Final answer:
Mars's atmosphere was lost due to its cooling, lower gravity, and thinning atmosphere, which led to reduced water vapor and a runaway refrigerator effect. As the atmosphere thinned, the surface water evaporated, conditions became harsher, and the Martian surface became uninhabitable.
Step-by-step explanation:
The cooling of Mars and the subsequent loss of its atmosphere were driven by a few key factors. Over time, Mars cooled significantly which caused most of the planet's water to freeze out of the atmosphere. This reduction in atmospheric water vapor diminished Mars's ability to retain heat, resulting in a runaway refrigerator effect, in stark contrast to the runaway greenhouse effect seen on Venus. Additionally, Mars's lower gravity facilitated the escape of atmospheric gases more easily than from Earth or Venus, leading to a gradual decrease in surface temperatures as the atmosphere thinned.
Five decades of robotic exploration have helped us understand that early Mars had conditions that could support liquid water and, potentially, life. But as Mars lost its atmosphere, conditions became harsher, the water dried up, and the remaining water became saltier and more acidic. This loss of atmospheric pressure, combined with exposure to solar radiation, made the Martian surface uninhabitable.
Ultimately, the fate of water on Mars was sealed by its inability to retain a thick atmosphere and maintain a substantial greenhouse effect. Unlike Earth, which sustains liquid water and a balanced climate, or Venus, which succumbed to a runaway greenhouse effect, Mars's environment became cold and dry due to these atmosphere-losing processes.