91Ó°ÊÓ

The conditions of pressure and temperature at which solid, liquid and gaseous states coexist is called the triple point.

A phase is stable along a range of temperature and pressure, which act as a transition line across which the phase change do occur. This is called as the phase equilibrium line.

Below the triple point pressure, the liquid state cannot exist. In the figure given, atmospheric pressure of Mars (given as 600 Pa = 0.006 atm ) is labelled. Given, the temperature range of consideration for mars is $-30°\mathrm{C}\mathrm{to}-100°\mathrm{C}$.

For each particular compound, there exist specific constant value of triple point pressure and temperature. For all the conditions of atmospheric pressures below the triple point pressure of the compound, the compound either exists as solid, or gas, and not liquid. This is clear from the figure.

The triple point of carbon dioxide is given in the table 18.3, as $5.17×{10}^5\mathrm{Pa}=5.17\mathrm{atm}$.

But the atmospheric pressure of Mars is 0.006atm , which is far less than the triple point (The region included is labelled inside the white circle). This implies that the state in which carbon dioxide is feasible to exist in the pressure of Mars is solid. Thus, they exist as dry ice on Mars.

Similarly, the triple point of water (from table 18.3 ) is$0.00610×{10}^5\mathrm{Pa}=0.00610\mathrm{atm}$

The atmospheric pressure of Mars is 0.006atm , which is slightly less than the triple point temperature of water. This means, mostly, the liquid state may not exist. But, if there is a rise in atmospheric pressure and temperature, the melting can occur, for ice to become water. That is still an extreme condition. Generally, water does not exist in liquid state, in a stable condition.

Hence, there could not be liquid $C{O}_2$on the surface of Mars and there could not be liquid water, in a stable state.