[X] My Assistant

[Seryddwr]

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.

[Harder]

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.

[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.

[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.