Composition of the Martian AtmosphereOn Earth, the atmosphere is composed of mostly of nitrogen (78%) and oxygen (21%). On Mars, the atmosphere is made mostly of carbon dioxide (95.3%), with a little nitrogen (2.7%), and a lot of dust particles mixed in. This means that there is no oxygen, which is essential to some forms of life.
A by-product of the presence of oxygen in an atmosphere is ozone, since oxygen can change over time to form ozone. Ozone is especially good at filtering out the sun's harmful ultraviolet radiation. Since Mars has no oxygen, this means that the surface of Mars is more exposed to these harmful rays of the sun than is the surface of the Earth.
There is very little water vapor in the atmosphere of Mars. Such small amounts of water in the atmosphere means that clouds are rarely seen in the Martian sky. And it is doubtful that it ever rains on Mars!