop of water. So, I got to thinking. How likely is liquid water on Mars?Â
The standard party line is that the temperature and pressure on Mars do not permit liquid water at the surface. But, how true is that?  Is that a blanket statement, or are there some rare times that liquid water is possible? What do you need for liquid water? Well, to answer that question, you need to study water.
Materials are typically in one of three phases of matter: solid, liquid, or gas. Going from one to the other is a matter of adding or subtracting energy from the molecules. It is also dependent to a degree on external conditions, such as pressure. Under pressure, you can force a gas into a liquid state by forcing the molecules closer to one another. Conversely, if you reduce the pressure, then you make it easier for a liquid to become a gas, at the same temperature. At what temperature does water boil? That’s easy, you might say. Water boils at 100° C (212° F). We drill this into our school children in science classes. But, it doesn’t always boil at that temperature. Rather, that is the temperature that pure water boils if the pressure is a standard 1.00 atmospheres. If the pressure drops, so does the boiling point. At high altitudes, water boils at a lower temperature. That is why baking directions on boxes of cookie mix tell you to bake at a lower temperature for longer at higher altitudes. Otherwise, instead of delicious cookies, you get dessicated cookie-like things. You can represent the relationship between state of matter, temperature, and pressure with a diagram called a phase diagram. The phase diagram of water is shown here.
Notice that there’s a point on the diagram where all three phases come together. This is known as the triple point of water.  At pressures below the triple point, water can not exist as a liquid — it is either a solid or a gas. The triple point of water is at 0.00604 Earth atmospheres and 0.01° C. Now, just about every textbook in introductory astronomy, and most everything that you read about Mars will state simply that it is too cold and too low pressure for Mars to have liquid water. Well, as it turns out, as often happens, that sort of blanket statement is a bit simplistic.
As it turns out, the average pressure on Mars is right at the triple point of water. Likewise, the atmospheric temperature is too low for liquid water. But, the important thing to remember is that these are average values. By definition, sometimes the values are above average and sometimes below average. The pressure in the Martian atmosphere varies wildly — far more (percentage wise) than does Earth’s. Mars also has vast differences in elevation, and the lower the elevation, the higher the pressure. As it turns out, at the high end of this range, the pressure is just barely above the triple point. And while the atmospheric temperature is a bit cold for liquid water, the Sun can warm the ground temperature sufficiently during the summer to a point where water would not be frozen. So, you can imagine that both events might happen to permit liquid water. The yellow box on the above phase diagram shows the approximate extremes of temperature and pressure.
Unfortunately, the highest pressures come at the lowest elevations, which are typically a ways to the south or north. Too far south or north, and it will simply be too cold for liquid water. Also, the pressure is typically lower in the middle of summer or winter. But, perhaps near the end of summer, the temperature will be warm enough and the pressure high enough for at least a few moments water to be liquid.Â
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The new gullies mentioned in the news release are located 35 to 40 degree south. Unfortunately, the southern part of Mars is the higher elevation. But, if you look at the coordinates given, one set of gullies falls near Hellas Planitia, one of the lowest parts of Mars. The other is in a slightly lower than average part of southern Mars a bit south of where Spirit landed (which is in a crater). That is also not too far from where air would flow down off of the Tharsis Plateau. So, perhaps certain freak conditions might make for slightly warmer and higher pressure than normal, permitting liquid water for just a short time at the surface. I really doubt that it stays liquid for long. Among other things, the temporary conditions permiting that phase will change. When the Sun sinks lower in the sky, the temperature will plummet. When it is winter, it is too cold. When the pressure changes, it is too low. And, the Martian atmosphere is pretty dry, so evaporation will be a major factor. But, for a moment …
Interesting. Mars is a bit more complicated that would first appear, like so many things. And that makes it all the more interesting.
-Astroprof
(Images courtesy of NASA)
Ed Minchau on December 8, 2006 at 7:11 pm: 1
Boy is my face red. I found another microscopic image of the same area on the rover, and it shows the same feature. It is highly unlikely that water condensed on the pancam and then dripped onto the top of the rover in the same spot and in the same quantity. More likely, I was wrong and it is indeed a part of the rover itself, perhaps some clear epoxy or something.
Ed Minchau on December 8, 2006 at 7:12 pm: 2
… and I should proofread before I hit Submit. That other image is from sol 327, two years before the sol 1006 image I showed on my site.
Astroprof on December 8, 2006 at 7:30 pm: 3
Well, it did look like water. And the gullies look like water did them. But, not everything is what it appears to be. It is easy to be fooled!
Dennis Acklin on December 9, 2006 at 5:32 pm: 4
the pressure is far greater in the Great Vally. thats where to look for liquid water
Astroprof on December 9, 2006 at 11:16 pm: 5
Valles Marineris also is close enough to the equator to be warm. I agree, that is the best place to look for occasional water.
mike on December 26, 2006 at 9:39 pm: 6
what would be the effect of mars’ gravity on the behavior of the water?
mike on December 26, 2006 at 9:43 pm: 7
would it effect the water as a whole entity (making it less dense) or effect each bond of the molecules changing the thermodynamic properties?
Astroprof on December 26, 2006 at 10:03 pm: 8
About the only effect of the lower Martian gravity would be to reduce the atmospheric pressure a bit, and to reduce the pressure underground. The triple point isn’t changed.