The gas-liquid-solid triple point of water is nothing but a temperature and pressure point where all the three states of matter with respect to water are present at the same time. This we have already discussed before in the article. In case of pressure below the triple point as in space, the solid phase i.e. ICE, when heated keeping the pressure constant directly, converts into gas or water vapor.This process is called sublimation. Now in case the parameters are above the triple point and keeping the pressure constant, the ice will first melt into water and then will get converted into water vapor.
In most of the cases, the gas-liquid-solid triple point is the minimum temperature at which the substance can exist in the liquid state. However, this is not true in the case of water. You see in the case of water, the melting point of solid ice will decrease with the pressure function. When we compress the water under constant temperature, then it first converts into solid from the liquid phase. This process is called liquefaction.
A very practical application of this phenomenon is seen in Mars Missions. The triple point of water is very efficiently used in measuring the sea level of Mars. This was done in the Mariner 9 mission of NASA. However, now laser altimetry is used to do the same.
At very high pressure the water shows a very complex behavior, which is still a point of research for scholars.
There exist 15 known phases of ice at high pressure.In case of water, the melting point increases with the rise in pressure. On raising the temperature above 273 K and increasing the pressures the water vapor first converts into liquid and then to ice. However in range of 251 -273 kelvins the water first takes solid phase followed by liquid phase and then again enters solid phase.
Further application of this phenomenon is found in calibrating the thermometers. For this, a device called triple point cell is utilized.
Hope our little informative piece of writing helps you out in your academic pursuits.