Low Atmospheric Pressure
The Martian atmosphere is composed mainly of carbon dioxide and has a mean surface pressure of about 600 pascals, much lower than the Earth's 101,000 Pa. One effect of this is that Mars' atmosphere can react much more quickly to a given energy input than that of Earth's atmosphere. As a consequence, Mars is subject to strong thermal tides produced by solar heating rather than a gravitational influence. These tides can be significant, being up to 10% of the total atmospheric pressure (typically about 50 Pa). Earth's atmosphere experiences similar diurnal and semidiurnal tides but their effect is less noticeable because of Earth's much greater atmospheric mass.
Although the temperature on Mars can reach above freezing (0 °C (32 °F)), liquid water is unstable over much of the planet, as the atmospheric pressure is below water's triple point and water ice simply sublimes into water vapor. Exceptions to this are the low-lying areas of the planet, most notably in the Hellas Planitia impact basin, the largest such crater on Mars. It is so deep that the atmospheric pressure at the bottom reaches 1155 Pa, which is above the triple point, so if the temperature exceeded 0 °C liquid water could exist there.
Read more about this topic: Climate Of Mars, Atmospheric Properties and Processes
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