According to this[1] the pressure on Europa is "one hundred billionth that of the Earth". Your graph uses a log scale, so maybe I'm measuring this wrong, but I'm pretty sure under so little pressure, water can be vapor at -50ºC. So the pressure is hardly negligible in this case.
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Only at one atmoshpere of pressure.
The difference is negligible: http://i.stack.imgur.com/n6LXj.gif
According to this[1] the pressure on Europa is "one hundred billionth that of the Earth". Your graph uses a log scale, so maybe I'm measuring this wrong, but I'm pretty sure under so little pressure, water can be vapor at -50ºC. So the pressure is hardly negligible in this case.
[1]http://hubblesite.org/newscenter/archive/releases/1995/12/te...