Today, it is thought that climate change took place on Mars about 3.7 to 3.8 billion years ago, when environmental conditions changed from a somewhat neutral, potentially life-sustaining and sporadically humid environment to a much more acidic, dry, cold environment that is hostile to life. The main reason for this, according to current knowledge, was the gradual loss of the Martian atmosphere and a change in the planet's volcanic activity.
One of the reasons why Mars lost its atmosphere is the loss of its magnetic field, which was active during the first 500 million years. As it grew ever weaker the solar wind was able to gradually split the molecules in the atmosphere, and the resultant ions were accelerated and lost to space. As a result, and also due to the declining volcanism, the atmosphere became thinner and thinner. In addition, Mars is only half the size of Earth, so its gravitational force is barely sufficient to bind atmospheric molecules to it. Below a certain atmospheric pressure, water can no longer remain liquid on the surface of a planet – it can remain as ice or gas. The lack of precipitation on Mars ultimately collapsed the water cycle.
Due to the current, very thin atmosphere, liquid water on the surface of Mars cannot be stable. Even if the temperature was favorable, it would evaporate immediately. However, beneath the surface it still seems to be abundant – in the form of water ice. The two polar ice caps on Mars also consist of a mixture of frozen water and carbon dioxide ice. Under very extreme conditions (for example, in very saline environments), liquid water could theoretically still exist for a short time on Mars.