Physical states of water
To fully characterise these changes of state with changing temperature and pressure, a phase diagram is used. A general phase diagram for water often looks like the small diagram on the right. This diagram shows the range of temperature and pressure conditions at which distinct phases of a substance can exist. Point T in this diagram is referred to as the "triple point" of water. It is the a specific temperature and pressure where three phases of water (solid, liquid, vapour) are stable. The pressure of this point is too low to be relevant to Earth (1/166 atmospheric pressure at sea level), but it is similar to the atmospheric pressure conditions on Mars. Point C is called the "critical point" of water, where temperature and pressure conditions are great enough that liquid and steam are indistinguishable from one another. The critical point is relevant to some geologic environments on Earth, such as deep sea hydrothermal vents.
The phase diagram at right is actually simplified. For planetary systems, the real phase diagram for water is more complex. In outer space, temperatures may extend to as low as absolute zero, and pressures may be at vacuum. So, please consider the customized, slightly quirky phase diagram below at and extended scale. In this diagram we use the more commonly used unit for pressure, a bar. The difference between atmospheres and bars is minor, 1 atm = 1.01325 bar.
We'll start with the axes. You'll notice that the temperature axis goes from -273°C (or 0 K) to 400°C. The pressure axis uses a logarithmic scale which means that every increment is 10 times more (or less) than the previous. That means that our pressure differential from sea level to the top of Mount Everest (1bar to 0.3bar) barely registers on that scale. But we do need that range if we want to consider conditions in the interior of
If you like more information about how phase diagrams work, we suggest you watch this video.