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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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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Oct 2 2012, 07:31 AM
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Member Group: Members Posts: 105 Joined: 13-July 05 From: The Hague, NL Member No.: 434 |
As mshell rightly mentions, the phase diagram is for pure water only. The implication is that in order to assess the equilibrium condition you have to use the partial pressure (for water) at Mars, rather than the absolute pressure. This is linear (every molecule has the same volume) so for agument's sake 1% H2O content in the Mars atmosphere means you have to read the phase diagram at 1% of the prevailing pressure to see what can exist in equilibrium. I think this definitely rules out liquid water.
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Oct 2 2012, 03:10 PM
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Member Group: Members Posts: 507 Joined: 10-September 08 Member No.: 4338 |
... partial pressure (for water) at Mars, rather than the absolute pressure ... The partial pressure determines whether evaporation occurs, i.e., vaporization of molecules from the surface of a liquid. The total pressure governs boiling, i.e., vaporization of molecules from the interior of a liquid. Even on Earth, the partial pressure of water is below the triple point most of the time. Water is generally unstable on Earth's surface, which is why hanging wet clothes out to dry is a reasonable thing to do. The issue is not whether water is stable (it is not), but whether it is replenished, and how long it can persist. The latter is affected, for example, by the area of the exposed surface, which is why water in a bottle with a narrow top evaporates more slowly than wet clothes. |
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