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The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure.

There is a lot of frozen ice on mars, but it can't melt, because of the low air pressure, it sublimates to steam if it's warm enough. But what air pressure is needed, to have 4°C fluid water on it's surface?

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    $\begingroup$ Thiss smells like homework. Have you tried looking at a phase diagram for water? $\endgroup$ – Sean Jan 6 '15 at 14:21
  • $\begingroup$ This isn't homework. I watched a documentary about Mars, and I'm curious, if it could have oceans, or not $\endgroup$ – Iter Ator Jan 6 '15 at 14:26
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    $\begingroup$ What @Sean means is that it is similar to a homework-like exercise. You are essentially asking us to do the math for you; to solve a problem for you. When what you should be asking is how such a problem could be solved by you; what the tools are to figure this out and what the physics behind them are $\endgroup$ – Jim Jan 6 '15 at 14:30
  • $\begingroup$ Sorry for that type of question. I haven't heard about triple point yet, so that is, what I was looking for. And now I can do the math, or look at the diagram $\endgroup$ – Iter Ator Jan 6 '15 at 14:42
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The triple point of water is at 611.73 Pa and 273.16 K (0.01 °C), so it's actually very close. There can be liquid water in the lowest laying regions, especially if it comes as brine, but it won't last long and the temperature has to be just right.

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Here is the phase diagram for water from Wikipedia: enter image description here

The average Martian surface temperature is 218K. From the diagram, you can see that at that temperature, there cannot be liquid water, however, you specified 277.15K. From the Clausius-Clapeyron relation, the required pressure for liquid water to form at 277.15K is 812.7 Pa. Given that the pressure on Mars ranges from 30 Pa to 1155 Pa (on Olympus Mons and in the Hellas Planitia respectively), and that the temperature on Mars can get as high as 300K in the summer, it is certainly possible to have liquid water on Mars temporarily at certain times of the Martian year.

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