Temperature and pressure at Gale, Suitable (for short periods) for liquid water? |
Temperature and pressure at Gale, Suitable (for short periods) for liquid water? |
Sep 30 2012, 03:23 PM
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#1
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Junior Member Group: Members Posts: 62 Joined: 11-July 11 Member No.: 6058 |
Just a quick query from someone with no background in science. Obviously, MSL has AFAIK not returned evidence of recent (i.e. years/decades) liquid water in its vicinity; however, I was interested by the following graphs:
08.21.2012: First Pressure Readings on Mars http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4501 08.21.2012: Taking Mars' Temperature http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4502 The first indicates that the pressure between 15 Aug and 18 Aug never dropped below c. 690 millibars; the second shows that, for a period of a couple of hours on 16 Aug, the temperature rose above freezing. If water had been present on the surface, then, would it have been liquid during this brief period? The pressure and temperature seemed to satisfy the conditions for liquid water as I understand them (indeed, the pressure seems to be high enough (just) on a 24-hour basis to allow for the presence of liquid water). Thanks in advance for your opinions (corroborative or not!) on this. |
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Aug 16 2013, 09:17 PM
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Member Group: Members Posts: 105 Joined: 13-July 05 From: The Hague, NL Member No.: 434 |
The dotted line at 32 F is too pessimistic when you're thinking about liquid water on Mars. A reasonable quantity of dissolved (soluble) salts would easily drop the freezing point to 25 F or lower, which brings most of the measured temps "above the line".
But pls. bear in mind that all the mentioned triple point- and other liq-vap data for water are measured in what's called a "closed system", i.e. 100% water inside a cylinder with a moving piston. An equivalent closed system at Mars would have to be underground. At the surface of Mars we have an open system with such an extremely low partial pressure of water (%water vapor in Mars atmosphere * abs. pressure at the surface of Mars) that we're nowhere near liq/vap equilibrium. Any water pushed to the surface of Mars would "explode" into vapor. |
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Aug 16 2013, 11:54 PM
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
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: QUOTE 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. -------------------- |
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Aug 17 2013, 04:11 PM
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#4
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Member Group: Members Posts: 105 Joined: 13-July 05 From: The Hague, NL Member No.: 434 |
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 |
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Aug 17 2013, 05:55 PM
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
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: QUOTE 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: QUOTE 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: QUOTE 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. -------------------- |
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