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I know that I can just read off the phase diagram for water (for the surface atmospheric pressure on each object). But could there possibly be some nuances that someone might miss just from viewing the diagram?

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Water does not form a liquid at very low pressure. Here is the this phase diagram:

enter image description here (image from http://www.lsbu.ac.uk/water/phase.html and appears to be under the creative commons non-commercial no-derivatives)

The lowest pressure at which water is liquid is that of the triple point: 273.15 K and 611.73 Pa. That pressure is about 0.006 atmospheres.

Wikipedia puts the surface pressure on Mars at around 636 Pa, from which we conclude that there is just barely room for water to be a liquid in a very narrow temperature band above the Earthly freezing temperature (and even this relies on getting the full pressure quoted above which won't be true except at the lowest elevations).

No asteroid has sufficient atmosphere to maintain liquid water. If you have ice and heat it, it will sublime (convert directly to vapor).

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The answer is correct for pure water. But water on Mars is likely to be very salty, and salty water can be liquid for up to 40 degrees or so below the triple point. –  Pete Jackson Aug 3 '11 at 20:35
    
That is a very good point, alas I don't know where to look for P--T--salinity phase diagram. –  dmckee Aug 3 '11 at 21:16

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