Mars used to be similar to Earth until its surface became rocky.
About 4 billion years ago, Mars lost its magnetic field
and that way it lost its life that existed before that happened.
It might have habitable microorganisms that lived in
water. The atmosphere in mars is just layers of gases.
Carbon dioxide 95%
Nitrogen 2.8%
Argon 2%
Oxygen 0.174%
Carbon monoxide 0.0747%
Water vapor 0.03% [Variable]
The atmosphere is much thinner than the atmosphere on Earth.
It has only 95 percent of carbon dioxide and less than 1 percent
of oxygen.
Credit: NASA/JPL-Caltech
(1, 2, 3)