Final answer:
Life isn't possible on Mars now due to the loss of its early atmosphere, harsh solar radiation, and absence of significant liquid water on the surface. Scientists are searching for fossil life, and exploration must be cautious to avoid contamination with Earth microbes.
Step-by-step explanation:
Life is not currently possible on Mars due to a number of harsh conditions. Mars has undergone significant changes through time. Initial warmer and wetter conditions that might have been conducive to life have transitioned to a much harsher environment. Mars lost most of its early atmosphere, and the surface water began to dry up, becoming saltier and more acidic until no significant liquid water remained on the surface. Moreover, the surface is bathed in harsh solar radiation, which has a sterilizing effect and would make the survival of life as we know it very unlikely. Past conditions on Mars were potentially similar to Earth, which has led scientists to believe that life could have existed there billions of years ago, prompting the search for fossil life instead of current life forms.
Adding to the complexity, the introduction of Earth microbes through human exploration could contaminate Mars and pose challenges in keeping potential Earth and Martian biospheres isolated. Any potential life forms on Mars might exist underground, away from the harsh surface conditions. Thus, the scientific focus has been on 'following the water,' looking for subsurface environments where water might exist in liquid form, offering a potential haven for life.